University of Groningen The interaction between water-soluble polymers … · 2016-03-07 · surfactants. Before definitely plunging into the matter of polymer-micelle interaction

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The interaction between water-soluble polymers and surfactant aggregatesBrackman, Josephine Charlotte

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THE INTERACTION BETWEEN WATER-SOLUBLE POLYMERS AND SURFACTANT AGGREGATES

RWKSUNIVERSITEIT GRONINGEN

THE INTERACTION BETWEEN WATER-SOLUBLE POLYMERS

AND SURFACTANT AGGREGATES

PROEFSCHRIFT

ter verkrijging van het doctoraat in de Wiskunde en Natuurwetenschappen

aan de Rijksuniversiteit Groningen

op gezag van de Rector Magnificus Dr. L.J. Engels

in het openbaar te verdedigen op vrijdag 30 november 1990 des narniddags te 2.45 uur precies

door

Josephine Charlotte Brachan

geboren op 18 mei 1962

te Haarlem

Promotor: Prof. Dr. J.B.P.N. Engberts

VOORWOORD

Nu zijn alle feiten en conclusies van vier jaar onderzoek vastgelegd op

vele bladzijden en blijft er nog CCn pagina over om de mensen te bedanken, die

ervoor gezorgd hebben dat die vier jaar zo plezierig zijn verlopen. Het

allerbelangnjkst daarvoor zijn de waterlabgenoten geweest. Frank, Wilfried,

Tino, en Saskia, mijn hartelijke dank voor wwel de gezelligheid als de

wetenschappelijke respons. Niet rninder gaat mijn dank en waardering uit naar

Jan Engberts, die mij het vertrouwen en de vrijheid heeft gegeven om mijn

eigen gang te gaan, en dat steeds met zoveel belangstelling heeft gevolgd.

De leden van de leescomrnissie, Prof.Dr. E.J.R. Sudholter,

Prof.Dr. J. Lyklema, en Prof.Dr. R.M. Kellogg, ben ik zeer erkentelijk voor de

vlotte correctie van, en nuttige cornmentaren op het manuscript.

Verder dank aan Willem Kuil, voor snelle levering van vele mooie

plaatjes; Marjan Ossebaard, voor het typewerk; Dr Jerzi Sek (Jerzi, thanks for

the fun with the rheology) en Prof.Dr. L.P.B.M. Janssen, voor de inwijding in

de viscometrie; Gert Haandrikrnan en Nico van Os, voor de micro-

calorimetrische experimenten; de werkgroepgenoten, voor de goede sfeer; en bovenal dank aan Jan Jaap.

CONTENTS

Chapter 1 A bird's-eye view of polymer-micelle interaction

1.1 Introduction

1.2 Aggregation of surfactants

1.3 Introduction to the polymers

1.4 The development of the polymer-micelle model

1.5 Aims of the study

1.6 Survey of the contents

Chapter 2 Interaction between nonionic surfactants and nonionic polymers: fact or fancy?

2.1 Introduction

2.2 The influence of polymers on the critical micelle concentrations

2.3 Clouding behavior and Krafft temperatures

2.4 Microcalorimetry

2.5 Discussion

2.6 Experimental section

Chapter 3 The influence of polymers on the micellization of cetyltrimethylammonium salts 35

3.1 Introduction 35 3.1.1 Cetyltrimethylammonium salts 3 5

3.1.2 Interaction of polymers with cationic surfactants 36

3.1.3 Rodlike micelles of CTAX salts 3 8 3.2 Critical micelle concentrations and aggregation numbers of CTAB 41

3.3 The sphere-to-rod transitions of CTATs

3.4 The polymer-induced transition from a non-Newtonian to a

Newtonian fluid

3.5 Experimental section

Chapter 4 The effect of headgroup charge on polymer-micelle interaction: n-dodecyldimethylamine oxide

4.1 Introduction

4.1.1 A brief glance at semipolar surfactants

4.1.2 The effect of protonation on the micellization of DDAO

4.2 Critical micelle concentrations

4.3 Aggregation numbers

4.4 Clouding of PVME and PPO

4.5 Experimental section

Chapter 5 The effect of headgroup charge on polymer-micelle interaction: mono-n-alkylphosphates

5.1 Introduction

5.2 Critical micelle concentrations

5.3 Clouding behavior of PVME 31 5.4 P-NMR investigations

5.4.1 Introduction 3 1 5.4.2 P-NMR study of n-decylphosphate1PVME

5.5 Preliminary experiments on the effect of PVME on sodium

didodecylphosphate vesicles

5.6 Experimental section

Chapter 6 The effect of headgroup charge on polymer-micelle interaction: spherical and rodlike micelles formed from Zalkylmalonate salts

6.1 Introduction

6.2 The aggregation behavior of mono- and di-salts of

2-alkylmalonic acids in aqueous solution

6.3 Aggregation of mono- and di-salts of 2-akylmalonic acids

in polymer solution

6.4 Clouding behavior of PVME 6.5 Experimental section

Chapter 7 SDS-induced enhancement of the viscosity and viscoelasticity of aqueous solutions of PPO

7.1 Introduction

7.2 The influence of SDS on the viscosity of a PEO solution

7.3 The influence of SDS on the viscoelasticity of a PEO solution

7.4 Experimental section

Chapter 8 An attempt to model polymer-micelle interactions quantitatively

8.1 Introduction

8.2 The models for polymer-micelle interaction developed by

Nagarajan and Ruckenstein

8.3 The 'dressed rnicelle' model of Evans and Ninharn 8.3.1 Theory

8.3.2 The 'dressed rnicelle' model applied to various

polymer-micelle systems

8.4 Comparison of the models

8.5 Experimental section

Chapter 9 Concluding remarks

9.1 Introduction

9.2 Conclusions 9.3 A criterion and a measure for polymer-micelle interaction 9.4 The driving force for polymer-rnicelle interaction 9.5 The role of the charge and structure of the surfactant headgroup

References

Summary

Samenvatting 161

CHAPTER 1

A BIRD'S-EYE VIEW OF POLYMER-MICELLE INTERACTION

1. I Introduction

Studies of the interaction between nonionic, water-soluble polymers and

micelles have their roots in biochemistry, for they originated from the study

of protein-surfactant interactionlv2. Polymer-micelle interaction3 in turn may

now serve as a simplified model for biological binding processes, for instance

to cell membranes. An important difference between proteins and nonionic

polymers is the complete absence of charged groups in the latter. Consequently

polymer-micelle interaction results from an accumulation of relatively weak

binding forces, which makes the association process even more intriguing.

At an early stage of the research in this field, it was recognized that

in the polymer-micelle complex the properties both of the micelles and of the

polymers are mutually modified2'. To mention the most important aspects in

view of industrial applications, the solubilization power as well as the

viscosity of an aqueous solution of polymer-bound micelles is higher than that

of the separate surfactant and polymer sol~tions*~'~. This commercial

interest is reflected in the fact that many of the early reports on

polymer-micelle interaction originated from industrial research

laboratorie~?-~. The properties of the polymer-micelle complex are very well

appreciated in formulations for paints and coating$, in cosmetic products10,

and in laundry detergents1'. Polymer-micelle interaction also plays a role in

tertiary oil recovery". Protein-swfactant complexes, in contrast, are used

in totally different applications, such as ele~tro~horesis'~ and the

reconstitution of membrane proteins13-15.

Although the applications of polymer-micelle complexes are numerous, many

problems are still unsolved. Particularly, the question of how the precise

chemical structure of the surfactant and the morphology of the unperturbed

micelle are related to the tendency for association with polymers poses a

challenge for chemists. The very limited choice of surfactants as well as

dubious generalizations in the study of polymer-micelle interaction3 certainly

obscured this problem. For example, the credo that mainly anionic micelles

interact with polymers but cationic micelles hardly and nonionic micelles not

at all, was deduced with sodium n-alkylsulfates, predominantly sodium

n-dodecylsulfate, as representatives of anionic surfactants, n-alkyltrimethyl-

ammonium bromide of cationic surfactants, and n-alkoxypoly(ethy1ene oxide)

ethers of nonionic surfactants. Not only is the generalization unwarranted,

but also the rationalization behind it is hampered by the limited choice of

surfactants.

Before definitely plunging into the matter of polymer-micelle interaction

in aqueous solution, an introduction to the individual constituents, e.g.

surfactants and polymers, is in order. Although many bookshelves could be

loaded with books concerning the most abundant constituent, i.e. waterI6, no

special section in this thesis is devoted to this common chemical.

Nevertheless, its importance for the subject will trickle through the entire

text.

At the end of this chapter the aims of the study described in this thesis

will be outlined and a survey of the contents is presented.

1.2 Aggregation of surfactants

Simple, single-chain surfactants, consisting of an akyl-chain with 8 to

18 carbons and a polar headgroup, may aggregate in water into micelles, above

the critical micelle concentration (cmc), which is really a critical

surfactant concentration for micelle f~rmation""~. However, why micelles are formed, what their structure is, and what their properties are, remain

questions that ensure a lively debate in the literature for several years to

come. Since different authors study different surfactants, which may very well

form different types of micelles, deviating results and discrepancies in

interpretation may be anticipated. 19,20b The driving force for micelle formation is long thought to be

hydrophobic interaction2'. Hydrophobic interaction is the breakdown of a part

of the hydrophobic hydration layers around the alkyl chains due to a reduction

in contact area between water and alkyl chains upon aggregation. Since water

molecules are less ordered but also less hydrogen-bonded to one another in bulk water than in such a hydrophobic hydration layer, entropy is gained but

enthalpy is lost upon hydrophobic interaction. It is argued, however, that the

entropy and enthalpy changes associated with hydrophobic interaction are 22 largely compensatory , and that the driving force for micellization is London

2Ob.19 dispersion interaction between the akyl chains . We note that ~ r i v a l o v ~ ~

has presented some views on the importance of London dispersion forces on

protein folding.

Another much disputed matter is the shape of the micelle. Particularly

the extent of water penetration24'26 and the roughness of the micellar

surface2' are hot items, but also the rnicroviscosity and degree of

ordering28d9 in the core, and the surface potentiaf031 are subject of

controversy.

A vast number of techniques has been used to study micelles. These can be

subdivided into (i) those that make use of probe molecules, (ii) those that

make uses of intrinsic properties of the system, and (iii) theoretical

modelling and calculations. The use of probe molecules necessitates either an

assumption on the location of binding in a micelle, in order to draw any

conclusions about the environment (water penetration, polarity,

microviscosity), or an assumption on the environment of the various locations

in a micelle, in order to specify the location. ~alasubramanian~~ and Drummund

and ~ r i e s e r ~ ~ have recently commented on these difficulties in interpretation.

Balasubramanian argues that most molecules that are not completely apolar will

be located at the micellar interface near the water since "the enormous

surface area to volume ratio that spherical micelles of nanometer radii

possess, amplifies the weak surface-active tendency of even a mildly polar

moiety". Of course, the most serious hesitation in the use of data from probe

molecules is the possibility that they alter the total system or at least the

local binding site. Therefore, the second class of techniques, which makes no

use of probes, is in principle preferable. However, these experiments also

leave ample cause for ambiguity. Consider the possibility that such a

technique reveals 'wetting' of some CH, groups near the end in the alkyl

chain. Does this mean that water penetrates the micellar core, or that the

alkyl chain folds back to the surface of the micelle?

Despite the problems in the interpretation of experimental data several

models have been proposed. The Hartley in which the micelle is

pictured as a kind of three-dimensional asterisk (Figure 1. la), has persisted

for half a century but has now been abandoned except in textbooks in general

chemistry35. ~ e n ~ e r ~ ~ , in 1979, gave the impetus for the development of new

micelle models in a beautiful review article. A 'Menger' micelle2' is

represented in Figure l.lb. Other authors, including ~ r o m h e r z ~ ~ , Dill and

lor^^^, and Cruen3' followed. Fromherz and Dill/Flory suggest the most

structured rnicelles, whereas Menger's rnicelle is the most chaotic one. ~ r u e n ~ ~

proposed an attractive mean (Figure 1.2). The surface of the micelle,

according to Gruen, is rather smooth, contrary to the micelle proposed by

Menger. The hydrocarbon chains are flexible. This flexibility combined with

packing requirements allows all chain segments to sample the surface, contrary

to the DillIFlory model. According to the model of Gruen, the chains are

somewhat straightened compared to alkyl chains in bulk liquid hydrocarbon.

For the understanding of polymer-micelle interaction, it is of importance

to note that most authors agree on a considerable extent of hydrocarbon-water

contact, whether or not by water penetration. Furthermore, it is generally

accepted that the size of the micelle is dictated by a balance of forces. The

Figure 1.1 Schematic representation of a micelle according to Hartley (a),

and according to Menger (b). Taken from ref. 34 and 27,

respectively.

Figure 1.2 Schematic representation of a rnicelle according to Gruen. Taken

from ref. 39c.

unfavorable hydrocarbon-water contact pushes the system to a smaller surface

area to volume ratio and thus to larger micelles. However this force is

opposed by headgroup repulsion, which tends to decrease the aggregation number

and increase the surface area to volume ratio. The result is a micellar system

of low dispersity. It should be mentioned that micelles are highly djmarnic: a

monomer remains in a micelle only for 10'~ to 10" s depending on the chain

length of the ~urfactant~~.

The above discussion focuses on spherical or spheroid micelles. However,

some single-chain surfactants aggregate into cylindrical aggregates (Figure

1.3a). The formation of these rodlike micelles usually requires high salt or

surfactant concentrations. The headgroups are more closely packed in a

cylinder than in a sphere and, concomitantly, headgroup interaction is more

substantial. Apart from this rather general phenomenon of micellar growth at

high salt or surfactant concentration, there are several combinations of

surfactants and counterions that aggregate into rods even at low

concentrations. This will be discussed in sections 3.1.3, 3.3, 3.4, and 6.2.

Double-chain surfactants often form bilayers in solution, which can be

closed to form vesicles (Figure 1.3b). In section 5.5 some preliminary results

on the interaction of polymers with this type of aggregate will be presented.

H (a) (b)

Figure 1.3 Schematic representation of (a) a rodlike micelle. (b) a vesicle.

1.3 Introduction to the polymers

The polymers PEO, PVME and PPO play a major role in this thesis, though

other polymers such as PVP, HPC and PVA-Ac have been used as well (Scheme 1).

In this section these polymers will be surveyed briefly4'.

Polyethylene oxide (PEO)~~, sometimes referred to as polyethylene glycol

(PEG), is the most familiar of the above mentioned polymers. Its low toxicity

and pseudoplastic properties (section 7.3) produce unique benefits for all

kinds of applications43. To mention a few: it is used in contact-lens fluid;

detergents and lotions, as adhesive, as thickener in acid cleaners and for

drag reduction, foam stabilization, lubrication and oil-well flooding. At room

temperature PEO is miscible with water in all proportions. It is interesting

to note that poly(methy1ene oxide) (PMO)~~, which contains a larger portion of

hydrophilic ether oxygens, is not soluble in water and neither is

poly(trimethy1ene oxide) (PTMo)~~. It has been argued that the exceptionally

good water solubility of PEO stems from its conformation that allows a

PMO PTMO

Scheme 1

PEO

H o { c H 2 - c H 2 0 & H

PVME

--ICH2- C H k

I OCH,

PVA

HPC

PPO

P V P

PVA- A c

hydrating water molecule to bridge two ether linkages4446, as shown in Figure

1.4. Upon heating, an aqueous PEO solution eventually becomes hazy, that is,

it exhibits a lower critical solution temperature (LCST)~*-". The LCST is

usually called cloud point or clouding temperature. At the cloud point a

microphase separation takes place in a polymer-rich phase and a water-rich

phase. It is believed to arise from a breakdown of the protective hydration

Figure 1.4 The hydration model of PEO. The line drawing is traced from a

photograph of molecular models. Taken from ref. 47.

sheath of the polymer4750. In pure water, the cloud point of a PEO solution

is near the boiling point of water. Addition of most salts lowers this

temperature. Of great interest to polymer-micelle interaction is the fmding

that anions exert a greater influence on the cloud point than cationss0.

PVP and PVA-Ac (Scheme 1) follow PEO in popularity, especially in

polymer-rnicelle research. PVP is composed of two kinds of groups: a dipolar

amide group, which is capable of hydrogen-bonding with water, and hydrophobic

groups such as the methylene and methine moieties in the ring and backbone.

Overall, PVP is considered to be less hydrophobic, i.e. more hydrophilic, than

P E O ~ ~ . The hydrophobicity of PVA-Ac depends on the degree of acetylation, or

more precisely, on the degree of the hydrolysis since it is prepared from

PVAc.

The isomeric polymers PVME and PPO (Scheme 1) are much more

hydrophobic than PEO, though they are still soluble in water. For instance,

PPO and PVME are also soluble in all kinds of organic solventss2, whereas PEO

is not. In contrast to PVME, the water solubility of PPO is limited to low

molecular weight samples (mw < 1500). The hydrophobicity of PVME and PPO is

reflected in the lower cloud temperature of 34 OC for P v M E ~ ~ ' ~ and 25 to

30 OC for PPO (sections 2.3, 4.4, 5.3, and 6.4). The clouding phenomenon of

PVME, that occurs quite near the biological temperature of 37 OC, even led to

the assumption that it may model the mechanism of temperature control in

homeotherrnic animals47.

Sandell and s or in$^ have concluded from a comparison of theoretical

calculations and experimental viscosity data on PPO that this polymer (which

is rather an oligomer) exists in aqueous solution as a tightly coiled disk

with most of the hydrophobic methyl groups in the center of the coil. In an

apolar solvent like benzene the disk is uncoiled and a looser Gaussian coil

configuration is formed48b. This uncoiling may also play a role in the binding

of PPO chains on the micellar interface. Lakhanpal et a1.49 have compared the

heats of mixing of PPO with water with those of PEO and concluded that for

both polymers hydrogen bonds between water and oxygen chain atoms are formed

almost quantitativelys0.

The final polymer to be discussed in this section, is an alkylated

cellulose, namely hydroxypropyl cellulose (HPC) (Scheme 1). HPC, as well as

its methylated or ethylated analogs, shows a strong tendency for

self-aggregation in aqueous solution (for HPC above 0 0~)5152. This does not

directly lead to clouding, though upon heating above 42 0c5' a microphase

separation readily occurs. It appears that, in nonpolar solvents, HPC

experiences extensive intramolecular hydrogen bonding, whereas in water,

intermolecular hydrogen bonding with solvent This easy

adaption of HPC to solvent polarity causes it to be soluble in an

exceptionally broad range of solventss2. Neverheless, HPC is a relatively

hydrophobic polymer.

The performance of these polymers in polymer-rnicelle interaction will be

explored in the next chapters. One should bear in mind that the structure of the polymer is at least as important as that of the surfactant in determining

the interaction.

1.4 The development of the polymer-micelle model

The recognition of the interaction between nonionic, water-soluble

polymers and surfactants occurred decades later than the notion that

surfactants proper form aggregates. But the morphology of the micelle has

attracted minimal comment until the eighties, despite the enormous number of

articles devoted to the properties of rnicelles. In contrast, the morphology of

the polymer-surfactant complex has puzzled chemists from around 1955 on, when

the pioneering work of saito4 was published, till the end of the seventies,

when an NMR study of the PEOISDS system by cabaneS4 f m l y established the

contemporary model. Of course many intruiguing questions have remained.

Particularly, the relation between the chemical structure of both the

surfactant and polymer, and the propensity for interaction, but also even more

fundamentally, the dominant driving force for interaction are still debated in

the literature.

In 1957, saito4 published the first extensive study on polymer-surfactant

complexation. Two major observations were (i) the increase of the viscosity of

an aqueous PVP solution upon addition of SDS and (ii) the increase in

solubilization power of a SDS solution upon addition of PVP. Though it was

suggested that the aggregation of surfactant molecules in the presence of

polymer resembles normal micellization, he proposed that, at a low

surfactant-to-polymer ratio, the surfactant molecules bind individually to the

polymer (as is the case for protein-surfactant interactions at low surfactant

concentration). This binding was thought to occur by dipolar interaction of

the surfactant headgroups with polar sites on the polymer, while the

surfactant chain was thought to lie parallel to the polymer chain. At a higher

degree of saturation the location of the alkyl chain would be altered.

However, saito4 wisely stated that the structure of the polymer-micelle or

polymer-surfactant complex had not yet been clearly established.

The major concept in the following decade, including the appearance of

Breuer and Robb's review article2, was the picture of individual molecules

along the polymer, with some kind of micellization occurring above the cmc of

the surfactant in pure

Many aspects of polymer-micelle interaction were revealed in that period,

including the fact that complexation takes place even below the normal cmc3'.

This has long been used as support for individual bindings6. But it was found

also that above a minimum molecular weight (mw) of the polymer, the

interaction is independent of mw7'83157, and that a certain saturation takes

place at increasing surfactant concentrations3157. The importance of

hydrophobic interactions2 for the polymer-surfactant complex formation was

deduced from the stronger interaction of more hydrophobic polymer$31 and of

surfactants with a longer allcyl chain3'. Much later, in 1987, the measurement

of heat capacities and apparent molar volumes also revealed a shift of these

thermodynamic properties upon addition of polymer, in the direction of

enhanced hydrophobic asso~iation~~.

ones^' reasoned in 1967 that the length of the polymer chain divided by

the length of the surfactant molecules, lying parallel to the polymer chain,

should determine the saturation concentration. Surprisingly, the reasoning

holds for his data on PEO/SDS. It would have been obvious to test the relation

by changing the alkyl chain length. Although later the increase in interaction

strength with increasing chain length was reported several times, the data on

the total amount of surfactant bound at the saturation concentration for

surfactants with a varying number of carbons atoms in the chain are

surprisingly few. The data of shinoda6 (amount of adsorption per gram of PVP:

9.0 mmol for SDS, 9.2 mrnol for C11H2,0S0,Na and 8.2 rnrnol for

C,,H2,0S0,Na), however, do not point to such a relation. Anyway, Jones' idea

has been neglected in the literature and later the entire concept of binding

of individual surfactant molecules to the polymer has been rejected.

The retreat from the 'individual binding' concept was initiated in 1971.

shinoda6 derived from cmc values for a series of homologous sodium alkyl

sulfates CnH2n+,0S0,Na (n = 10, 11, 12) in the absence and presence of PVP

that the free energy of transferring a CH, group from the aqueous solution to

either the aggregate (polymer-micelle complex) or the rnicelle is in both cases

1.1 kT. He deduced from these data that C H2n+10S0,Na molecules adsorbed on

PVP contact each other, and are not uniformly distributed on the PVP macromolecule, right from the initial stages of adsorption. In the same year,

~ a n ~ e ' commented on the discrepancy between the viscosity increase upon

polymer-surfactant complexation, which indicates coil expansion, and the

increased solubilizing power of the polymer-surfactant complex, which involves

a compact structure of the complex. He also stressed the cooperative nature of

the complex formation, which is apparent from the existence of a critical

concentration for its formation. These arguments appear to require the

conclusion that rnicelles bind to the polymer. Nevertheless, ~ a n ~ e ' explains

the discrepancy in the following way: at the concentration (below the

unperturbed cmc) at which solely the complex can be responsible for

solubilization of dyes, the polymer (PVP) exists partly as a compact knot in which surfactant molecules penetrate. The rest of the polymer is unoccupied

and may expand due to electrostatic repulsion of either polymer-bound micelles

or individually bound ions, without disturbing the solubilization process.

The idea that surfactants bind to polymers in clusters took ~ x o l d ~ * ~ ~ ~

and the next issue gradually became apparent. Tokiwa and Tsujii (1973)', as

well as Fishman and Eirich (1975f9 assumed without any discussion that the

surfactant micelles encompass portions of the polymer chain. shirahamaS6,

however, in 1976 suggested binding of the polymer at the micellar surface

(above the cmc) leading to a stabilization through reduction of the core-water

contact but he did not yet believe in the existence of micelles below the

unperturbed cmc. He also predicts lower aggregation numbers for the

polymer-bound rnicelless6.

Cabane (1977)~~ definitely established the polymer-micelle model as it is

quite generally accepted to date (see for instance the excellent review

article of coddard3). Figure 1.5 and 1.6 give schematic representations of the

model. He studied the PEOISDS system with 13c-, 'H-, and ? 3 ~ a - ~ ~ ~ . Only the

first three carbon atoms of SDS, counted from the SO, headgroup, exhibit I3c chemical shifts which are affected by the presence of PEO. cabaneS4 concluded

that in the polymer-micelle complex the major part of the alkyl chain resides

in an environment indistinguishable from a normal micelle, which is a micellar

core. The first three carbon atoms are influenced by the polymer because the

Figure 1.5 Schematic representation of a polymer-micelle complex according

to Cabane. Taken from ref. 60.

(a) (b) Figure 1.6 Schematic representation of a polymer-rnicelle complex according

to Nagarajan. The probably more realistic representation (a) from

1989 compared to the artistic representation (b) from 1985 shows

the development of the model. Taken from ref. 61e (a) and ref.

6 1 b (b) respectively.

polymer binds at the micellar surface, which in an unperturbed micelle

(according to Cabane) is occupied by -SO,- groups for only one third. The

other two thirds of the surface contain primarily the first chain segments

(according to Cabane). The NMR signals of the polymer are barely influenced by

complexation with rnicelles. That is interpreted as an indication that only a

fraction of the polymer is actually adsorbed onto the rnicellar surface,

whereas the rest protrudes as loops in the aqueous surroundings. This was to

be expected because total adsorption and thus a restricted mobility of the

polymer chain would be very unfavorable for entropic reasons. cabaneS4 also

mentions two sound, common sense reasons why PEO should bind at the micellar

outer sphere. First, PEO does not dissolve in hydrocarbons, and will

therefore, not penetrate into the micellar interior; second, since most

probes, even those with only a slightly polar character reside in the micellar

outer layer, so would the hydrophilic polymer, PEO. Both reasons also hold for

PVP. Hydrophobic polymers like PPO, PVME, PVA-Ac and HPC, however, do

dissolve in organic solvents. Nevertheless these polymers are sufficiently

polar to dissolve in water, and, like polar probes, will seek the outer layer

of the rnicelle.

Very recently, Kwak et a1.62 published an NMR study on the system a-phenyldecanoate/PEO and concluded that PEO resides in the interior of the

micelle. The conclusions were based on 'H aromatic ring current-induced shifts

of the PEO protons. However, the argumentation hinges on the debatable

assumption that the phenyl moieties do not fold back to the surface of the

micelles.

Two additional indications that both hydrophilic polymers and relatively

hydrophobic polymers bind to the micellar surface are the smaller aggregation

numbers of polymer-bound micelles 60.63-66 and the variation in interaction

tendency with headgroup charge 4,85358.67 (section 3.1.2). The decrease in

aggregation number was initially only documented for the systems pEOISDS60,63~6667 pVplSDS63~646667

, , and PVAISDS~~ but has recently also

been reported for PPOISDS~~ '~~ and, in this study, for CTAB in the presence of

PPO and PVME (section 3.2). This is in accord with the presence of the polymer

at the micellar surface, whereas solubilization in the core is expected to

lead to an increase in aggregation number. The effect of short-chain and

long-chain alcohols and alkanes on the aggregation number of micelles supports

these ~onsiderations~''~~. Short-chain alcohols, which reside at the micellar

surface decrease the aggregation number7', whereas alkanes which reside in the

core increase the aggregation number6'. The finding that cationic surfactants

usually interact more weakly with polymers than anionic surfactants do, (which

will be discussed in detail in section 3.1.2). also points to a location of

the polymer in the same region as the headgroups, whatever the origin of the

difference is.

Gilanyi and in 1981, began an endeavor to find a quantitative model for the prediction of binding isotherms and critical concentrations.

Their model was based on the mass-action law for micelli~ation~~. Like all

other models for polymer-micelle interaction 61.71.72 published until today the

predictions were checked with experimental data on SDS micelles bound to the

hydrophilic polymers (PVA, PVP, and PEO). The models of ~uckenstein~l and

~ a ~ a r a j a n ~ l were checked on the system PEO/SDS, and the model of ~ v a n = ? on

the system PEO/C~(DS),~~ (see also Chapter 8). Gilanyi and also made

the important point that the formation of regular (free) micelles may take

place at a surfactant concentration below the saturation concentration of the

polymer, since the activity of the surfactant rises as the polymer is loaded 60,61b with micelles . The activity of the surfactant ions may thus reach the

critical value for formation of free micelles before binding of micelles to

the polymer is completed.

Several authors have studied the influence of polymers on the properties

of probe molecules bound to micelles. The various probes, such as the fluorescence probe, pyrene64'74, the kinetic probes, 1-benzoyl-3-phenyl-

1,2,4-triazole and 1-benzoyl- 1,2,4-tria~ole~~, and several persistent

nitroxide ESR spin indicate a more open and water-penetrated

structure of the polymer-bound micelles.

Details concerning the polymer-micelle interaction have been provided by

modem techniques like NMR self-diffusion77, electric birefringence7', 79 ultrasound absorption , and the use of surfactant-ion selective

electrode^^^'^^. Nevertheless, a consistent explanation for the influence of

the precise chemical structure of surfactant and polymer on the interaction

tendency and a quantitative model that is applicable to more systems than

PEO/SDS alone are still lacking. A clarification of just these problems is of

the utmost importance for the further understanding of polymer-micelle

interaction and development of the model.

1.5 Aims of the study

The main incentive of the work presented in this thesis is to obtain a

better understanding of the relation between the chemical structure of both

surfactant and polymer, and the tendency for polymer-rnicelle interaction.

Particular emphasis is placed on a systematic search for the influence of the

charge of the surfactant on the interaction. Therefore, the first aim was to

scrutinize the interaction of nonionic surfactants with polymers, even though

such interaction was reported to be absent. Subsequently, the influence of

charge variation was investigated. In order to avoid dramatic changes in

chemical structure upon charge variation, surfactants were chosen or

developed, that can be protonated or deprotonated in a reasonable pH range.

Since the properties of the polymer-bound micelles should be compared to those

of the unperturbed micelles, the aggregation behavior of the surfactant in

aqueous solution in the absence of polymers has been explored also. We did not investigate charge variation with micelles formed from a mixture of nonionic

and ionic surfactants, because the 'average structure' of the headgroup will

vary with charge in that case.

The systematic investigation of charge variation could be combined with

the aim to enrich the limited choice of surfactants, already studied for their

interaction with polymers with novel surfactant molecules. The structure of

the polymer has also been varied, with the purpose of monitoring possible

differences for the various polymers in sensitivity towards micellar charge.

Since, altogether, a large set of data on the interaction between various

polymers and micelles built from (monovalent) surfactant ions was accumulated

during the study, an attempt was made towards quantitative theory to model the

interaction process.

Another important goal of this study was to extend the field of

polymer-micelle chemistry and investigate the interaction of polymers with

other than spherical micellar aggregates, such as rodlike micelles and

vesicles.

Furthermore, the question whether the viscoelasticity of a polymer (PEO)

solution is influenced, as is the viscosity, by the binding of micelles, is

attacked. Also, the effect of micelles on the shear dependence of the

viscosity of a polymer solution was deemed worthwhile for study.

Finally. attempts have been made to bring about the disruption of the

polymer-micelle complex by shear forces.

1.6 Survey of the contents

Chapter 1 contains a general introduction in the field of polymer-micelle interactions. The micellization of surfactants in .aqueous solutions, as well

as the polymers, that are applied in this study, are briefly reviewed in

separate sections. After this acquaintance with the constituents, the development of the current model for polymer-micelle interaction is briefly

presented. Detailed discussions of specific aspects of the interaction may be found in the other chapters. Based on the state of the art at the beginning of

the work presented here, the aims of the study are briefly discussed. Chapter 2 deals with the interaction of nonionic micelles with polymers.

Hitherto such interactions were considered to be absent, partly because no

reduction in cmc upon addition of polymers had been observed in previous

studies. This criterion for polymer-micelle interaction is shown to be

incorrect. The interaction between PPO and micelles of n-octylthioglucoside was revealed by microcalorimetry and the measurement of clouding and Krafft temperatures. The absence of stabilization of the micelles, i.e. reduction of

the cmc, is discussed in terms of a favorable free energy for transfer of

chain segments of the polymer from the aqueous to the micellar phase.

Chapter 3 is largely devoted to the interaction of the relatively

hydrophobic polymers PPO and PVME with spherical and rodlike micelles of

cetyltrimethylammonium salts. Aggregation numbers for spherical micelles

formed from cetyltrimethylarnmonium bromide are presented. They appear to be smaller in the presence of PPO and PVME than in aqueous solution. Rheological

measurements were applied to obtain information on the effect of polymers on

(i) the transition from spherical to rodlike micelles of

cetyltrimethylarnmonium tosylate and on (ii) a viscoelastic gel-like solution of cetyltrimethylammonium salicylate. For the first time rodlike micelles

entered the field of polymer-micelle interaction. Rodlike micelles are

transformed into spherical, polymer-bound micelles in the presence of an appropriate polymer.

Chapters 4, 5 and 6 contain investigations into the effect of micellar charge on polymer-micelle interaction with the aid of surfactants that allow

variation of the charge without concomitant, large variations in structure.

The influence of polymers on the aggregation behavior of n-dodecylammonium

oxide, in neutral form and at various degrees of protonation, is presented in

Chapter 4. A new method for the measurement of the crnc was developed that

makes use of the sudden change in pH of the solution at the crnc of an acidic

or basic surfactant. Again the crnc of the neutral form of this surfactant was

not perturbed by the presence of PPO or PVME, but the occurence of interaction could be deduced from the reduction in aggregation number of the micelles.

Furthermore, the clouding behavior of PVME and PPO was altered by the presence

of the surfactant. Stabilization of the micelle by the presence of polymer

takes place at increased micellar charge.

In Chapter 5 the effect of polymers on the micellization of

n-decylphosphate surfactants is presented. The charge of the surfactant was

varied from -1 to -2 units. The newly developed pH-method proved indispensable

for the determination of the crnc in the presence of polymers. Surprisingly,

the crnc values revealed that the interaction with PEO, PVME, and PPO appears

to become weaker with increasing charge. Aggregation numbers, clouding

temperatures for PPO and PVME, and 3 1 ~ - ~ ~ ~ longitudinal relaxation times are

presented as well. The final part of the chapter describes a preliminary

report on the interaction of PVME with vesicles formed from sodium

di-n-dodec ylphosphate.

In Chapter 6 the aggregation behavior of n-dodecylmalonates with a charge

of -1 and -2 is discussed. The surfactant properties of the mono salt of

n-dodecylmalonate have not been investigated before, probably because of the

high Krafft temperatures of the alkali salts which are usually employed. We

used the mono-tetramethylammonium salts and obtained viscoelastic solutions at

room temperature at extremely low concentrations (- 10" M). This was hitherto

unknown for anionic surfactants. The visual observed viscoelasticity

disappears upon addition of PVME or PPO, indicating that the originally formed

rodlike micelles are transformed into polymer-bound spherical micelles. The

interaction of polymers with the mono- and di-tetramethylammonium salts have

been investigated in aqueous solutions by 'H-NMR, crnc measurements, and the

determination of the influence on clouding of PVME.

Chapter 7 is devoted to the rheology of the PEOISDS system. Both the

apparent viscosity and the viscoelasticity, measured as the first normal

stress difference, of an aqueous solution of high molecular weight PEO were

monitored as a function of the SDS concentration and shear rate. The shear

rate dependence of the apparent viscosity could very well be analyzed using a

power-law model of non-Newtonian behavior. The first normal stress difference

increased upon binding of SDS micelles. At SDS concentrations near or above

the saturation concentration, the viscoelasticity leveled off to a constant

value at a certain critical shear stress. This was interpreted as a disruption

of the polymer-micelle complex by shear forces.

In Chapter 8, all cmc data and aggregation numbers on combinations of

polymer and monovalent surfactants, that have been collected in this study are

fitted to a quantitative model, namely the 'dressed rnicelle' model of ~vans '~ .

The implications of the results for the polymer-micelle model are discussed. A

comparison is made between this quantitative analysis and those reported in

the literature for the PEO/SDS system.

Finally, the impact of the results and observations from this study on

the ideas about polymer-micelle interaction are discussed in the concluding

chapter 9. Most of the work described in this thesis has been published, or will be

published in the near future (Chapters 2'l, 382'83, 484, 585, 686, 7". gE8).

CHAPTER 2

INTERACTION BETWEEN NONIONIC SURFACTANTS AND NONIONIC POLYMERS:

FACT OR FANCY?

2.1 Introduction

Nonionic swfactants form an important class of components for industrial

and research purposes89. They are mild detergents with applications in

cosmetic products and microemulsions. Their 'mildness' as surfactant has also

led to applications in biochemical research, since these surfactants may

solubilize proteins from cells or membranes without destroying the tertiary

structure89. Especially surfactants based on sugar enjoy a

lively interest at present, both for biochemical applications13'15 and for

industrial uses8'. Sugars are readily available as a renewable ~ o u r c e ' ~ ' ~ ~ and

biodegradable, which is of particular promise.

Nonionic surfactant are actually subdivided into two subclasses. These

are (i) the true nonionics, which have a polar, hydrophilic headgroup such as

a poly(ethy1ene glyco1)ether or sugar moiety, and (ii) the zwitterionic

surfactants of which the headgroups contain two opposite charges in near

proximity. They bear no net charge. To avoid confusion, the term 'nonionic'

will be reserved for the first subclass.

Zwitterionic surfactants sometimes behave similarly to nonionic 96 surfactantsg5 and sometimes similarly to ionic surfactants . For instance,

neither zwitterionic nor nonionic surfactants show a progressive decrease in

free surfactant concentration above the cmc as ionic surfactants dog5. On the

other hand studies with a kinetic probe by Bunton et al?6 revealed that

micelles formed from sulfobetaine and betaine surfactants behave like cationic

micelles with complete (100 %) counterion binding. Malliaris et alm9' found

that the aggregation numbers of zwitterionic and ionic micelles decrease upon

increasing temperature, whereas the nonionic surfactant, Triton X-100, showed

an increase in aggregation number with increasing temperature. The latter

observation 3s probably more characteristic for poly(ethy1ene glyco1)ether

type surfactants than for nonionic surfactants in general. The aggregation

number (n) for P-D-n-octylglucoside, for instance, does not show a consistent

increase (n = 68 (20 OC); n = 84 (30 OC); and n = 72 (50 OC))~~.

The interaction of zwitterionic surfactants with nonionic water-soluble

polymers has scarcely been investigated, whereas nonionic surfactants, usually

represented by the poly(ethy1ene glyco1)ether type, have already gained a poor 3.61,71 reputation as far as interaction with polymers is concerned . They are

usually considered to be totally indifferent to polymers, although

n-nonylphenol-poly(ethy1ene glyco1)ether interacts with P E O ~ ~ and hydroxyethyl

cellulose (HEC)'", and in this study interaction of PPO with

P-D-octylthioglucoside has been established8'. The former complexation has

been attributed to an affinity of the phenol moiety for PEO'", since several

polymers, including PEO, are known to interact with p-substituted phenols102.

By contrast, viscometric measurements did not provide evidence for interaction

between n-octylphenolethoxylate and PEO~~'. Whether viscometry is the method of choice to detect polymer-micelle

interaction with nonionic micelles, is disputable. The intermicellar .repulsion

between polymer-bound micelles, which lies at the origin of the polymer

expansion and concomitant increase in viscosity, may well be small or

insignificant for uncharged micelles. Particularly when the polymer coil in

aqueous solution is already quite expanded, like that for PEO, the method is

not sensitive enough. Nevertheless, viscosity and clouding point measurements

have provided evidence for interactions between surfactants of the

poly(ethy1ene glyco1)ether type and some mildly hydrophobic (co)polymers and

poly(carboxy1ic acid)sMg. This association resembles the formation of 103-105 interpolymer complexes between PEO and poly(carboxy1ic acid)s . The

presence of an alkyl chain in the surfactant will enhance this interaction,

just as the extra methyl group in poly(methacry1ic acid) enhances interaction

as compared to poly(acry1ic acid). This association, however, is not quite

comparable with, for instance, that of PEOISDS for which the micellar

character of SDS is decisive.

In order to explain and quantify the influence of surfactant headgroup

structure on interaction with polymers, both ~ a ~ a r a j a n ~ ' and ~uckenstein~'

developed detailed models. Based on different points of view, both authors

stress the importance of the relative contribution of stabilization of the

water-hydrophobic core interface by the polymer on the one hand and the

unfavorable interaction between surfactant headgroups and polymer segments on

the other. ~ a ~ a r a j a n " proposes that the latter interaction stems from steric

repulsion, whereas ~uckenstein" suggests that the interfacial tension between

the headgroups and water is unfavorably influenced by polymer association.

Since nonionic surfactants invariably possess bulky headgroups, the area of

hydrophobic core-water contact is limited and, as a result, association with

polymers is predicted to be i n ~ i ~ n i f i c a n t ~ " ~ ~ . However, in Nagarajan's

original model the free energy of transfer of the polymer from the aqueous

phase to the rnicellar pseudophase was not taken into account. In the most

recent paper on Nagarajan's model6" this contribution is accounted for in the

term a (section 8.2), which is related to polymer properties. In pol

Ruckenstein's treatment71 this quantity is implicitly accounted for in the

experimental method for estimating the change in interfacial tension induced

by the polymer. However, this experimental method cannot be used for

water-soluble polymers such as P P O ' ~ ~ and H P ~ ~ , which are soluble in nonpolar

solvents. Exactly these polymers are known to show the strongest interaction

with sodium dodecylsulfate ( s D s ) ~ ~ ' ~ ~ ' ~ ~ and cetyltrimethylammonium bromide (aAB)53,67.76 and are likely candidates for favorable interactions with

nonionic surfactants. A computer simulation by Balazs and H U " ~ on the effect

of surfactants on the aggregation of associating polymers (polymers with a

'sticker' at each end) also revealed the importance of steric hindrance

exerted by the surfactant headgroup.

In the present chapter we provide strong evidence for the association of

PPO with rnicelles formed from the nonionic surfactant P-D-octylthioglucoside

(OTG) (1). It is suggested that the predicted destabilizing effect of PPO on

the Stem layer of the rnicelle is overcompensated by a favorable free energy

of transfer of the polymer from water to the micelle.

After completion of our work, winnilclog has also provided strong evidence

that interaction between HPC and OTG takes place.

2.2 The influence of polymers on critical micelle concentrations

One of the most convincing indications for the formation of polymer-bound

micelles has always been a reduced value of the cmc in the presence of

polymer2'3'5. We will show that this criterion appears not to be generally

valid.

The determination of the cmc of a surfactant in an aqueous polymer

solution is often not a trivial matter. The very presence of polymers, in

particular the rather hydrophobic ones such as PPO and HPC, excludes many

techniques. For instance, several fluorescent- or UV-probes, which bind to

surfactant aggregates and are used to determine the. cmc, also bind to these

polymers, which obscures the results. Furthermore, the presence of some of the

polymers hampers the interpretation of surface tension measurements, because

they are surface-active themselves. Often NMR methods cannot be used because

of the overlapping resonances of the polymer. NMR resonances of polymers are 109,110 usually broad and often complicated due to dyad and triad splitting ,

The hydrophilic polymers, like PEO and PVA are less problematic, but they show

also the weakest tendency for interaction with micelles. The problems that

arise with conductivity and other techniques, used in the case of ionic

surfactants, will be discussed elsewhere (sections 3.2 and 5.2). Table 2.1 lists cmc values of several zwitterionic and nonionic

surfactants in water and in the presence of various polymers. Because of the

Table 2.1 Cmc values (mM) for micelles in water and in the presence of polymersa.

Surfactant T,OC H 2 0 PEO PPO HPC PVA-PVAc PVP

CBE3g OTG

OTG

a) Polymer concentration: 0.5 g . d ~ - l for the zwitterionic surfactants and OTG;

1.0 g . d ~ ' for C,E,. b) Brornophenol blue absorption method. c) Surface tension

method. d) Pyrene fluorescence method. e) Microcalorimetric method. f) In D20:

8.2 rnM. g) C,E, denotes n-C,H,,O(CH,CH,O),H.

high Krafft temperatures of the zwitterionic surfactants, the measurements

were performed at elevated temperatures. This excluded the use of PPO and HPC

because of clouding of these polymers at those temperatures. Four techniques

were used to obtain the data listed in Table 2.1. (i) The bromophenol blue

method, which relies on a shift in the absorption spectrum of the dye upon

binding to the micelles (Figure 2.1). The probe is negatively charged but is

readily stabilized in nonionic micelles. (ii) The surface tension method. This

method depends on the fact that the surface tension of a solution decreases

Figure 2.1 Bromophenol blue absorption and turbidity in an aqueous

0.5 g . d ~ - l PPO solution as a function of the OTG concentration.

A710 is a measure for the turbidity (inset); Aazo-A710 denotes

the 620 nm absorption of bromophenol blue corrected for the

turbidity.

steadily at increasing surfactant concentrations, until, at the cmc, a

constant value is reached. The presence of surface-active impurities may be

detected as a minimum in the surface tension versus concentration curve.

(iii) The pyrene fluorescence which is based on a change in the

fine structure of the spectrum upon binding of the probe to the rnicelles

(Figure 2.2). The sudden decrease in the relative peak intensities of the

h.nm

Figure 2.2 Fluoresence spectrum of pyrene dissolved in (a) water and (b) 30

mM OTG solution.

first (h = 372 nm) and third (h = 383 nm) peak indicates the cmc. (iv) The

microcalorimetric method, which will be discussed in detail in section 2.4.

For all the surfactants listed in Table 2.1 the cmc values are virtually

unchanged in the presence of polymer. Even HPC?~, which is able to lower the

cmc of SDS by a factor of 15, and that of (XAB by a factor of 4, has no effect

on the cmc of OTG. Although the conclusion that these nonionic and

zwitterionic surfactants do not interact with the polymers would be pleasingly

in accord with theories for polymer-rnicelle interaction (vide it

is definitely not true for the combination PPODTG. It is known that the

rather hydrophobic PPO is folded spirally in tightly coiled discs in aqueous

solution48. We find that even at 25 OC these discs tend to aggregate slightly,

producing a slightly visible turbidity. This turbidity is, however, suddenly

reduced upon addition of OTG in a concentration equal to or beyond the cmc,

presumably because of interactions between the polymer and OTG micelles

(Figure 2.1, inset). The change in turbidity (and, consequently, in background

absorption) necessitated a correction in the analysis of the VIS-absorption

data of dissolved bromophenol blue used for the cmc determination (Figure

2.1). This was the first indication that, despite the unchanged cmc, PPOIOTG

interaction might take place.

Recently, interaction between HPC and OTG micelles has also been

establishedIo8, though the cmc is again not influenced.

2.3 Clouding behavior and Krafft temperatures

The decrease in turbidity of a PPO solution in the presence of OTG

micelles is a result of the perturbed clouding behavior of PPO due to the

presence of OTG micelles (Table 2.2). Clouding of PPO in H 2 0 and D 2 0 is a

gradual process taking place in a temperature range of over 10 OC. However, in

the presence of OTG clouding occurs abruptly within 2 OC, indicating a more

Table 2.2 Clouding temperatures of PPO'.

Medium Clouding Temperature, OC

Hzo 26-37 D2° 26-37 H 2 0 + OTG (20 mM) 30 D 2 0 + OTG (15 mM) 25

cooperative process. In D,O, OTG shifts the clouding of PPO towards lower

temperatures, which is expected in view of the low solubility of OTG in D 2 0

below 30 OC. Probably OTG is preferentially solubilized in the polymer-rich

phase. We note that the Krafft temperature of OTG in D 2 0 (30 OC) is shifted to

a value below 20 OC by the presence of 0.5 g . d ~ ' of PPO (Table 2.3)..

It is not possible to decide whether the effect of PPO on the Krafft

temperature of OTG in D 2 0 originates from a decrease of the cmc induced by the

presence of PPO or from an increased solubility of surfactant monomers. A

reduction of the cmc is not likely in view of the data shown in Table 2.1,

but, unfortunately, the cmc of OTG cannot be determined at the same

temperature in D,O in the absence and presence of 0.5 g . d ~ - ' of PPO. Either

the temperature is below the Krafft temperature or above the clouding point of

PPO.

Comparison of the clouding behavior of Triton X-100 in H,O and D 2 0 also

revealed a lower cloud point in D 2 0 than in ~ ~ 0 " ' . This means that the

solubility of Triton X-100 (monomers and micelles) is lower in D 2 0 and a

similarly decreased solubility of OTG in D 2 0 may cause the increase in Krafft

temperature. The rationalization behind the lower solubility, suggested by

Pandit and caronia1 ' ', is based on the enhanced structuredness of D 2 0 compared

to ~ ~ 0 " ~ . Although this results in a better solubility of hydrocarbons in D 2 0

than in H20, the effect is more than compensated by the hydration of the polar

headgroups, which occurs to a greater extent in H20.

Table 2.3 Krafft temperature of OTG.

Medium Krafft Temperature, OC

H z 0 < 20 "2O 30 H 2 0 + PPO (0.5 g.d~- ') < 20

D 2 0 + PPO (0.5 g . d ~ ' ) < 20

2.4 Microcalorimetry

The remarkable association between PPO and OTG micelles is definitely

confirmed by microcalorimetric measurements. In a typical experiment, 10 yL aliquots of a concentrated OTG solution ( [OTG] >> cmc) were injected into the

perfusion cell which contained 2 mL of the polymer solution or water. When OTG

solution was injected into the PPO solution, the microcalorimetric response

curve consisted of an endothermic peak followed by an exothermic peak. The

endothermic peak increases in size whereas the exothermic peak diminishes and

eventually disappears with increasing fmal OTG concentration (Figure 2.3). This phenomenon is attributed to rapid endothermic polymer-micelle association

near the injection point followed by a slower disintegration of the complex

and dilution of the surfactant molecules in the entire solution. The total

dilution enthalpies shown in Figure 2.4 are summations of the areas of the

endothermic and exothermic peaks.

The curve for OTG dilution in H,O can be characterized by three regions.

In the premicellar region I, the injected rnicelles disintegrate completely and

the enthalpy change for demicellization and loss of intermicellar interactions

is recorded. Region I1 is the transition region around the cmc. In the

posttransition region 111, the injected micelles remain intact and only a very

small enthalpy change for reduction of intermicellar interaction is measured.

The enthalpy of micellization calculated as the difference in dilution

enthalpy between region I and 111, is +4.5 lcJ.m01-~, a normal value for a 113,114 nonionic surfactant .

Comparison of the curve for the PPO solution with the curve for H,O

reveals that PPO exerts only a small endothermic effect on the premicellar

enthalpy of dilution. Furthermore the transition region is located in the same

concentration range, indicative of an unchanged cmc. However, a clear

endothermic effect, +4.3 kJ.mo1-I, is observed in the posttransition region of

the PPO solution. We contend that this value represents the enthalpy of

interaction between PPO and the OTG rnicelles. Interestingly, ~ h i r a h a m a ~ ~ also

found an endothermic enthalpy for interaction between PEO and SDS micelles (in

a 0.1 M NaCl solution). Krescheck and ~ a r ~ r a v e s " ~ found an endothermic

Figure 2.3 Top: Microcalorimetric response curve upon injection of a

concentrated OTG solution into a PPO solution with the final OTG

concentration remaining below the cmc. The numbers refer to the

titration steps, i.e. 9 corresponds to the ninth titration step,

see also Figure 2.4. Each response consists of an endothermic and

an exothermic peak. Bottom: similar data but now the final OTG

concentration is beyond the cmc. Note the increase of the

endothermic signal relative to that shown in the top part. The

exothermic effect has disappeared completely beyond titration

step nr. 18. Signal noise is caused by the stirrer. Temp.: 25 'c.

binding of sodium octyl- and decylsulfates to PVP, but an essentially athermal

binding of SDS to PVP. The dilution enthalpy curve for injection of a solution

of the OTG micelles into 0.5 g . d ~ - l of PEO equals that of water, and,

therefore, there is no rnicrocalorimetric or other evidence for interaction

between PEO and OTG.

0 3 6 9 12 15

f inal concentration of OTG in mmol l l i te r

Figure 2.4 Enthalpy of dilution as a function of the final OTG concentration

in water or in an aqueous solution of PPO at 25 OC.; (m) in

water, cmc = 8.05 x 10"; in PPO solutions: (A) exothermic

effect, (V) endothermic effect, (o) summation of exothermic and

endothermic effect. The numbers (9-12; 17-21) correspond with the

titration steps indicated in Figure 2.3.

2.5 Discussion

Since the Gibbs energy of micellization of OTG is unchanged by the

presence of PPO, the endothermic interaction enthalpy is apparently

compensated by a positive entropy change. This AHIAS compensatory behaviorz2

probably originates largely from the release of water molecules from the

hydrophobic hydration shells of the polymer discs upon interaction with the

micelles.

The different behavior of PEO and PPO most likely reflects the difference

in free energy of transfer of the polymer from water to a more apolar

environment. PPO is more soluble in hydrocarbons than in water, contrary to

PEO, which does not dissolve in the usual solvents other than water. PPO

solubilization in or at OTG micelles may thus provide a favorable free energy

that may compensate for the disturbance of the OTG Stem region. More or less

the same situation applies to HPC"'. AS ~uckenstein~' and ~ a ~ a r a j a n ~ ' have

pointed out, the presence of a polymer among the bulky headgroups of the

nonionic surfactant will cause a destabilization of the Stem region due to

polymer-headgroup repulsion.

The question arises whether PPO, interacting with OTG micelles, resides

at the micellar surface like PEO in the system PEOISDS, or deeper in the

micellar core. The latter possibility is not likely for the reasons mentioned

in section 1.4, but additional evidence is called for. Aggregation numbers may

give a clue, because if PPO resides in the core an increase in aggregation

number is expected instead of the usuaI decrease found in most polymer-ionic

micelle complexes. We have made an attempt to measure aggregation numbers of

OTG micelles in the absence and presence of PPO using quenching of the

fluorescence of bis-(2,2'-bipyridyl)-mono-(4,4'-didecyl-2,2'-bipyridyl)

rutheniurn(I1) perchlorate by Pmethylanthracene (see section 3.2). We obtain

an aggregation number of 156 + 10 for OTG micelles, which is rather high

compared to the values of 68 - 8478, or 87'16, for P-D-n-octylglucoside (with

an ether instead of a thio linkage) determined by light scattering and

sedimentation techniques. In the presence of 0.5 g . d ~ l of PPO, we find a

value of 96 + 3. Although the exact values may be slightly in error (section

3.2), we submit that the trend is obvious, and points to location of PPO in

the outer region of the micelle. Thus PPOIOTG interaction probably resembles

the classical PEOISDS association. The most important conclusion from this

chapter is, however, that polymer-micelle interaction is not necessarily

accompanied by a reduction in cmc.

2.6 Experimental section

Materials. The surfactants C,E, (supplied by B. Kwant, University of

Groningen) and OTG (n-octyl-P-D-thioglucopyranoside, Sigma) were used as

received. The zwitterionic surfactants were synthesized and kindly provided by

K. Hovius and A. ~uiterman"~. PEO (weight-averaged mw 10,000, Fluka), PVP

(Kolloidon-90, BASF), and PVA-PVAc (acetate content 17 %, Mowiol 3-83,

Hoechst) were purified by fractionation, followed by deionization. PEO was

dissolved in chloroform and precipitated in petroleum-ether (bp 40-60 OC)

under rigorous stirring. PVP was dissolved in chloroform and precipitated in

ether. PVA-Ac was dissolved in DMF (75 OC) and, after cooling, precipitated in

ether. Aqueous solutions (5% wfw) of the respective precipitates were

deionized by stirring with cationic (Dowex-SOW) and anionic (Dowex -3 or -1)

ion-exchange material until the specific conductivities of the solutions were

below 10 pC2-'.cm-'. The deionized solutions were dialized against

demineralized water in cellulose acetate tubes for 25 h. Then the solutions

were freeze-dried and in the case of PVA-PVAc, dried over P,O, in vacuo. PPO

(weight-averaged mw 1,000 , Janssen) and HPC (weight-averaged mw 100,000, Aldrich) and the probes bromophenol blue (Merck) and pyrene (Aldrich) were

used as received.

Cmc measurements. Spectrophotometric measurements of the cmc were

performed by determining the absorption of bromophenol blue at a suitable

wavelength between 600 and 620 nm at a probe concentration of 6 x M using

a Perkin-Elmer h5 spectrophotometer. In the case of OTG in the presence of PPO

(measurements at 610 nm), a small correction had to be made to account for the

change in turbidity. This was done by subtraction of the absorption at 710 nm,

outside the bromophenol blue absorption band (Figure 2.1). Surface tension

measurements were carried out by using the Wilhelmy-plate method. Plots of

surface tension vs. C,E, concentration showed no minimum. Fluorometric

measurements of the cmc were performed by monitoring the fine structure in the

fluorescence spectrum of pyrene53.74, using a SLM-Aminco SPF-500 cTM spectrofluorometer. Sample solutions were made by adding aliquots of a

surfactant stock solution to 2 ml of pyrene-saturated water ([pyrene]

ca. M) or a polymer solution, prepared with pyrene-saturated water. At

this concentration hardly any excimer formation occurs. Spectra were recorded

from 365 nm to 400 nm in 0.05 nm steps. The emission band-width was 1 nm, and

the excitation took place at 335 nm with a band-width of 5 nm. The cmc was

indicated by a drop in the intensity ratio of the first (372-373 nm) and third

(383 nm) peak, I, 1 Im, from 1.8-2.0 to 1.2-1.3. In all cases thermostated

sample solutions were used.

Microcalorimetry. Microcalorimetric measurements were performed by

G. Haandrikman and N.M. van 0 s of the Koninklijkel Shell Laboratorium,

Amsterdam, to whom we are much indebted. A LKB 2277 heat-flow

microcalorimeter, described elsewheren8, was used. Because of the long time

needed to complete a dilution curve, storage of the OTG stock solution between

injections in a cool atmosphere (0 - 4 OC) appeared to be necessary in order

to prevent bacterial degradation.

Clouding points and Krafft temperatures. Clouding points and Krafft

temperatures were determined by recording the transmission at 500 nm of

vigorously stirred dispersions as a function of temperature using a

Perkin-Elmer h5 spectrophotometer. The clouding point of PPO was taken as the

temperature representing the midpoint of the change in transmission in the

case of a narrow transition region (1-2 OC) or as the temperature range in the

case of a broad transition region (ca. 10 OC). The Krafft temperature of OTG

is taken as the o w t of the sudden increase in transmission in a 20 rnM OTG

dispersion in H,O or a 15 mM OTG dispersion in D,O.

Aggregation numbers. For a discussion of the method, see section 3.2. The practical aspects are described in section 3.5.

CHAPTER 3

THE INFLUENCE OF POLYMERS ON THE MICELLIZATION OF CETYLTRIMETHYLAMMONIUM SALTS

3.1 Introduction

3.1.1 Cetyltrimethylammonium salts

Cetyltrimethylarnrnonium salts (CTAX), particularly the bromide and

chloride, are by far the most widely studied cationic surfactants. Although

this onesided interest probably stems from their easy availability, they are

indeed interesting surfactants. The formation of viscoelastic solutions at

extremely low concentrations (ca. lo4 M) in the presence of salicylate

anions119 is an especially fascinating phenomenon. Notwithstanding these

interesting properties, CTAX salts as well as the relatively few other

cationic surfactants that have been investigated, have a poor reputation in

the field of polymer-rnicelle chemistry3. This stems from the fact that they

give only significant interaction with rather hydrophobic polymers, though

recently a modest propensity for binding to more hydrophilic polymers has been 120-122 detected .

After a brief overview on CTAX/polymer interaction and on the formation

of rodlike rnicelles in the presence of specific counterions, this chapter will

deal with the interaction of rnicelles of CTAB (X = bromide), CTATs (X =

tosylate), and CTASal (X = salicylate) with polymers. The relatively

hydrophobic polymers PVME and PPO both induce a decrease in

aggregation number of CTAB and an increase in the concentration at which the

transition from spherical to rodlike micelles takes place for CTATs. These

findings support the view that the disappearance of the gel-like and

viscoelastic properties of a CTASal solution in the presence of PVME or PPO originate from a preferential binding of the polymers to spherical micelles

rather than to rodlike micelles.

3.1.2 Interaction of polymers with cationic surfactants

The early work of saito4 already revealed that the cationic surfactants

n-dodecylarnmonium chloride (DAC), n-dodecyl- and stearyltrimethylammonium

chloride (DTAC and STAC) and 1-cetylpyridinium chloride do not interact with

PVP, PVA, and PEO. saito4 was also the first to suggest that the large size of

the trimethylammonium headgroup might be the cause. This explanation does not

hold, however, for DAC. Furthermore he found that the hydrophobic polymer PPO

(mw 2,000) does interact with DTAC and DAC.

Several other authors also allude to the fact that the interaction of

polymers with micelles formed from anionic surfactants is much more facile

than with micelles composed of cationic surfactants 3,858,65,71.80,123.124

The criteria used to establish the absence of interaction between micelles of

cationic surfac~ants and hydrophilic polymers include viscometry of PEO~" and

PVP~ solutions, the study of thermodynamic quantities5s, fluorescence probe 53 measurements , and solubilization e ~ ~ e r i m e n t s ~ ' ~ .

Recently, however, potentiometric studies with ion (surfactant) selective

electrodes by ~ h i r a h a r n a ' ~ ~ " ~ ~ and others78 have revealed a feeble and

virtually non-cooperative interaction of CTAB with PVA and PEO~. There are no

indications for binding to PVP. Furthermore, Perron et a1.58 found a slight

influence of PEO, but not of PVP, on the heat capacity of a CTAB solution.

Since cooperativity of the binding process is almost absent, it has been

suggested that surfactant monomers bind to the macromolecule 78,122,125

Anyway, the interaction is about a hundred times weaker than with SDS. The

cooperativity and the amount of binding of cationic surfactants to PVA is

greatly enhanced when the hydrophobicity of the macromolecule is increased by 78,125 acetylation (up to 12%) . This is in accord with the finding that

hydrophobic 124 polymers like pp04,65,67,68 , H P C ~ ~ and

ethylhydroxyethylcellulose ( E H E C ) ~ ~ ' ~ ~ interact with cationics similarly to

SDS.

Interaction with cationic surfactants is also promoted by the presence of

a strongly interacting counterion such as SCN- ". Saito and coworkers" found

that DA+SCN;, as judged by reduced viscosity data, interacts quite strongly

with PVA-Ac (with 30% acetate residues) and PVP whereas the corresponding

chloride shows comparatively weak interactions. Regrettably, this line of

studies has not been continued.

Three explanations have been advanced for the origin of the weakness of

the interactions between polymers and cationic surfactants: (i) the bulkiness

of the cationic headgroup, (ii) a positive charge (vide infra) on the polymer,

and (iii) a difference in interaction of cations and anions with the hydration

sheath of the polymer. The first explanation, which is the oldest and still

the most popular one, has been proposed by saito4, and has later been adopted

by ~ a ~ a r a j a n ~ l . The bulkiness of the headgroup of most cationic (and nonionic)

surfactants is assumed to hamper the presence of a polymer at the

hydrocarbon-water interface. Furthermore, a bulky headgroup quite effectively

shields the hydrophobic core. Thus, the stabilization of the core-water

interface by the polymer is less relevant in that case than in case of

micelles of which the core is less shielded by the headgroups. Small angle

neutron scattering studies indeed reveal that the trimethylammonium headgroup

in micelles of n-tetradecyltrimethylarnmonium bromide does not even leave 126,127 enough space for penetration of water molecules between the headgroups .

An opposite situation applies for sulfate surfactants, where extensive

core-water contact is suggested by NMR and small-angle neutron scattering

mea~urements'~~. ~uckenstein~l suggests a more indirect interaction, but also

considers the bulkiness to be critical. There are, however, several

observations which cannot be reconciled with the bulkiness playing a major

role. These include: (i) DAC and protonated n-dodecyldimethylamineoxide

(DDAOH+) micelles (Chapter 4) do not interact with PEO, PVP, and PVA or only

very weakly and (ii) the finding that the bulkiness of the hydrophobic

polymers does not prevent interaction with, for instance, CTAB , having a

voluminous trimethylammonium headgroup.

The second involves electrostatic repulsion with a

proposed slightly positive charge on the polymers. This charge is thought to

originate from protonation of the ether oxygens in the case of polyethers and

the amide moiety in the case of PVP. The pH dependence of the interaction

between SDS and PEO was used by schwuger8 to support this view. Moroi and

~ a i t o l * ~ used the same concept to explain the difference between DTAB and SDS in mixed micelle formation with nonionic micelles of the

poly(oxyethy1ene)alkylether type. The very low pKa value of an ether or amide

(pKa (CH,CONH,)H+ = 0.3), however, raises serious doubts about the importance

of protonation at neutral pH.

The third explanation is based on the different influence of cations and

anions on the hydration sheath of the polymer, and thus of headgroup-polymer

interaction. To support this view witte6' refers to the work of ~ a ~ ~ e r ' ~ ' , who

studied the role of electrolytes in the flocculation behavior of polymers. The

decrease in clouding temperature of PEO by the addition of salts also

indicates the more pronounced influence of anions compared to cationS42~8,~o.130 . tama an" stated that "the suppression of the cloud point

temperature of PEO .... appears to be a combined cationlanion effect, but the

anion effect seams predominant". It is noteworthy that the sulfate ion belongs

to the more effective anions, whereas for instance NH,' is one of the least

effective cations5'. The ion-polymer interactions are usually thought to occur

via hydration shell overlap effects.

Probably the size of the headgroup and the interaction of the headgroup

with the hydration sheath of the polymer are the main reasons for weak

interaction of cationic micelles with polymers. The electrostatic repulsion

between polymer and micelles may modify interactions at low pH, but do not

seem to be decisive under neutral conditions. Altogether, only an appreciably

hydrophobic polymer can overcome these factors by a favorable free energy of

transfer of polymer segments from the aqueous to the micellar phase and

interact also with cationic (and nonionic) micelles.

3.1.3 Rodlike micelles of CTAX salts

So far, only the interaction of spherical cationic micelles with polymers

has been discussed. Certain cetyltrimethylammonium salts, however, are well

known for the formation of rodliie micelles (Figure 3.1). For instance, for

the salicylate salt, Hirata et al. 132.133 even published electron micrographs

of these rods, but these results have been criticized and are most likely

artifacts associated with chemical staining. The use of cryo-transmission

electron microscopy avoids these artifacts and direct imaging of the rods has

become feasible134. From these direct images of the rods, it appeared that the

diameter (45 to 60 A) is in agreement with expectations134, contrary to the

diameter of 100 to 120 A reported by ~ i r a t a ' ~ ~ .

Surprisingly, ~ a ~ a r a ~ a n ~ " is the only author who considered rodlike

micelles in the study of polymer-micelle interactions. He predicted

theoretically that rodlike micelles of SDS formed in the presence of NaCl

would be transformed to polymer-bound ellipsoidal micelles in the presence of

PEO. Rodlike rnicelles of C,,E, would be unaffected by PEO. However,

Nagaragan did not publish or perform any experiments to test his predictions.

Consequently, this section deals only with the structure and properties of

rodlike micelles, of CTA' in particular, in the absence of polymer.

The formation of rodlike micelles from CTA' depends mainly on the 135.136 structure and concentration of counterions . Whereas CTAB forms rodlike

Figure 3.1 Schematic representation of a network of rodlike micelles in a

viscoelastic surfactant solution. Taken from ref. 13 1.

micelles only at high concentrations 20a.13711.138,139 CTASa113 1,140-142

produces viscoelastic solutions, indicating the presence of rods, even below

1 m ~ " ~ . Other hydrophobic counterions, such as m- and p-chlorobenzoate135,

t o ~ ~ l a t e ' ~ ~ ~ , benzene~ulfonate'~~~, naphthalenesulfonate143, and

o-iodopheno1133. also induce rod formation, though not as effectively as

salicylate. Interestingly, the precise substitution pattern of the aromatic

ring of the counterion is of decisive importance 135,136,138,144 . For instance,

m- and p-hydroxybenzoate136"38"44 are no more effective than bromide,

whereas the o-isomer is very effective. On the other hand o-chlorobenzoate is 135.138 ineffective, but the p- and rn-isomers are . Rao et al. 135.145 revealed a

correlation between the orientation of the counterion at the rnicellar surface

and its rod-inducing efficiency. He suggested that the salicylate and

m-chlorobenzoate ions, which protrude from the rnicellar surface, serve as 143,254 bridges between beads (Figure 3.2). However among others ,

disagrees with this view and argue in favor of the 'rod' model. The matter is

still actively debated in the literature. At this moment 'rods' are still most

\ Micelle / Micelle u Figure 3.2 Schematic representation of the orientation of (a) salicylate,

and (b) m-hydroxybenzoate molecules, embedded into a CTAB micelle, and (c) a chain of (TAB rnicelles linked through

salicylate ions. Taken from ref. 145.

generally accepted, largely because the 'suing7-model seems too elaborate as

an explanation for such a common phenomenon as large aggregates producing

viscoelastic solutions143.

Apart from the question, whether 'strings' or 'rods' are present in

CTASal solutions, the viscoelasticity at the extremely low concentrations

remains a puzzle. Wemerstrom and ~ravsholt '~' once proposed the formation of

periodic colloidal structures induced by long-range intermicellar

interactions. However, in that case smaller aggregates at higher

concentrations might be expected to show viscoelasticity as well, which is not

always found. Hoffmann 144,148 suggested that shear-induced phase transitions

take place in dilute aqueous surfactant solutions. This suggestion is based on

flow birefringence and rheological measurements. The anisotropic shape of the

shear-induced structures is thought to be responsible for the viscoelasticity

and birefringence.

3.2 Critical micelle concentrations and aggregation numbers of CTAB

Table 3.1 lists the cmc values of CTAB in the absence and presence of

polymers. These cmc values were determined by conductivity measurements, which

constitute a well-known method for study of ionic surfactants. Usually, a

Table 3.1 Critical rnicelle concentrations of CTAB in the absence and

presence of polymersa at 25 OC.

medium

- --

cmc, mM

Hzo 0.95~ PVME 0.46

PPO 0.37

PEO 0.95~

a) Polymer concentration: 0.5 g .d~- l . b) Taken from ref. 68.

41

clear break is observed in the conductivity vs. concentration plot, indicative

of the cmc. In the presence of PVME or PPO, however, the conductivity changes

gradually. The most likely cause for this behavior is the fact that the degree

of counterion binding is less for the polymer-bound than for the normal

micelles. The reduction in counterion binding will be most pronounced just

above the cmc when the polymer to micelle ratio is relatively large and the

aggregation number relatively small compared to the ratio and aggregation

number near saturation (vide infra). At increasing concentrations the

aggregates grow and the counterion binding will increase in order to reduce

electrostatic repulsion between the headgroups, which come nearer together.

Furthermore, the cooperativity of the monomer-to-aggregate transition will be

less for smaller aggregates and this will also widen the concentration range

for the transition.

The hydrophobic polymers PVME and PPO induce a reduction in cmc, which

points tot polymer-micelle interaction. PEO does not exert any influence on

the cmc. Though that observation does not exclude interaction (Chapter 2),

literature data reveal that there is no interaction comparable to PEO/SDS or

CTAB with hydrophobic polymers (section 3.1.2). However, some very weak

interaction of PEO presumably with surfactant monomers may occur. PVP definitely does not associate with CTAB (section 3.1.2).

The aggregation numbers of CTAB micelles in the absence and presence of

polymers have been measured as well, using the Turro and ~ e k t a ' ~ ~ method,

extended by Warr and Grieser 120.151.179 for application to cationic and

nonionic micelles. The method is based on static quenching of the fluorescent

probe ~ u ( b i ~ ~ ) r (la) or an analog (lb) by the hydrophobic quencher

9-methylanthracene. It is required that both fluorophore and quencher are

completely bound to the rnicelles according to a distribution following Poisson

statistics. The static character of the quenching process implies that the

distribution remains the same during the time needed for the excited probe

molecule to relax to the ground state. Furthermore, a monodisperse size

distribution of the rnicelles is tacitly assumed. In the case of anionic

rnicelles, the probe l a binds electrostatically. For nonionic and cationic

micelles the arnphiphilic analog l b or a homolog is used, which binds

hydrophobically.

If the fluorophore is only luminiscent when it occupies a micelle, devoid

of quencher molecules, then the measured intensity ratio, I([Q])/ I(O), of the

luminiscence intensities in the presence of quencher to that in the absence of

quencher, is determined by equation 3.1, in which [Q] is the quencher

concentration and [MI the micelle concentration. The latter concentration is

related to the surfactant concentration according to equation 3.2, in which n is the aggregation number.

WI = [surfactant] - cmc

n

The method is very practical. However its applicability is the subject of 149-154 severe debate , and the method should be employed with care. Lianos and

~ a n a ' ~ ~ , for instance, found a too low value for the aggregation number of

SDSINaCl using ~u(bi~~)~/9-9-meth~lanthracene. This was attributed to a failure

of the assumption of static quenching at high aggregation This

failure and the errors induced by polydispersity do not immediately result in

non-linear Stern-Volmer plots, which are used to obtain the aggregation

number"' (Figure 3.3). Furthermore, high fluorophore-to-micelle ratios should

be avoided152, although later this prerequisite also has been disputed150.

Another method that is often applied is based on pyrene excimer 155-157 formation . Moroi et al.15' used both static quenching of Ru(bipy)? and

10' [Q] x ( [CTAB] - crnc 1 - 1

Figure 3.3 Plot of In I(0) - In I([Q]) vs. [Q] I ([CTAB] - cmc). These data

are obtained for a 20 mM CTAB solution containing 0.5 g . d ~ - l of

PVME at 25 OC.

pyrene excimer formation to obtain aggregation numbers for micelles of SDS and

alkylsulfonic acids and found a satisfactory correspondence. Lissi and ~ b u i n ~ ~

used the ~u(bi~~)~l9-meth~lanthracene system for the determination of the

aggregation numbers for PEOISDS and PVPISDS. If the experimental conditions

are chosen with care, that is, low aggregation numbers and a low fluorophore

concentration, good results can be obtainedlS3.

A value of 70 (Table 3.2) was found for the aggregation number of CI'AB micelles at 25 OC using quenching of the probe lb . Other values reported in

the literatux-e33 include 5415' (steady-state fluorescence, pyrene); 88Is7,

90lS7, 82149 and 96 f 10lS9 (time-resolved fluorescence, pyrene); 104'~'

Table 3.2 Aggregation numbers of CTAE? in the absence and presence of

polymersa at various surfactant concentrationsb.

[CTAB], mM H,O PVME PPO PEO PVP

a) Polymer concentration: 0.5 g . d ~ - l . b) Temperature: 25 OC.

(time-resolved fluorescence, 1-rnethylpyreneltetradecylpyridinium chloride);

80 f 116' (time-resolved fluorescence, 1,5-dimethylnaphthalenelcyclic

azoalkane); and 95162 (light scattering); and 145lZ7 (small angle

neutron scattering). The reader will have noticed that a value must be grossly

in error to fall outside the literature range (54-145). Several authors have

questioned the use of pyrene for tetradkylarnrnonium surfactants since both

specific in tera~t ion '~~ as well as the induction of micellar growth 150,164

have been observed. Scattering techniques are only applicable to polymer-bound

rnicelles if several conditions are met165, which is probably not the case with

hydrophobic polymers like PPO and PVME. The use of pyrene

fluorescence in CTAB solution also has some disadvantages (vide supra).

Therefore, we contend that quenching of l b by 9-methylanthracene is a good

choice and most likely suitable for studies of the influence of polymers on

the aggregation number of CTAB rnicelles, as we have performed.

Addition of PVME or PPO results in a appreciable reduction of the

aggregation number (n) (Table 3.2). Such a decrease in n has also been found

for SDS in the presence of P E ~ ~ ~ * " ~ ~ ~ ' ~ ~ ~ , PV?'~, and P P ~ ~ ~ ~ ~ . We find

that the aggregation number of CTAE? rnicelles is not altered by the presence of

PEO and PVP. This was anticipated since these polymers do not interact

significantly with CTAB micelles. Reduction in n upon binding of the

hydrophobic polymers is understandable since the interacting polymers, which

most likely reside at the hydrocarbon core-water interface54, require space in

order to keep headgroup-polymer repulsion to a minimum. The interheadgroup

repulsion will be reduced, and the increased area of hydrophobic core-water

contact will be shielded by the polymer. Altogether comparatively large

surface-to-volume ratios of the micelles will be favored in the presence of an

interacting polymer. This implies the formation of a smaller aggregate.

The dependence of n for the polymer-bound rnicelles on the surfactant

concentration (observed for both CTABIPVME and CTABPPO), has precedent in

the literature on anionic micelles of SDS in the presence of polymers57965.

Lissi and ~ b u i n ~ ~ , however, have reported n values independent of the

surfactant to polymer ratio (SDSPEO and SDSPVP) in the range of 40 to 100 9%

saturation of the polymer. The saturation concentrations for the combinations

CTABPVME and CTABiPPO are unknown. In comparison to SDSPEO, and in

view of the results presented in the next section, the polymer to surfactant

ratios at which the aggregation numbers in Table 3.2 are measured, are likely

to cover at least part of the range of 40 to 100 % saturation. The same

applies for the aggregation numbers reported by witte6'*' and others57w, so

we conclude that the majority of the studies reveal a concentration-dependent

n value. The polymer-bound aggregates probably grow as the degree of

saturation of the polymer is increased due to a decrease of the local

concentration of polymer segments at the micellar interface165, and an

increase in intermicellar repulsions. However, the listed aggregation numbers

represent a number-average. Even if the size of the polymer-bound micelles

were constant and free CTAB rnicelles of n = 70 were formed abruptly above the

saturation concentration, a gradual increase in n with CTAB concentration

would be observed above the saturation concentration.

Altogether, it is clear that CTAB micelles interact with PVME and PPO, which results in a decrease in both the cmc and the n value, and the

interaction process apparently closely resembles the interaction of polymers

with rnicelles of SDS.

3.3 The sphere-to-rod transition of CTATs

One of the counterions that is able to induce the formation of rodlike

micelles formed from cetyltrimethylamrnonium surfactants is tosylate. Sepulveda

and coworkers166 first introduced C'I'ATs for the measurement of the degree of

dissociation of CTAX, in which X represents inorganic counterions. Later they

studied the rheology of solutions of CTATs and of other CI'AX surfactants. They

also reported cmc values, degrees of dissociation, and the transfer free

energy for the counterion from water to the micelle13'.

The tosylate ion is less rod-inducing than the salicylate (Sal) ion. As a result globular micelles of CTATs are initially formed above the cmc

(2.6 x M'~'~). These rnicelles start to grow above a critical rod

concentration (crc) of around 15 Thus, CTATs provides the possibility

of studying the sphere-to-rod transition and the influence of polymers on the

concentration at which this transition takes place. In the case of CTASal

rodlike rnicelles are formed directly above the cmc.

Sepulveda et z ~ 1 . I ~ ~ established the transition concentration, by

examining the increase in relative viscosity (crc ca. 20 mM), the decrease in

partial molar volume (crc = 8-12 mM) and the break in the plot of UV absorbance versus concentration (crc = 15 mM). The observation that the

transition concentration determined from viscosity data is higher than that

determined by partial molar volume (or NMR line broadening) measurements is

not a matter of serious concern. The cause for this common discrepancy is that

the viscosity data reflect the interaction between rods and depend mainly on

the overlap concentratibn whereas the other methods reflect changes in the

local structure of the aggregate, such as surfactant packing and mobility.

We could not reproduce the break in W absorbance around 15 mM, reported

by ~ e ~ u l v e d a ' ~ ~ ' . The absorbance at 262 nm, the absorption maximum of a CTATs

solution, measured using a cuvette with pathlength of 1 mrn in order to keep

the absorbance below 1 absorption unit, appeared to have a linear relation

with the concentration of CTATs up to 30 mM. Since ~ e ~ u l v e d a ' ~ ~ ~ did not state

the wavelength at which his measurements were performed, a difference in

wavelength between his and our measurements might lie at the origin of our

deviating results. However, we find it more likely that the break in absorbance found by ~ e ~ u l v e d a ' ~ ~ ~ is due to a deviation from the Lambert-Beer

law of the measured absorbances at high CTATs concentration and,

concomitantly, high absorbances. Most spectrophotometers are linear up to 3

absorption units (A), for the very good ones this may be up to 5 A. The values

reported by Sepulveda, however, go up to 9 A, with the break occurring at 4 A. This high value causes us to view his results with considerable reserve.

Another method that is often used to monitor the sphere-to-rod transition

is line-broadening of the 'H-NMR signals 142,145 of the alkyl chain protons of

the surfactant upon rod formation. Figure 3.4 shows the concentration

dependence of the combined line width of the resonances of the CH, and CH,

groups of CTATs in D,O, at half height of the CH,-signal. Line-broadening

Figure 3.4 The combined line width of the 'H NMR signals of the CH, and CH,

groups, at half height of the CH, signal, of CTATs in D,O at

25 OC as a function of the CI'ATs concentration.

takes indeed place between 15 and 20 mM, which is in good accord with the results of the partial molar volume measurements of ~ e ~ u l v e d a ' ~ ~ ~ . The NMR

line-broadening method, however, was not reliable for measurements in the

presence of PVME (or PPO), because of overlapping surfactant and polymer

signals. Therefore, we used viscosity measurements to obtain the concentration

at which the sphere-to-rod transition of CI'ATs takes place, in the absence and

presence of PVME. For these rheological measurements we used a shear-viscometer that can be

equipped with different measuring devices. Two of these have been used, one

having cone-and-plate geometry and the other cylindrical geometry. Provided

that the rheometer is also equipped with a special sensor, the former allows

the measurement of first normal stress difference^'^^, indicating

viscoelasticity, as well as shear stress, from which the apparent viscosity

can be calculated according to equation 3.3.

apparent viscosity = shear stress1 shear rate (3.3)

The latter measuring device only allows the measurement of shear stress but

produces more accurate data.

Usually the apparent viscosity of a solution of rodlike rnicelles drops

rapidly when the shear rate is increased. (This, as well as other rheological

peculiarities, will be discussed in more detail in section 3.4) Only at low

shear rates (or at very high shear rates) is the viscosity Newtonian, that is,

independent of shear rate. In Table 3.3 these low shear (Newtonian)

viscosities are listed for solutions containing various concentrations of

CTATs in H,O, and in the presence of 0.25 and 0.5 g . d ~ - ' PVME (measured with cylindrical geometry). For the highly viscous solutions, shear rates as low as

6 x s-' have been used. It is hard to associate the sphere-to-rod transition to a well defined concentration, since the viscosity increases

non-linearly with the CTATs concentration (Figure 3.5). The viscosity of a 15

rnM CTATs solution in H,O is already four times as high as that of water (I cP). At 18 rnM CTATs, a first normal stress difference, indicating viscoelastic behavior and thus the presence of rods, can be observed above a

Table 3.3 Apparent viscosities of CI'ATs in aqueous solutions in the absence

and presence of PVME at 25 OC.

- -

mM Hzo 0.25 g.dl-l PVME 0.5 g . d ~ ' PVME

shear rate of 476 s-' (using cone-and-plate geometry). Such viscoelastic

behavior can also be observed visually as the recoil of trapped air bubbles

when a swirling motion of the solution is abruptly stopped. From 20 mM CI'ATs

onwards, thixotropic behavior is definitely displayed using a cone-and-plate

measuring device (between 119 s1 and 476 il) and from 25 rnM CTATs onwards

using a cylindrical measuring device (between 60 s-' and 119 i l ) . Thixotropic

behavior is the occurrence of a decrease of the apparent viscosity with

increasing time and is revealed in this case after a stepwise increase in

shear rate (see, for example, Figure 3.10). The thixotropy as well as the

viscoelasticity and non-Newtonian behavior are indicative for changes in the

internal structure of the solution. Those changes originate from alignment and

disruption of the rodlike micelles by the shear forces 136.141.144.168-171

Although the transition concentration for CTATs cannot be clearly defined

it seems obvious from Figure 3.5 that the presence of PVME shifts the

sphere-to-rod transition to higher concentrations. However, there may be a

pitfall in this alluring conclusion. In 1985 Hoffmann et al.l4' stated "that

viscosity, c P

Figure 3.5 The viscosity at low shear rates (Newtonian behavior) of CTATs in

H,O (o), 0.25 g . d ~ - l PVME (m), and 0.5 g . d ~ l PVME (o) at 25 OC,

measured with cylindrical geometry. Extrapolation of the lines is

based on the data from Table 3.3.

all theories which try to explain the viscoelastic properties of rnicellar

solutions on models that are based on the existence of well-defined rods,

without taking into account the transient nature of the micelles, sooner or

later must fail". He illustrated this statement with the behavior of

n-tetradecylpyridinium salicylate and n-tetradecylammonium salicylate. These

compounds have similar cmc values, critical rod concentrations, and light

scattering behavior, which suggests that the micellar structures and the

interactions between them should also be the same. In spite of these

similarities, the viscosities of aqueous solutions of these two compounds

differ by almost two orders of magnitude. The differences between the

structural relaxation times of the micelles was shown to lie at the origin of

this difference. For these surfactants the relaxation time stems from the

kinetics of formation and dissociation of the micelle, whether stepwise per

monomer or via coalescence or fragmentation of the entire micelle, and not

from the rotation of the rods. Since this relaxation time may be influenced by

the presence of additives 131,148.172 such as n-butanol or n-pentanol, it is

conceivable that the shift in concentration where the viscosity increase of

the CTATs solution takes place, caused by PVME, is also due to these kinds of

e f f e ~ t s ~ ' ' ' ~ ~ and not to a shift in concentration of the sphere-to-rod

transition. However, we submit that this is not the case (vide infra) and that

indeed a shift in transition concentration upon PVME transition takes place.

We propose that PVME preferentially binds to spherical rnicelles of CTATs, for

which the surface to volume ratio is more favorable for interaction with the

polymer. Headgroup-headgroup repulsion and headgroup-adsorbed polymer

repulsion will be less compared to those for polymer-bound rodlike aggregates,

while the extra hydrocarbon core-water contact is stabilized by PVME. When the

CTATs concentration exceeds the saturation concentration of PVME, free

micelles will be formed, which grow into rods upon increasing the

concentration.

This view is based on circumstantial evidence: (i) a reduction in the

size of the aggregate is also found for CTAB micelles in the presence of PVME,

(ii) the transition regions in the viscosity plots of the CTATs solution in

H,O, 0.25 g . d ~ - l aqueous PVME, and 0.5 g . d ~ " aqueous PVME are virtually

superimposable, (iii) the shift in transition concentration is almost

proportional with the polymer concentration, which points to saturation of the

polymer playing a role, and (iv) in section 3.4 it will be shown that

0.5 g . d ~ - ' ethanol, t-butanol or non-interacting polymers do hardly or not

perturb the viscosity of a CTASal solution, contrary to interacting polymers

such as PVME.

We draw the conclusion that rodlike rnicelles of CTATs are transformed

(via the monomers) to polymer-bound spherical micelks in the presence of

PVME. This is a novel finding in the field of polymer-micelle interactions.

3.4 The polymer-induced transition from a non-Newtonian to a Newtonian

fluid

Cetyltrimethylarnmonium salicylate is the archetype of a cationic

~urfactant'~' that forms rodlike micelles even in dilute (ca. 10" M)

solution^"^. At higher concentrations CTASal solutions become viscoelastic

and behave strongly non-Newtonian. The maximum in viscosity lies at a

[s~~-]/[CTA+] ratio below one169'1. One does not need special apparatus to

observe the high viscosity and viscoelasticity of such a curious mixture. It

is also easily seen that the presence of 0.5 g .d~- ' PVME or PPO completely

eliminates the gel-like properties and reduces the viscosity to about that of

water. Addition of the more hydrophilic polymers PEO or PVP does not induce

such a transition. Although the change in the properties of the CTASal

solutions induced by PVME or PPO strikes the eye, rheological measurements

were performed to quantify the effect. As a matter of fact, the f fASal system

spoils a beginning rheologist since the more attractive and special aspects of

rheology are encountered without recourse to further examination of the field.

The same shear viscometer as used in the study of CTATs (section 3.3) was

used. The (apparent) viscosities of micellar CTAB solutions in the absence and

presence of sodium salicylate, polymers and low molecular weight additives are

listed in Table 3.4. These values have been obtained using a measuring device

with cylindrical geometry. The CTABINaSal solutions, whether or not in the

presence of PVP, ethanol, or t-butanol, and, to a slightly lesser extent,

CTA3/NaSaVPEO (20k), exhibit genuine non-Newtonian behavior. That is, the

apparent v isc~si t ies '~~ vary dramatically with changing shear rate (Table 3.4,

and Figures 3.6 and 3.7). The details of this behavior will be discussed

note (1): This observation seems to support Rao's 'string'-model'35, since

intermicellar interactions induced by protruding salicylate ions are

anticipated to be optimal below a complete saturation of the CTA' rnicelle with

salicylate ion. However, a second maximum in viscosity is observed in the

presence of an excess of salicylate ions in the case of n-tetradecylpyridinium

mice l~es '~~ .

Table 3.4 The effect of sodium salicylate and several monomeric and

polymeric additives on the viscosity of a micellar CTAB solution.

EnABl, [Nasal], mM rnM additivea viscosity, cP

PVME

PEO

15

15 PVME

15 PPO

15 PEO

15 PVP

15 EtOH

15 t-BuOH

a) [additive] = 0.5 g .d~- ' . b) Shear rate = 0.2985 s-I. c) Shear rate = 477.6 s".

later. By contrast, the apparent viscosities of CTAB/NaSal in the presence of

PVME or PPO, and of CTAB solutions without NaSal, are orders of magnitude

lower and are independent of shear rate, indicative of Newtonian behavior.

This polymer-induced transition from a non-Newtonian to a Newtonian fluid

is, like in the case of CTATs, attributed to preferential binding of spherical

rather than rodlike micelles onto the hydrophobic polymers. This is completely

consistent with the reduction in aggregate size of CTAB micelles in the

presence of PVME and PPO, and the shift to higher surfactant concentrations

for the sphere-to-rod transition of CTATs by PVME. The hydrophilic polymers

PEO and PVP do not bind CTAX micelles and, therefore, do not exert dramatic

effects on the rheology of a solution of these aggregates. Our results do not have any implication for the choice between

'strings-of-spheres' or 'rods' as model for CTASal aggregates. Interactions of

strings-of-spheres with hydrophobic polymers would also be disastrous for

l o g ( a p p a r e n t viscosity

l o g ( s h e a r r a t e

Figure 3.6 Double logarithmic plot of apparent viscosity vs. shear rate for

the following aqueous solutions of CTAB (25 mM): no additives,

(m); + PEO 20k (0.5 g.dL-l), (A); + PVME (0.5 g . d ~ ' ) , (0);

+ NaSal (15 mM), (n); t NaSal (15 mM) and PEO 20k (0.5 g .d~- l ) ,

(A); and + NaSal (15 mM) and PVME (0.5 g.d~''), (0). The data

were measured with cylindrical geometry.

their structural integrity. The aggregation number of the individual spheres

would be expected to decrease, but, more importantly, the presence of polymer

loops around the micelles would severely hamper string formation, and thus

reduce the viscosity of the solution.

The shear rate dependence of the viscosity of the CTABINaCl solutions is

not at all affected by PVP, ethanol, or t-butanol (Figure 3.6 and 3.7), and

Log (apparent viscosity)

log (shear ra te )

Figure 3.7 Double logarithmic plot of apparent viscosity vs. shear rate for

the following aqueous solutions of CTAB (25 mM)/ Nasal (15 mM):

no additives, (0); + P V P (0.5 g.d~-'), (0); + ethanol (0.5 g.dL-I), (A); + t-Butanol (0.5 g.dL-I), (0); + PPO (0.5 g.d~- ') , (0 ) . The data were measured with cylindrical

geometry.

only slightly by PEO. Generally speaking, three regions may be discerned in a

plot of apparent viscosity versus shear rate167. At very low shear rates,

Newtonian behavior is displayed. Our data do not include low enough shear

rates to observe this region. In the second region, the internal structure of

the solution is altered by the shear forces. In the case of rodlike rnicelles,

this causes a drop in apparent viscosity, due to aligning and disruption of

the rods. This region is very obvious in Figures 3.6 and 3.7 In the third

region, at high shear rates, the structural changes are completed and

Newtonian behavior can be observed again. For the present system, this

transition occurs around a shear rate of 100 s l . Wolff et a ~ . " ~ reported the

same shear rate of 100 i l , above which Newtonian flow was observed for 20 to

25 mM CTAB containing 9 to 11.3 rnM of 9-anthracenecarboxylic acid.

A closer inspection of our data in the second region, in which structural

changes occur, leads to surprising results. In this region (below 84 f'), the

shear stress (0.8 Pa, cylindrical geometry) does not change at all with

changing shear rate (Figure 3.8), which implies a power law exponent of zero

(see section 7.2). This is very un-liquid-like behavior. However, visual

observation convinced us that the CTAB/NaCl solution is not a solid body. The

shear stress obtained with cone-and-plate geometry reveals the same plateau

Log (shear stress)

Log (shear r a t e )

Figure 3.8 Double logarithmic plot of shear stress, measured with

cylindrical geometry, vs. shear rate for aqueous solutions of

CTAB (25 mM)/NaSal (15 rnM) without additives (0) and in the

presence of 0.5 g . d ~ - l of PEO 20k (o).

region below 476 i1 at a shear stress of 1 Pa (Figure 3.9). In the presence

of PEO only a slight shoulder can be detected (Figure 3.8 and 3.9). This

anomalous shear stress behavior has, to the best of our knowledge, only been

noted before by ~trivens'~', also for the CTASal system. A phenomenon known as

' ~ a l l - s l i ~ ' ' ~ ~ may lie at the origin of these observations. It is generally

postulated that (apparent) slip effects are due to the formation of a thin low-viscosity fluid layer near the wall of the flow channel. We lack the

necessary equipment to study this effe~t"~. Wunderlich et al.170 tried to

relate the dependence of the flow curve of a CTASal solution on the measuring

Log (shear stress 1

- 1 0 1 2 3 4

Log (shear ra te

Figure 3.9 Double logarithmic plot of shear stress, measured with

cone-and-plate geometry, vs. shear rate for aqueous solutions of

CTAB (25 mM)/ Nasal (15 rnM) without additives (a) and in the

presence of 0.5 gd1-l of PEO 20 k (m).

device to slip effects on the walls. The attempt was in vain, however, since

the curves could not be explained in terms of the slip velocity concept.

A rate-independent shear stress is only expected for a solid body167,

which clearly does not apply for a CTABINaCl solution, as mentioned before. A

definite feature of the liquid-like properties of this solution is the

observation of thixotropy (Figure 3.10) and rheopexy. Rheopexy (the opposite

of thixotropy) denotes an increase in viscosity with increasing time at a

constant shear rate. For CTAB/NaCl solutions, this is observed upon stepwise

decreasing the sheai rate. It results from the reversibility of the alignment

and breakdown of the rodlike micelles. For the CTAB/NaCl solution thixotropy

app.visc.. Pas

0 2 6 6 8

time, min.

Figure 3.10 Thixotropic behavior of CTAB (25 mM)/ Nasal (15 mM)/ PEO 20k

(0.5 g.dl"). At t = 0 rnin., the shear rate is switched from 1.19

to 2.38 s-'.

and rheopexy is observed below a shear rate of 476 il, and for the

ffAB/NaSal/PEO system below 76 i1 using cone-and-plate geometry. The

disappearance of these phenomena coincides with the end of the shear stress

plateau. The 3 to 5 minutes, which are needed in the case of stepwise

increased shear rates, are normal times for CI'ASal solution^'^' and are not

greatly affected by PEO. The visually observed viscoelasticity has also been quantified through

the measurement of first normal stress differences using cone-and-plate

geometry (Figure 3.11). Especially at shear rates below 2000 i', the first

f i rst normal

stress difference, Pa

shear r a t e , s-'

Figure3.11 Plot of first normal stress difference, indicating

viscoelasticity, vs. shear rate, for aqueous solutions of CI'AB (25 mM)/ Nasal (15 mM) without additives (0) and in the presence

of 0.5 g.dl-' PEO 20 k (A). The uncertainty of the data is ca.

50 Pa.

normal stress differences of the CTAB/NaSal solution, in the presence of PEO,

are significantly larger than those of the aqueous CTAB/NaSal solution.

However, below 200 i' in the presence of PEO, the first normal stress

difference drops sharply to values that are too low to measure. The higher

first normal stress difference of the CI'AB/NaSaVPEO solution compared to that

of the CI'AB/NaSal solution is probably related to the fact that the viscosity

of the former solution is also higher in this range of shear rate (Figure

3.6).

The (slight) influence of PEO on the rheology of the CTAB/NaSal solution

may be due to (i) interference of the polymer chains with the intermicellar ordering and flow of the rods, or (ii) to a modest interaction of PEO with

CTAX monomers or aggregates either influencing the structural relaxation time

or the structures themselves. There appears to be no reason, however, why

interference should occur for PEO but not for PVP, whereas there are

indications that PEO has a very small but detectable effect on CTAB

aggregation, which PVP has not (see section 3.1.2). Thus the latter

explanation is more likely.

We conclude that PVME and PPO induce a breakdown of rodlike rnicelles of

CTASal into polymer-bound spherical micelles, while PVP, ethanol, and

t-butanol do not affect the rods at all. Presumably, PEO undergoes a modest

interaction with CTASal, which results in a slight altering of the detailed

rheology .

3.5 Experimental section

Materials. CTAB (Merck) was purified as described by Duynstee and ~ r u n w a l d ' ~ ~ .

CTATs (Sigma), Nasal (Merck), and PPO (weight-averaged mw 1,000, Aldrich)

were used as received. PEO (Fluka) and PVP (Kolloidon-90, BASF) were purified

as described in Chapter 2. PVME (50 % (wlw) solution in water, inherent

viscosity 0.57, Aldrich) was freeze-dried. The yellowish residue was dissolved

in ethanol and heated with activated carbon. After filtration, the solvent was

evaporated and the residue was dissolved in water, dialyzed and freeze-dried.

The polymer was stored as a 20 % (wlw) solution in water. The molecular weight

of PVME (27,000) was determined by viscosity measurements in butanone. The

intrinsic viscosity equals K x (mw)a, in which K = 137 x mLg-' and

a = 0.56 at 30 0d2 for this combination of polymer and solvent.

The quencher Pmethylanthracene (Janssen) was used as received. The

fluorophore bis(2,2'-bipyridyl)-mono(4,4'-didecyl-2,2'-bipyridyl)ruthenium(II) perchlorate was a gift from Dr. L.A.M. Rupert of the Koninklijkel Shell

Laboratorium, Amsterdam. The water used in all experiments was demineralized

and distilled twice in an all-quartz distillation unit.

Conductivity measurement. Conductivities were measured using a Wayne-Kerr

Autobalance Universal Bridge B642 fitted with a Philips electrode PW 9512101

with a cell constant of 0.71 cm-I. The solutions were thermostated in a cell

at 25 4 0.1 OC for at least 15 min. before measurements were initiated. The

conductivity cell was equipped with a magnetic stirring device. The surfactant

concentrations were varied by the addition (micro-syringe) of appropriate

portions (10 to 50 p.1) of a concentrated solution of the surfactant to the

conductivity medium. Concentrations were corrected for volume changes. Cmc

values were taken from the intersection of the tangents drawn before and after

the frrst break in the conductivity vs. concentration plot. In the case of PPO and PVME solutions no clear break could be observed since the conductivity

varies non-linearly with the concentrations above the cmc. In these cases, the

cmc values were taken from the discontinuity in the plot of the first

derivative of the conductivity vs. the concentration. These values deviate

from those determined by witte6', who took the observed deviation from the

first linear part of the conductivity plot.

Fluorescence measurements. Stock solutions of fluorophore and quencher were

prepared in 96 % Uvasol-grade ethanol (Merck). In a typical experiment, 2 pl of the fluorophore stock solution was injected into 2 ml of the surfactant

solution, yielding a probe concentration of to 10'~ M. Subsequently 2 p1 aliquots of the appropriate quencher solution were injected. The concentration

of the quencher solution was chosen to yield a quencher-to-micelle ratio of

ca. 0.8 after injection of between 8 and 20 pl. The solution in the cuvette

was stirred with a magnetic device, and thermostated at 25 f 0.1 OC.

Fluorescence intensities were measured using a SLM-Arninco (SPF-SOOC)

spectrofluorometer. Excitation and emission wavelengths were 453.5 nm and 626

nm, respectively. The aggregation numbers were determined from plots of

In I(0) - In I([Q]) versus [Q] / ([CI'AB] - cmc) according to the method of

Turro and ~ekta"'.

UV measurements. UV measurements were performed on a Perkin-Elmer 15

spectrophotometer, using cuvettes with a 1 rnm pathlength. Some practice in

inserting cuvettes in the cuvette-holder of the spectrophotometer is needed to

obtain results that are reproducible to within 0.001 A.

Rheological measurements. Solutions were prepared at least one hour in

advance. CTATs solutions were prepared by dilution of a clear stock solution

of 40 mM CTATs with either water or an aqueous polymer solution. CTASal

solutions were prepared from appropriate fresh stock solutions of CTAB, Nasal,

and polymer. Rheological measurements were performed on a Brabender Rheotron

rheometer with either cone-and-plate geometry (W) or cylindrical geometry

(Al). The rheometer was equipped with a Normal F-sensor which allows the

measurement of first normal stress differences when cone-and-plate geometry is

used. The sample solution was thermostated at 25 f 0.1 OC during the

measurements.

CHAPTER 4

THE EFFECT OF HEADGROUP CHARGE ON POLYMER-MICELLE INTERACTION:

n-DODECYLDIMETHYLAMINE OXIDE

4.1 Introduction

4.1.1 A brief glance a t semipolar surfactants

The next three chapters deal with the effect of the surfactant headgroup

charge on polymer-rnicelle interaction. Each chapter is concerned with a

surfactant system of which the charge can be varied by protonation or

deprotonation, without a drastic change in the structure and volume of the

(unhydrated) headgroup. The present chapter is concerned with a study of the surfactant n-dodecyldimethylamine oxide (DDAO), which on protonation affords

N-hydroxyl-n-dodecyldimethylarnmonium chloride (DDAOH+).

Dipolar DDAO belongs to an interesting class of nonionic surfactants,

sometimes referred to as the semipolar subclass180. Other members are the phosphine oxides, arsine oxides and sulfoxides. The sulfoxides differ from the

others in that the heteroatom has a coordination number of three instead of

four.

From a comparison of the phase behavior (liquid crystalline), basicity,

dipole moments, and hydrogen bonding ability, the order of hydrophilicity of

these compounds is found to follow the order in dipole moments's0. Thus the

hydrophilicity decreases in the order N + 0 > As + 0 > P ---+ 0 > S --+ 0

(Table 4.1). The more hydrophobic character of the phosphine oxide surfactants

compared to that of the amine oxide is also reflected in the much higher

solubilization capacity of the formerlS1. For instance, tetradecyl-

dimethylphosphine oxide may solubilize six times as much decane as the

corresponding arnine oxidelE1. The comparison of these two surfactants in their

tendency to interact with polymers would be a very interesting topic for

future research. We have studied the effect of protonation on polymer-rnicelle

interaction for the surfactant DDAO. In principle, the pKBH+ value of the

corresponding arsine oxide would allow a similar kind of study (Table 4.1), but hardly anything is known about the aggregation behavior of this

surfactant.

Table 4.1 Bond dipole moment and basicity data for Group V and VI oxides.Taken from ref 180.

reduced bond basicity

moment p: pKm+

a) Reduced bond moment pr = pobmd I dM O(in A) x 4.80.

65

4.1.2 The effect of protonation on the micellization of DDAO

The convenient pKB of amine oxide surfactants has led to many studies on 182.183 the effect of charge variation (0 + +1) on micellar properties , such

as the cmc lMvl 85, aggregation n ~ m b e r ' ~ - ' ~ ~ , counterion binding1 87, surface

tensionlg8, and phase behavior 185,189.190 . Charge variation also aided in distinguishing electric and non-electric contributions to the free energy of

191,192 micelle formation . For most studies an indication of the degree of protonation (P) at a well

defined DDAO concentration suffices to describe the trend in the change of

properties upon charge variation. But, naturally, the potentiometric behavior

of DDAO has challenged several investigators to find a precise theoretical

d e s ~ r i ~ t i o n ~ ~ ~ ~ ~ ~ ~ ' ~ " . In such a description, the concept of two pKA values

is often used, one for the surfactant in the monomeric state and one for the

surfactant in the micellar state188"". This concept is easy to visualize since obviously the electrostatic repulsion in the micelle will reduce the

ability of a DDAO molecule to take up a proton. However, Rathman and

~ h r i s t i a n ' ~ ~ have recently rejected this concept and managed to describe the

behavior using the (pseudo)phase separation model and a single pKA value, while calculating the activities of the surfactant directly from experimental

titration curves.

The influence of protonation on the cmc and especially on the aggregation

number reveals many of the interactions and forces that play a role in the

micellization of DDAO. The cmc of the nonionic form (2 mM, Table 4.2) lies in

between those of a comparable cationic surfactant (~-c,,H,,N+(cH,),B~-, cmc

17.5 r n ~ ~ ~ ) and a nonionic surfactant of the conventional type

(n-C,2H2,0CH2CH,0),H, crnc 0.1 d 2 ) . This is due to the highly polar N 0 bond which causes dipolar headgroup repulsions. Upon protonation, the cmc of

DDAO increases 184.186.193 , as expected, since the solubility of the monomers

becomes higher, while in the micelle the inter-headgroup electrostatic

repulsion increases (Table 4.2 and sectiog 4.2). Upon addition of salt (NaCl) both the cmc's of the nonionic and cationic form are decreased 184.186.193 . The effect of NaCl on the nonionic form is caused by a salting-out effect 186,188

Table 4.2 Literature data on the cmc and aggregation number (n) of DDAO, at

various degrees of protonation, in aqueous solution.

degree of protonation cmc, n Reference

P mM

0 2 76 Herrmann (1962) '~

0 2.01 76 Ikeda ( 1 9 7 9 ) ' ~ ~

0 2.1 76k7 Faucomprd (1987)~ '~

0 1.45 Ikeda (1978) '~~

0 1.9 Rathman (1990) '~~

0.5 2.36 52 m a (1979) '~~

0.5 2.16 Ikeda (1978)lg8

8 89 Henmann (1 962)lW

6.55 48 Ikeda (1979) '~~

6.40 Ikeda (1978) '~~

4.7 Rathman (1990) '~~

For the cationic form the influence of salt is mainly to reduce electrostatic

repulsion between headgroups due to an increase in counterion binding and bulk

ionic strength. However, there is a deviation from the theoretical prediction

for the influence of charge on the cmc, which probably originates from the

fact that the degree of ionization of the micelle changes with counterion

concentration even at a constant degree of neutrali~ation'~~.

It was shown by 1kedalg6 that the aggregation number of DDAO micelles in

salt-free aqueous solutions decreases upon increasing the degree of

protonation (Table 4.2). These results are in contrast with those of

~ e n m a n n ' ~ ~ (1962). Our results (section 4.3) are intermediate between those

of ~ e r r m a n n ' ~ ~ and ~ k e d a ' ~ ~ and show also a decreasing trend. The decrease in

aggregation number is thought to result from electrostatic repulsion which is

the size-limiting factor in the absence of saltlg6.

Many more data have been reported on the aggregation number in salt

solutions, The micelles, even in nonionic form, grow with increasing salt

(NaC1, NaBr) concentration. It is peculiar that above a salt concentration of

0.01 rnM NaCl, the maximum aggregation number is found at P = 0.5120,186 . In

0.2 M NaCl and at P = 0.5 rodlike rn i~e l l es '~~ are formed with aggregation

numbers as high as 6001g6. Due to the presence of salt, the dominating

influence of electrostatic interactions is diminished, although these

interactions are still size-limiting at j3 = 1. At j3 = 0.5, however, hydrogen

bonding between DDAO and DDAOH' headgroups prevails over electrostatic

interaction and stabilizes the rodlike micelles 184.190 (Figure 4.1). At P = 0,

the interaction between the (hydrated) headgroups is mainly dipolar in

character.

The hydrogen bonding between the neutral and protonated amine oxide also

influences the phase behavior of these surfactants. For instance, ~ e r r m a n n ' ~ ~

found that a micellar solution of DDAO in the presence of 0.2 M NaBr shows

phase separation between pH 4.4 and 5.4, which is the pH region of

half-protonation. 1maelgo observed a similar liquid-liquid phase separation at

half protonation for oleoyldimethylamine oxide. Furthermore, the contraction

of DDAO monolayers upon protonation is attributed to intermolecular

hydrogen-bonding'96. The same kind of complex formation has been revealed by 197.198 other half-ionized molecules, such as fatty acid soaps ,

Figure 4.1 Schematic representation of hydrogen bonds- formed in micelles of

DDAO at different degrees of protonation. (left) P = 0; (middle)

p = 0.5; (right) P = 1. Taken from ref. 256.

The study of polymer-micelle interaction with DDAO at various degrees of

protonation in salt solutions would be useful in order to probe the importance

of inter-headgroup interaction. However, this chapter contains only such a

study in the salt-free system, which is a better starting point and provides a

good insight into the effect of micellar charge on polymer-micelle

interaction. The interaction of DDAO with the polymers PVME, PPO, and PEO will

be discussed. The properties of the system that have been measured include cmc

values, aggregation numbers, and cloud points of PVME and PPO.

4.2 Critical micelle concentrations

Cmc values for DDAO at various degrees of protonation were measured using

the pH-method developed for the phosphate surfactants (Chapter 5). This method

is especially powerful for cmc determinations in polymer solutions, in which

many other methods fail. The method is based on the abrupt change in pH upon

increasing the surfactant concentration above the cmc (Figure 4.2 and 4.3).

[OOAO] .mM

Figure 4.2 Plot of pH vs. DDAO concentration. Data for DDAO at P = 0.47 in

0.9 g d ~ - ' PPO solution.

Figure 4.3 Plot of pH vs. DDAO concentration. Data for DDAO at P = 0

in H20.

The pH change is thought to originate from a reduced tendency of the

surfactant in the micellar state to take up extra charge by protonation (in

the case of DDAO) or deprotonation (in the case of the phosphates). One reason

is the presence of many charged headgroups close together, and another reason

is that the low local polarity. More precisely, the phenomenon is a matter of

surfactant activity (section 4.1.2). An attractive aspect of the method is

that it makes use of an intrinsic property of the surfactant, and thus avoids

the problems associated with the use of probe molecules. A drawback is,

however, that for DDAO the method fails at the extremes of P = 0 and P = 1 in

the presence of polymers. At P = 0 the pH change is very small albeit in the

most sensitive pH range (pH ca. 7). At P = 1 (pH 2 - 3) the swamping amount of

H+ masks the pH change. In the absence of polymers, however, we have succeeded

in measuring the cmc even at p = 0 and p = 1. Remarkably, at P = 0 the pH

changes to higher values at the cmc, opposite to the above considerations

(Figure 4.3). The surfactant picks up H+ from the bulk solution upon

micellization. This probably reflects that a small degree of protonation is

favorable as a result of inter-headgroup hydrogen-bonding interaction.

The crnc values in the absence and presence of polymers are listed in

Table 4.3. The degree of protonation reported in this table is calculated from

the pH at the crnc using a pKA value of 5.0, which seems reasonable in view of

the data in Table 4.4. The degree of protonation is adjusted by varying the pH

of the concentrated DDAO stock solution. The pH at the cmc, and thus P, is not

noticeably affected by the presence of polymer.

The crnc values of DDAO in H20 , measured by using the pH method, are

relatively low in comparison with those reported in the literature (Table

4.2). A similar observation was made in case of the phosphate surfactants

(section 5.2). Presumably the method responds to even the first stage of

aggregation.

Within the limit of reproducibility (5 % at P = 0.24, 2 % at P = 0.5, and

0.75) the crnc at P = 0.24 is not influenced by the presence of polymers. At

higher degrees of protonation the crnc is reduced in the presence of PPO and

PVME, but unaffected by the presence of PEO. Although it is tempting to

conclude from the crnc data that the stabilization of the micelles by PPO and

PVME increases with increasing rnicellar charge, a more quantitative conclusion

Table 4.3 Cmc valueshf DDAO, at various degrees of protonation, in the

absence and presence of polymers.

b PC

polymer

0.0 0.24 0.47 0.75 0.98

- 1.7 1.53 1.80 2.54 4.74

PPO - 1.46 1.33 1.63 PVME - 1.56 1.70 2.08

PEO 10 k 1.61 1.84 2.55

a) In mM. b) Polymer concentration: ca. 0.9 g .d~- l (see experimental section).

c) Calculated from the pH at the crnc using pKa = 5.0.

Table 4.4 Literature data on the pKa values of R(CH3)2NO~* in aqueous solutions.

--

pKa Reference

cH3 4.65 Nylen (1 9 4 1 ) ' ~ ~

'zHs 5.13 Nylen (1941) '~~

n-ClzH2s 5 .O Hemnann (1962)lg4

n-C12Hzs 4.95 Tokina (1966) '~~

n-C12H25 4.78 (5.63") Maeda (1 974)191

n-C12H,, 5 .O Rathman (1990) '~~

a) pKa of the surfactant in the micellar phase.

should be based on a comparison of free energies of micellization in the

presence and absence of polymer. In a first approximation, the free energy of

rnicellization is related to the crnc expressed in mole fraction units according

to equation 4.1~'~.Thus, the change in standard free energy of the micelles

AGO = RT In (crnc) mic (4.1)

due to the presence of polymer is given by equation 4.2, in which crnc

represents the crnc in the polymer s~lution~"~.The quantity AGO. - AGO m~c-pol mic

AGO mic-pol

- AGO. = RT In (cmc / cmc) m1c P

denotes the change in standard free energy when 1 mole of surfactant molecules is transferred from regular micelles to polymer-bound micelles, plus the change in free energy of the polymer induced by this process. The results for

AGO mic-pol

- AGO are presented in Table 4.5 and c o n f m the intuitive mic

conclusion from the crnc data, namely, that the stabilization is more

pronounced at higher micellar charge. At first sight this seems to agree with current views on polymer-micelle interaction. Deeper thought reveals that,

although indeed the interaction with the ionic surfactant is stronger than

with the nonionic surfactant, any rationalization based on headgroup volume is

misplaced. Protonation will barely influence the size of the headgroup, but

the hydration shell will be affected. This would be expected to lead to larger

(hydrated) size of the cationic headgroup. Apparently, the size of the

cationic headgroup will not be much different from that of a trimethylammonium

group. Since the size of the headgroup obviously does not play a dominant

role, the effect must have a different origin. We contend that the increase in

stabilization of the micelles by interaction with polymers at increasing

micellar charge stems from an increasing reduction of electrostatic repulsion.

Especially at higher micellar charge the formation of smaller, polymer-bound

micelles will be favored, since electrostatic repulsion is diminished while

the increase in hydrocarbon-water contact area is stabilized by the polymer.

Since hitherto the influence of charge has been studied by comparing

polymer-micelle interaction for SDS, CTAB, and Triton X-IOO~, with completely

different headgroups, too much emphasis has been placed on headgroup structure

and size, instead of on the role of charge proper.

Table 4.5 AGO mic-pol

- AGO for DDAO micellesa, at various degrees of mic

protonation, in the presence of polymers.

PPO -0.1

PVME 0.1

PEO 1Ok 0.1

a) In ld.mo1-l, estimated error 0.1 kJ.mo1-'. b) Polymer concentration: ca.

0.9 g . d ~ - l (see experimental section). c) Calculated from the pH at the cmc

using pKa = 5.0.

In view of the results presented in Chapter 2, however, one should be

careful not to link the occurrence of polymer-micelle interaction too heavily

to the stabilization of the micelles (see also section 9.3). Therefore,

aggregation numbers have been measured to decide whether or not the absence of

a reduction of the cmc in case of DDAO (any P) 1 PEO and DDAO (0 = 0.24) / polymer points to the complete absence of polymer-rnicelle interaction.

4.3 Aggregation numbers

The aggregation numbers of DDAO at various degrees of protonation have

been measured by static fluorescence quenching using the system bis

(2,2'-bipyridyl) - mono (4,4'-didecyl-2,2'-bipyridyl) ruthenium(I1)l 9-methyl- 178,179 anthracene . As pointed out in section 3.2, this method should be

applied with care. The same fluorophore/quencher system has been used by Warr and ~ r i e s e r ' ~ ' for the determination of the aggregation numbers of DDAO at

various P's, using dynamic fluorescence. Since their measurements were

performed in salt solutions, in which rodlike micelles are formed, the

possibility of polydispersity necessitated the analysis of dynamic

fluorescence decay curves. Unfortunately the data on aggregation numbers in

salt-free DDAO solutions is limited. However, our data on DDAO in the absence

of polymer (Table 4.6) agree well with those reported in the literature (Table

4.2).

The aggregation numbers of DDAO in water (Table 4.6) show a decreasing

trend with increasing p as expected in view of the enhanced electrostatic

repulsion. The slightly higher aggregation number at P = 0.47 compared to

those at p = 0.24 and 0.75 would be in accord with inter-headgroup

hydrogen-bonding being maximal. The effect is too small, however, to exclude

the possibility of an experimental artifact. We emphasize that the possibility

of systematic errors that may obscure a comparison is appreciably higher

within a horizontal row of Table 4.6 than within a vertical column.

The data in Table 4.6 nicely illustrate that an unperturbed cmc may have

different origins. In the case of DDAO/PEO at various degrees of protonation,

Table 4.6 Aggregation numbers of micelles of DDAO, at various degrees of

protonation, in the absence and presence of polymers.

Pb polymera [surfactant],

mM 0.0 0.24 0.47 0.75 0.98

30

20

PPO 20 PVME 20

PEO 10 k 20

a) Polymer concentration: 0.5 g .d~- l . b) Calculated from the pH at the cmc

using pKa = 5.0. c) Calculated on the assumption that the cmc in the presence

of polymer equals that in H20.

the unperturbed aggregation numbers (within confidence limits) clearly

indicate the absence of interaction. In the case of DDAO/PPO and DDAOIPVME at

low degree of protonation, in contrast, the reduction in aggregation number

definitely suggests polymer-micelle association, but this interaction does not

lead to stabilization of the micelle. As discussed in Chapter 2, this probably

originates from counteracting contributions to the total free energy from the

changes in free energy of surfactant molecules and polymer upon transferring a

mole of surfactant molecules from normal to polymer-bound micelles.

Steric hindrance between the hydrated nonionic headgroups and polymer

segments will be unfavorable, whereas the transfer of polymer segments, in the

case of PPO or PVME, to the micellar phase will be favorable. Furthermore,

there will be no favorable loss of electrostatic repulsion like at higher P. The decrease in aggregation number in the presence of PPO and PVME

becomes more pronounced at higher P. This is not surprising since a reduction

in electrostatic repulsion by increasing the surface to volume ratio of the

micelles will be more important at higher micellar charge. The influence of

PPO and PVME on the aggregation number is, within the confidence limits,

equal. even though AG:~+~, - AGO. is clearly more negative for PPO than for mlc

PVME. This trend in stabilization is consistently found (Chapter 3, 5, and 6).

It may point to stronger hydrophobic interaction for PPO compared to PVME, and

to a slight difference in morphology of the polymer-micelle complex due to the

lower molecular weight of PPO (mw 1,000) compared to PVME (mw 27,000). This

matter will be discussed in the next section in which the differences in

clouding behavior of PPO and PVME are presented.

4.4 Clouding of PVME and PPO

P V M E ~ ~ ~ ~ and PPO both have a cloud point just above 30' C. Clouding

behavior indicates a microphase separation into a polymer-rich and a

water-rich phase. It is thought to result from a breakdown of the hydration

layer at higher temperatures, which facilitates interpolymer intera~tion~~.

Especially for the hydrophobic polymers PVME and PPO, the unfavorable entropy

associated with hydrophobic hydration and the cooperativity of interpolymer

Londen dispersion forces, may drive the system towards microphase separation.

For the determination of cloud points many slightly different methods may

be found in the literature47'103"04042w . These have in common that they rely

on a change in light transmission upon clouding. We have taken the cloud point

as the temperature at which the transmission at 400 nm passes through 50 %.

For PVME, clouding occurs within such a narrow temperature range, that the

outcome is hardly dependent on the method. For PPO, however, clouding takes

place in a temperature range of over 10 OC, but it appeared to become more

cooperative when micelles are bound to the polymer (see also section 2.3). The midpoint of the clouding phenomenon of PPO (32 OC) is lower than that

of PVME (34 OC). For higher molecular weights of PPO (mw > 2,000) the polymer

becomes insoluble in water. It is known, that PPO coils up in aqueous solution

into tight disks with most of the hydrophobic methyl groups in the center of

the One might expect that this renders PPO more hydrophilic, since

mainly polar ether groups reside at the outside. The exact spacing of the

ether groups, however, appears to be decisive for the solubility in water (see

section 1.3, PEO and PMO). Altogether, PPO appears to be more hydrophobic than

the isomeric polymer PVME.

The cloud point of PVME is raised in the presence of DDAO micelles to an

extent that is almost proportional to the charge on the micelles (Figure 4.4).

It has been noted before3, that the cloud point of partly hydrolyzed pvAC20'-203 or methylcellulose20' may be elevated through binding to ionic

micelles. Our results for PVME/DDAO clearly illustrate that intermicellar

electrostatic repulsion is the main reason for such an elevation. Since the

micelles are bound to the polymer, the polymer chain segments will be held

apart if the micelles repel each other. At /3 = 0, in contrast, intermicellar

interaction is small or absent according to light scattering data190 and the

T , O C

Figure 4.4 The cloud point of PVME in the presence of DDAO at various

concentrations of DDAO, as a function of P. (a) 1.25 mM, (0); (b) 2.5 mM, (A); (c) 5 mM, (A); (d) 10 mM, (I); (e) 15 mM, (0);

(0 20 mM, (0).

A

Figure 4.5 The cloud point of PPO in the presence of DDAO at various

concentrations of DDAO, as a function of P. (a) 0 mM (+);

(b) 1.25 mM, (0); (c) 2.5 mM (A); (d) 5 mM (A); (e) 10 mM, (m);

(f) 15 mM, (0).

effect on clouding of PVME is nil. The clouding behavior of PPO is altered by DDAO in a completely different

way (Figure 4.5). At P = 1, the cloud point is raised slightly less than that

of PVME. But quite unexpectedly, also at P = 0 the cloud point is elevated,

even somewhat more than at p = 1. At p = 0.5 and a DDAO concentration of 10 or

15 mM a shallow minimum in cloud point vs. P is observed, which may result

from the optimal interheadgroup hydrogen-bonding at that P, leading to reduced

intermicellar repulsion. The deviating behavior of PPO, compared to the anticipated characteristics of PVME, may stem from a difference in aggregate

morphology. PPO appears to be more hydrophobic than PVME, thus a smaller

number of chain segments will protrude as loops in the solution surrounding

the micelles. Furthermore, the PPO sample used by us has a much lower

molecular weight (mw 1,000) than PVME (mw 27,000). So it may well be possible

that a PPO chain is adsorbed on no more than one single micelle, instead of

the usual case of a number of micelles bound to one polymer chain. Therefore,

clouding of PPO may be hampered, even by binding to nonionic DDAO micelles,

due to compartimentalization. The fact that the nonionic surfactant OTG does

not really raise the clouding point of PPO (Chapter 2) may be due to

preferential solvation of OTG in the polymer-rich phase, which will stimulate clouding. Nevertheless, it remains unclear why the cloud point elevation by

DDAO is stronger at P = 0 than at P = 1. The opposite is to be expected for

the following reasons. The number of micelles will be greater at P = 1, since

the difference in cmc is negligible, whereas the aggregation number of the

micelles at p = 1 is smaller. This implies not only more segregation of PPO

chains, but also more hydrophobic interface to adsorb PPO and prevent it from sticking out into the solution. Nor does the temperature dependence of the

micellar aggregation give a clue either. Hoffmann et a1.1a9 found an increase

in aggregation number at higher temperatures for n-tetradecyldimethylarnine

oxide at p = 0. Since a decrease in aggregation number may be expected for the

ionic form, this temperature dependence does certainly not explain the problem at hand. One, admittedly ad hoe, explanation for the clouding behavior might

be that the higher ion concentration at the micellar surface at P = 1

depresses the cloud point more than that increased intermicellar repulsion raises it. More work is needed, however, to provide a definite explanation of

the clouding behavior of PPO.

4.5 Experimental section

Materials. DDAO (Fluka) and PPO (weight-averaged molecular weight 1,000,

Aldrich) were used as received. The purification of PVME and PEO has been

described in sections 3.5 and 2.6, respectively. The water used in all

experiments was demineralized and distilled twice in an all-quartz distillation unit.

Preparation of stock solutions. To a precisely measured weight of DDAO (ca.

250 mg) was added an appropriate amount of aqueous 0.2 N HC1 and water until a

total volume of 25 rnl, which was again precisely weighed. The pH of the stock

solution containing 40 mM of DDAO (corrected for the 7 % H,O content of the

commercial surfactant) was measured. The stock solutions, producing P = 0, 0.24, 0.75, and 0.98 at the cmc, had a pH of 7.31, 5.21, 3.19, and 2.05, respectively, and were used for all experiments. They were stored at -20 OC.

Cmc measurements. An amount of stock solution was injected stepwise in 7.5 rnl

of water or a 1 g . d ~ - l aqueous polymer solution, by an home-made apparatus,

~ 0 ~ e c t e d to a PC. Since the stock solution did not contain any polymer,

dilution of polymer upon injection of the DDAO solution was unavoidable.

However, at the cmc, the added amount of stock solution never exceeded 0.8 ml,

corresponding to dilution to a polymer concentration of 0.9 g . d ~ l , and at the

end of the experiment 2.5 ml, corresponding to a polymer concentration of 0.75 g .d~- ' . Each injection of 0.85 mg was followed by a delay time of 2 sec,

after which the pH was measured with a Corning 130 pH meter, connected via an

analog-digital converter with the PC. The pH was plotted against the

surfactant concentration, which is corrected for volume changes. From this plot the cmc was determined as the intersection point of the tangents drawn

before and after the sudden change in pH (Figure 4.2). The cmc values were

obtained at 25 OC.

Aggregation numbers. Sample solutions were made from appropriate amounts of

DDAO and polymer stock solution and water. Measurements were performed as

described in section 3.5.

Cloud point measurements. Cloud points were determined as the temperature at

which the transmission at 400 nm passes through 50 % following the procedure

described in section 2.6. The polymer concentration was 0.5 g .d~- ' .

CHAPTER 5

THE EFFECT OF HEADGROUP CHARGE ON POLYMER-MICELLE INTERACTION:

MONO-n-ALKYL PHOSPHATES

5.1 Introduction

The enormous number of articles that appears on the surfactant sodium

n-dodecylsulfate fills several pages per year in Chemical Abstracts. In

contrast, reports on sodium mono-n-allcylphosphate surfactants are extremely scanty. For the mono-n-octyl, -decyl, and -dodecyl phosphates together only

four or five articles per year are listed in Chemical Abstracts, together with several patents. The commercial interest that led to the patents on these

surfactants stems from the combination of adequate surface-active properties

and low degrees of skin irritation and damage204. It is unlikely that the

difference in scientific interest between the sulfates and phosphates originates solely from the fact that the pure mono-n-alkylphosphates are not

commercially available, but it is hard to find another reason. The phosphate

surfactants can be prepared in one step from pyrophosphoric acid and the

appropriate alcoholm5. Purification by recrystallization yields the

n-alkylphosphoric acid, which can be neutralized by sodium ethanolate to afford the sodium salt. Hydrolysis of the mono-anion is negligible at room

temperature (for n-decylphosphate at pH 4.5 the rate constantm is 8.2 x i' at 100 OC, corresponding to a half life time of approximately

24 hours), whereas the di-anion is totally unrea~tive~'~.

The structural charge (ZJ of m ~ n o - a l k ~ l ~ h o s p h a t e s ~ ~ which possess two

acidic protons, can be varied according to

where pKl is ca. 2 and pK2 is ca. 7". Both the mono-anion and the di-anion

behave as surf act ant^^^^. We emphasize that in the corresponding micelles the

actual charge of the surfactant molecule will be somewhat lower than the

structural charge which is the average charge of the mono-alkylphosphate anion

in the undissolved state2''. For example, Chevalier et aL208 reported for

n-octylphosphate micelles at Zo that the counterion binding P = 0.4, whereas

at Z = 2.0 the P value was 0.7, slightly dependent on concentration. 0 The aggregation behavior of the mono-anion is quite comparable to that of

other monovalent anionic surfactants, such as SDS~'~. The cmc and headgroup

area per molecule in the micelle lie in the same range2'' for both

surfactants. Furthermore, Romsted et aL2I1 studied specific counterion

effects on indicator equilibria in micellar solution, and found no special

effects of the phosphate mono-anion headgroup as compared to the corresponding

sulfate.

Increasing the structural charge to Zo = 2, results in a substantial

increase in headgroup repulsion, as evident from the larger headgroup area in 212,213 m i c e l l e ~ ~ ~ ~ and monolayers , lower aggregation numbers of the

m i c e l l e ~ ~ ~ ~ , and a higher cmc209'214. The cmc of the di-anion is comparable to

that of other divalent surfactants, such as the n-alkylsuccinates and

malonate~~'~. The high enthalpy of micellization at Z = 2 209 and the reduced

enthalpy loss upon micellization per CH2 group212b'1p, compared to the values

at Zo = 1, indicate the more pronounced hydration of the highly charged

headgroups and the larger area of contact between alkyl chain segments and

water208. The difference in rnicellar properties for the mono- and di-anions

also leads to a reduced solubility power for benzene in the highly charged

micelles208.

In view of the results discussed in the previous chapter, inter-headgroup

hydrogen-bonding may be expected to occur for the alkylphosphates. This is

indeed observed2I3, but mainly in studies of m ~ n o l a ~ e r s ~ ' ~ . The interaction is

most effective at half-ionization of the first acidic proton. In contrast to

DDAO (Chapter 4). only a few reports have appeared on aggregation numbers, and

the available data are mainly for rn~no-n-oct~l~hosphate~~~. No anomaly in the

trend of aggregation number with increasing charge has been found yet (section

4.1.2). However, interheadgroup hydrogen-bonding may explain that the rather

short-chain n-octylphosphate at Zo = 1 is already capable of forming rodlike

micelles (in the concentration range from 0.6 M to 2 M), as deduced from 3 I P NMR ~ ~ - h e a s u r e m e n t s ~ ~ .

In this chapter, we compare micellization and interaction of micelles

with the nonionic polymers PEO, PVME, and PPO for mono-n-octylphosphate

(Zo = 1) and mono-n-decylphosphate (Zo = 1.0, 1.1, 1.5, and 2.0). A new method, based on pH changes upon increasing the surfactant concentration, was developed and used for the determination of the cmc. Furthermore, the clouding

behavior of PVME in the presence of mono-n-decylphosphate will be described.

Apart from the possibility of charge variation, an incentive to choose the

phosphates has been the presence of the 3 1 ~ nucleus, which is suitable for NMR measurements219. This chapter also contains a study of the influence of PVME

on the NMR properties of the 3 1 ~ nucleus of the surfactant molecule in the

micellar state. The chapter ends with some preliminary results on the

interaction of PVME with di-n-dodecylphosphate vesicles. This study is by no means complete but is presented in the hope of stimulating further research in

this field.

5.2 Critical micelle concentrations

We have measured cmc values of mono-n-octyl- and mono-n-decylphosphate

at various values for the structural charge of the headgroup in the absence

and presence of PEO (10k and 20k), PVME, and PPO. Conductornetry, one of the most popular methods for the measurements of the cmc of ionic surfactants,

does not produce reliable results for the phosphates in the presence of PEO.

PPO, and PVME. In these solutions there is no well-defined break in the plot

of conductivity vs. surfactant concentration. Reasons for the failure of the conductivity method probably involve the variable and weak counterion binding

as well as the low aggregation numbers of polymer-bound micelles. Even in the

absence of polymers the break in the conductivity plot of, for instance,

mono-n-octylphosphate (Zo = I)~", is not as pronounced as usually found for

ionic surfactants. Therefore, a new method has been developed that takes advantage of the abrupt change in the pH as a function of surfactant

concentration upon micellization (Figure 5.1)"~. The pK of the phosphate headgroup is higher for the surfactant molecule in the rnicelle than for the

surfactant monomer because inter-headgroup interactions inhibit dissociation.

Furthermore, the intrinsic pKa of the phosphate is likely to increase, e.g.

the surfactant becomes less acidic, because of the low local polarity at the

micellar surface. The increase of the pH upon rnicellization is most pronounced for the n-alkylphosphates at Zo = 1 but also clearly recognizable for n-decylphosphate at Zo = 1.5 and 2.0. In Table 5.1 cmc values for the two

n-alkylphosphates (Zo = 1.0) are listed which have been determined by

different procedures (Figures 5.1, 5.2, and 5.3). It is evident that the cmc

obtained by the pH method coincides rather closely with the surfactant

concentration at which the first deviation occurs from the initially linear

relation between conductivity and surfactant concentration.

Cmc values for the two n-allcylphosphates, in the absence and presence of PPO, PEO 10k, PEO 20k, and PVME are presented in Table 5.2. For comparison,

Table 5.1 Cmc values of n-alkylphosphates, determined by different methods.

surfactant T, OC cmc, Methoda Reference mM

This study

207 This study

This study

209

This study

207 This study

This study

209

a) A: conductivity. B: pH method. C: first change in the conductivity vs. concentration plot, see text and Figure 5.3.

Figure 5.1 Typical plot of pH vs. surfactant concentration. (o) Data for n-C,,H,,OPO,HNa, 25 OC, (0) data for n-C,,H,,OPO,HNa in 0.5 g d ~ - ' PPO, shifted one pH unit upward.

[surfactant I ,mM

Figure 5.2 Typical plot of specific conductivity (K) vs. surfactant

concentration. Data for n-C,,H,,OPO,HNa at 25 OC.

cmc values are also listed for sodium n-decylsulfate (SDeS), and sodium

n-dodecylsulfate (sDs)~~. For the ionic surfactants a reduction of the cmc is

taken as evidence for polymer-micelle interaction. Obviously, PPO and PVME

interact with the surfactants listed in Table 5.2. For PEO 10k or 20k only

interaction with SDeS, SDS, and n-decylphosphate at low structural charge is

revealed.

A quantitative comparison of polymer-micelle interaction for different

combinations necessitates an analysis of the free energy changes for

micellization, in the presence and absence of the polymer. In a first

approximation, the change in standard Gibbs energy of the micelle due to the

association to a polymer is given by756 equation 5.1 (see also section 4.2),

AGO mic-pol

- AGO. = RT In (cmc / cmc) mu: P

d H I d [surfactant 3

first change

[surfactant] .mM

Figure 5.3 Typical plot of d~ / d[surfactant] vs. surfactant concentration.

Data for n-C,,H,,OPO,HNa at 25 OC.

where cmc is the cmc in the presence of polymer and cmc the corresponding P

value for the unperturbed micelle. These standard free energy changes are

given in Table 5.3 and denote the changes in standard free energy when one

molecule is transferred from normal to polymer-bound micelles plus the change

in free energy of the polymer induced by this process.

The data in Table 5.3 show that an increase of the surfactant alkyl chain

length has the effect of enhancing polymer-micelle attraction. Comparable

effects have been found previously3~6~8. For example, we find that

n-octylphosphate micelles (Zo = 1) do not interact with PEO 10k or 20k,

whereas micelles formed from n-decylphosphate (Zo = 1) are stabilized by PEO

20k. In addition, interactions involving n-alkylsulfate micelles are more

pronounced for SDS than for SDeS. These results are usually rationalized by

assuming that hydrophobic interactions largely determine the complexation

process31. Although it is most likely that hydrophobic interaction is an

Table 5.2 Cmc values of various surfactants in the absence and presence of

polymersa.

surfactant

cmc, mM

zd H,O PPO PEO PEO PVME

a) Polymer concentration: 0.5 g.d~-l. b) Structural headgroup charge. c) Cmc

values determined using the pH method. d) Cmc values determined using

conductometry. e) Interaction has been definitely established, but no accurate

cmc values could be obtained. f) Taken from ref. 67.

important driving force, in view of the hydrophobic polymers showing stronger

micelle stabilization, there is a flaw in the above reasoning on the effect of

the chain length. Thus, ~sraelachvili~~~ has pointed out that the surface area

per surfactant molecule in the rnicelle is virtually unaffected by changing the

chain length (above a certain length). If this area is the same, so is the

area of hydrocarbon core-water contact, which is thought to determine the

interaction with polymers. Therefore, the possibility of hydrophobic

interaction of the micelles is not significantly affected by the alkyl chain length. The same theoretical difficulties are encountered in solubilization studies22'. Again, solubilization is usually enhanced when the alkyl chain

Table 5.3 Free energy of rnicelle stabilization by polymers' for various

surfactants.

AGO mic-pol

- AGO 1d.mo1-' mic'

surfactant z," PPO PEO 10k PEO 20k PVME

a) Polymer concentration: 0.5 g .d~- l . b) Structural headgroup charge. c)

Estimated error, based on the reproducibility (3%.) of the cmc: 0.15 k.J.mo1-'.

length is increased. For solubilization, a relation with rnicellar size was

eventually used, to explain the effect for substrates that are located in the

core or deep in the palisade layer221. For substrates located at the rnicellar surface the same difficulties as with the polymer-micelle interaction, still

remain. Also catalytic effects increase with increasing chain length222. Anyway, it is clear that a relevant comparison of headgroup effects on

polymer-rnicelle interaction should be confined to surfactants of the same alkyl chain length.

Comparison of the data for n-decylphosphate (Zo = 1) and -sulfate,

clearly shows that even small changes in headgroup structure have a pronounced

effect on polymer-rnicelle interaction even within the class of anionic

surfactants. The difference in the (unhydrated) headgroup volume between

-OPO,H- and -OS03- is relatively small as demonstrated by the limiting partial

molar volumes of HSO; (35.67 cm3.mol-l) and H2m4- (29.1 cm3.mol.')".

However, the effect of the headgroup on polymer-micelle interactions is

considerable and, in contrast to expectation, the interaction is stronger for

the micelles formed from the n-akylsulfates. For exarnple,we note that PEO 10k

and 20k exert a relatively large and nearly equal stabilizing effect on SDeS

micelles. By contrast, the stability of sodium n-decylphosphate micelles

(Zo = 1) is hardly affected by PEO 10k whereas the stabilizing effect of PEO

20k is 50 9% smaller than that for the sulfate.

We have considered the possibility that the different behavior of

n-alkylphosphates and -sulfates is caused by hydrogen-bonding interactions

between phosphate headgroups224, disfavoring penetration of polymer segments

beyond the micellar surface. If this being the case, then di-sodium

n-decylphosphate micelles with Zo = 2 are expected to interact more strongly

with PEO because at this structural charge inter-headgroup hydrogen bonding

will be reduced. In addition, the enhanced inter-headgroup repulsions will cause a decrease in aggregation number208 and an increased hydrophobic

core-water contact. ~ a ~ a r a j a n ~ ' suggested that the latter factor is expected

to enhance polymer binding. However, in contrast to expectation, the

interaction with polymer at ZO = 2 is weaker than that at lower Zo values.

Particularly for PEO 10k and 20k, but also for PVME, this trend is clear. One

might object that in the A G : ~ ~ ~ , - AGO analysis several simplifications mic

have been incorporated, such as assuming that the activity of the surfactant

in the monomeric state equals the concentration, and leaving counterion

binding out of consideration. Nevertheless, the trend in interaction with PEO

is already quite clear from the complete absence of a reduction of the cmc at

Zo = 2.0. For PVME, the effect of n-decylphosphate at various Zo values on the

clouding behavior of the polymer strengthens the conclusions on micelle

stabilization (section 5.3). Probably, the unexpected decrease in interaction

tendency relates to the strong hydration of the phosphate headgroup at Zo = 2.

It is likely, that this hydration will hamper the presence of the polymer in

the headgroup region because overlap of the hydration spheres of the

headgroups and the polymer requires too much free energy.

It is obvious from the data in Table 5.3 that the hydrophobic polymers

PPO and PVME exert a stronger influence on the micellization process than PEO.

The difference in hydrophobicity between both types of polymers is illustrated

by the fact that PPO and PVME are soluble in apolar solvents whereas PEO is

not. We contend that the larger value of A G : ~ ~ - ~ , - A G O , for PPO and PVME mlc

largely stems from the favorable free energy change for these polymers upon

formation of polymer-bound micelles.

As noted earlier, for SDS and SDeS the change in molecular weight of PEO

from 10,000 to 20,000 has no effect on polymer-micelle interaction. This is

consistent with previous results3, which indicate that PEO/SDS interactions

become independent of molecular weight of PEO above a value of about 4,000. By

contrast, the interaction of PEO 20k with n-decylphosphate micelles is

slightly stronger than that for PEO 10k. Apparently, the minimum molecular

weight of PEO necessary to render PEO n-decylphosphate interactions

independent of molecular weight is higher than that for S D S ~ " ~ .

5.3 Clouding behavior of PVME

To support our conclusions regarding the decrease in interaction tendency

with increasing charge for n-decylphosphate, we have studied the clouding

behavior of PVME in the presence of this surfactant. For the method used to

obtain clouding temperatures the reader is referred to section 4.4. The

results for the clouding temperature of PVME in the presence of

n-decylphosphate at Zo = 1.1, 1.5, and 2.0 are presented in Figure 5.4. Should

the extent of polymer-micelle interaction be comparable at the various

structural charges, one would expect the surfactant with Zo = 2 to induce the

greatest increase in clouding temperature, since electrostatic interaction

between the micelles will be most pronounced at the highest charge of the

micelles. The curves in Figure 5.4 reveal that, in contrast to expectation,

the increase in clouding temperature becomes less pronounced at higher

structural charge. This indicates at frst sight that probably the interaction

occurs to a lower extent at higher charge. However, if above cmc all P

surfactant molecules bind to the polymer until the saturation concentration is

reached, one would still expect the initial slope of the curve obtained at

Zo = 2 to be the largest, even if (cmt-cmc ) would be smaller. It must be P

30 I I I I

0 50 100 150 200

[surfactant] , mM

Figure 5.4 Clouding temperatures of PVME in the presence of n-decylphosphate at Zo = 1.1 (o), Zo = 1.5 (a), and Zo = 2.0 (A),

pointed out that above the cmc the monomer concentration (and activity) will P

increase upon increasing the total surfactant concentration until the

concentration is reached at which the formation of normal micelles concurs

with the formation of polymer-bound micelles71. Therefore, one anticipates a smaller influence on the clouding temperature at higher Zo, if the rise in

monomer concentration above the cmc is steeper, due to a decrease in micelle P

stabilization upon binding to the polymer.

This explanation is in accord with that deduced from the cmc data.

However, it should be mentioned that, due to the higher cmc at ZO = 2, the P

ionic strength of the solution will also be higher, compared to that at Zo = 1.1 or 1.5. At higher ionic strength the electrostatic repulsion between

polymer-bound micelles will be slightly reduced and thus the effect on the clouding temperatures will be less. Nevertheless the slope of the clouding

temperature vs. concentration plot diminishes upon going from Zo = 1.1 to

Zo = 1.5, whereas the cmc's do not differ much. This indicates that the decrease in polymer-micelle interaction is the main factor.

5.4 3 1 ~ - ~ ~ ~ investigations

5.4.1 Introduction

NMR spectroscopy has become a popular technique to investigate surfactant

systems2252z. However, for the study of polymer-micelle interaction it is not often used. It is true that ~ a b a n e ' s ~ ~ "c- and 'H-NMR study of the

2 -

PEOISDS system had a great impact on the development of the model for polymer-micelle interaction. Nevertheless, NMR studies for other polymers than

PEO, such as PVME, PPO, HPC, and PVA, are hampered by the occurrence of broad

polymer resonances. The phosphate surfactants seemed to present a good

opportunity to circumvent these problems by studying the 3 1 ~

There are, according to ~ i n d m a n * ~ , two main disadvantages in the use of NMR spectroscopy for the study of surfactant systems: (i) the low sensitivity and

(ii) the fact that the primarily studied system is the nuclear spin system, so that the chemically relevant information is obtained through indirect

mechanisms. The first problem iS not of major importance for our system of

n-decylphosphate (Zo = 1.1) micelles and PVME, since the concentration ranges were such that adequate spectra could be obtained within an acceptable time

span, using a Fourier Transform spectrometer. The second problem makes it

difficult to guess within any certainty what to expect from an NMR "experiment.

For instance, what is the effect of the presence of a polymer in the headgroup

region of the micelle on the 3 1 ~ resonance of the headgroup? Before discussing

this matter, a brief overview of the relevant aspects of 3 1 ~ - ~ ~ ~ spectroscopy

will be presented. The influence of charge on the chemical shift of the 3 1 ~ resonance in a

phosphate group is small (4 ppm downfield on going from the mono- to the

di-anion) and depends mainly on the 0-P-0 bond angle and not on the charge

directly (there is no shift upon going from the free acid to the 219 mono-anion) . The chemical shift of the 3 1 ~ resonance in phosphate esters is

modestly dependent on solvent and temperature. The latter dependence has a

stereo-electronic origin219.

hac chat^^^^ and coworkers report an increase in the 3 1 ~ chemical shift of

pyridinium n-octylhydrogenphosphate upon micellization. This has been

attributed to the position and orientation of the pyridinium groups at the

micellar surface, which lead to shielding of the 3 1 ~ nucleus. Chachaty 228,229

does not report such shifts for n-octylphosphate (Zo = 1) with inorganic

counterions.

The three-bond coupling constant, '1, or 3 ~ p m in the case of

n-decylphosphate depends on the dihedral angle, and Karplus-like curves have 3 been established219. OkabayashiUO observed from JpOC13C coupling constants

of mono-alkyl and di-alkylphosphates that upon micellization the percentage of

trans conformations increases, although the population of the trans form is

already rather high in the monomeric state. Chevalier and chachaty207 3 concluded on the basis of the J- and 3~pmH coupling constants for

n-octylphosphate that the changes upon micellization are rather small,

indicating that the geometry of the polar headgroup is not significantly

affected by micellization.

A very useful NMR property of 3 1 ~ for the study of surfactants is the

longitudinal relaxation time of the nucleus m228s229. For instance, these

longitudinal relaxation times (TI) revealed a sphere-to-rod transition of 13 n-octylphosphate, which was not apparent from density or C chemical shift

207 measurements . Furthermore, T1 measurements may be used for self-diffusion 227.229 studies, and provide order parameters . For the determination of order

parameters, Chachaty 229,23 1 has also used paramagnetic ions to influence the

relaxation rate.

Most studies have been focused on the NMR properties of n-octylphosphate

(Z, = l), but, in order to obtain significant polymer-micelle interaction, we

used n-decylphosphate at Zo = 1.1 and 1.5. The influence of the binding of

PVME on the chemical shift. 'JmH and Tl of the "P nucleus in the micellized

state has been determined and the results will be discussed in the next

section.

5.4.2 3 1 ~ - ~ ~ ~ study of n-decylphusphatelPVME

No significant change in 3 1 'J- or the P chemical shift of

n-decylphosphate at Zo = 1.1 and 1.5 are found upon addition of 0.5 g . d ~ - '

PVME, either below or above the cmc. In view of the small changes in these

properties upon micellization anyway (section 5.4.1), it is not surprising

that the slight difference in micellar aggregates, e.g. normal micelles vs. polymer-bound micelles, produces no appreciable effects.

The T, relaxation rates of the "P nucleus of n-decylphosphate at

Zo = 1.1 have been measured as a function of the surfactant concentration in D,O/H,O (1:3 vlv) in the absence and presence of 1 g .d~ ' l PVME at Larmor

frequencies of 120 MHz and 80 MHz respectively (Figures 5.5 and 5.6).

Figure 5.5 The longitudinal relaxation rate, T~-', of the "P nucleus of n-decylphosphate (Zo = 1.1) as a function of the surfactant

concentration: (o) in H 2 0 P 2 0 (3: 1 vlv), and (A) in H , 0 P 2 0 (3: 1

V/V) containing 0.5 g . d ~ - ' of PVME. Data obtained at a Larmor

frequency of 120 MHz.

3 1 Figure 5.6 The longitudinal relaxation rate, T ~ ' ~ , of the P nucleus of n-decylphosphate (Zo = 1.1) as a function of the surfactant

concentration: (a) in H20/D,0 (3:l v/v), and (A) in H20/D20 (3:l

V/V) containing 0.5 g.d~- ' of PVME. Data obtained at a Lannor

frequency of 80 MHz.

Extrapolation to zero of the linear portion below llcmc in the plots shown in

Figures 5.5 and 5.6. yields ~;'(mic), the relaxation rate in the micellized

state. Above llcmc, the relaxation rate of the monomers, ~~' '(mon), is

obtained. The results are listed in Table 5.4, together with some results of

chachatyZm for n-octylphosphate (Zo = 1). Since our TI values lie in the same

range as those reported in the literature, which were measured in D20 , we

conclude that the presence of 75 vol. % H 2 0 does not greatly influence the TI,

as was anticipated. A large percentage of H,O was chosen in order to make the results comparable to the cmc measurements, which were performed in pure H,O . Although the shapes of the plots of the "P - Tl values vs. concentration are

different for the measurements in the absence and presence of PVME, due to the

3 1 Table 5.4 Longitudinal relaxation rates, Tl(rnic) , of the P nucleus of n-alkylphosphate surfactants, C H2n+10P0,HNa, in the micellar state.

n System Larmor freq., T1(mic), $' Reference

MHz

10 H,O/D,O~ 120 1.43f 0.02 This study

10 H, OD, O/PVME'" 120 1.- 0.013 This study

10 H,OD,O~ 80 1.32f 0.02 This study

10 H, OD, O/PVME~'~ 80 1.32f 0.02 This study

8 Dz" 202 1.96 207 8 Dz" 36 2.0 207

a) H,O/D,O ratio is 7525 v/v. b) Polymer concentration: 0.5 g ,d~ - l .

change in cmc, the Tl(mic) values are virtually the same. This is unexpected

in view of the factors that determine the relaxation rate. Thus, the

relaxation of the 3 1 ~ nucleus is mainly due to the chemical shift anisotropy (CSA) and the dipolar coupling (DIP) with neighboring protons (eq. 5 . 2 1 ~ ~ .

For an isotropic rotational motion, as is the case for monomeric surfactant,

the expressions for T~(csA)-~ and T , (Dl~) - la re~~:

T,(DIP)-' = 0.1 $ 'y: h2 Z r-6 (J(wH - "4 + 6J(wH + WJ + 3 J ( 9 ) (5.4) I P-H.

in which y is the gyromagnetic ratio, Ho is the magnetic field strength, 6 ,

6Y, and 6 are the principal values of the chemical shift tensor, J(o) is the

spectral density, w is the Larmor frequency, z is the rotational correlation

time, and rp-Hi is the internuclear distance. In the micellar state slow

tumbling (several ns) of the m i c e l l e ~ ~ ~ ~ is assumed with an isotropic

reorientation of the molecule about a preferred axis, which is that of the

fully extended hydrocarbon chain. This makes the expressions for the

correlation time and T~(CSA)-' much more complicated2~. However, T~(DIP)-'

may be estimated using eq. 5.4 and the expression for zo given by Chevalier

and chachatym7. We calculate zo = 1.64 x 10.~ s for n-decylphosphate, which

is comparable to the value of 1.25 x s for n-octylphosphate, calculated

by the above authors. The correlation time is a sum of two terms, which are

proportional to R3 and R2 respectively, in which R is the radius of the

micelle. This correlation time is an important factor in the expression for

T~(DIP)-'. Since the polymer-bound micelles are smaller, implying a smaller

radius. and, therefore. a lower b T~(DIP)" is expected to be influenced. It

may also be argued that tumbling will be slower because the polymer-bound

micelles will be restricted in their mobility. Unfortunately, the value of zo for the polymer-bound micelles is not known.

At 120 MHz, the contribution from dipolar relaxation for the unperturbed

micelles of n-decylphosphate is calculated to be 0.15 s-'. Thus T~(CSA)-' may

now be obtained by substracting T~(DIP)-~ from TI-', which yields s value of

0.55 s-I. This implies that, at 120 MHz, the relative contribution of the

chemical shift anisotropy is large, almost 80 %. The chemical shift anisotropy

is less sensitive to z At lower field strength the relative contribution of 0'

T~(DIP)-' increases. Therefore, we have also measured T,(mic)-' at 80 MHz. An

attempt to measure this quantity at 36 MHz failed, due to the decrease in

signal-to-noise ratio. At 80 MHz T~(DIP)-' is estimated to be 0.8 s-'. Since

the experimental value of Tl(mic)" is 0.76 s-', this must be an

overestimation, but it indicates that at low Larmor frequency the magnitude of

~ ~ ( r n i c ) " is dominated by T](DIP)-'. Although the relative contribution of

T](DIP)-' is large and this dipolar relaxation is strongly dependent on 7,.

still no difference in T1(mic)-' for the measurements in the absence and

presence of 1 g .d~- ' PVME are revealed. One must conclude that the changes in

aggregate morphology upon binding of the micelles onto the polymer are not

reflected in the NMR properties of the 3 ' ~ nucleus.

5.5 Preliminary experiments on the effect of PVME on sodium didodecylphosphate vesicles

The interaction of vesicles with polymers has been reported in the

literatureB3. These studies focus on PEO as a polymer that dehydrates the

phospholipid headgroups. Often very high polymer concentrations (up to

30 wt. %) were used. The fusion of vesicles may be either inhibited or induced

by the presence of PEO, depending on the vesicle system and the polymer 234-236 concentration and molecular weight .

In this section some preliminary observations on the interaction of PVME

with vesicles formed from the synthetic amphiphile sodium didodecylphosphate

( D D P ) ~ ~ ~ will be described. The vesicles were prepared in 5 mM Hepesl 5mM sodium acetate buffer (pH 7.4) by the ethanol injection method238 at 55 OC, in

the absence of polymer, because of serious clouding of PVME at that

temperature. The elevated temperature is necessary, because the vesicles

should be prepared above the gel to liquid-crystalline phase transition, which occurs around 29 O P 3 ' . After cooling to room temperature, the vesicle

solution was divided into two portions and diluted with either buffer solution

or PVME in buffer solution, to a final concentration of PVME of 0.5 g d ~ ' ' .

Samples for electron microscopy were prepared directly after mixing and

after an incubation time of 45 min. Samples were also taken after heating the

PVME/vesicle solution quickly to 55 OC. Uranyl acetate as well as ammonium

molybdate were used as staining compounds. In all cases, larger and more

sausage-shaped aggregates were observed in the PVh4l3/vesicle solution, besides

the normal spherical vesicles, which are also found in the absence of PVME.

The length of those sausage-like vesicles was 400 to 600 nm and the diameter

was around 100 nm. The vesicles in the absence of PVME were 50 to 60 nm in

diameter. Unstained and hence 'uncontaminated' samples prepared from a new

vesicle preparation showed qualitatively the same differences. The vesicles

in the absence of PVME had diameters between 80 and 200 nm, whereas those in

the presence of PVME were sausage-like and had a diameter of about 330 nm and a length of about 1500 nm. Probably, some sort of aggregation or fusion of DDP

vesicles is induced by PVME.

The clouding behavior of PVh4E in the presence of ca. 6 mM DDP vesicles is

very peculiar. When a vesicle/PVh4E (0.5 g . d ~ l ) solution is slowly heated from

room temperature to 55 OC, clouding occurs at 36 OC, quite near the clouding temperature of 33 OC of PVME in the same buffer solution in the absence of vesicles. However, if the vesicle1PVME solution is quickly heated to 55 OC, it

remains clear at that temperature for several hours. If the clear solution at 55 OC is quickly cooled (in ice) the solution stays clear, whereas slow

cooling results in clouding, until at low temperature the solution clears

again.

It seems that during a slow (few minutes) transition of the vesicles from

the gel state, in which the molecules are quite rigidly packed, to the

liquid-crystalline state, the type of binding or location of PVME in or at the

vesicles is significantly altered. This does not occur when intermediate

stages between the gel and liquid-crystalline state are avoided. Obviously,

more work is needed to clarify what exactly is happening.

5.6 Experimental section

Materials. Mono-n-octylphosphoric acid (mp 31.5 - 32.5 OC, lit.209 31.5 OC)

and mono-n-decylphosphoric acid (mp 47.7 - 47.9 OC, lit? 48.0 OC) were prepared according to the procedure of Nelson and ~ 0 ~ ~ ' ' . A 0.5 mol aliquot of

the appropriate alcohol was dissolved in 200 ml of benzene. Part of the

benzene was evaporated as an azeotrope with water, using a rotatory evaporator, to remove traces of water from the alcohol until a final volume of

about 115 ml was reached. To this solution 11 1 g of H,P,O, (0.6 mol) was added

and the slurry was mechanically stirred for 4 days at room temperature. The

viscous but clear solution was diluted with 650 ml of ether and washed with

450 ml of water. The ether layer was added dropwise to a solution obtained by adding 126 ml of NaOH (50 wt. % in H20) to 1 1 of water. The aqueous layer was acidified with concentrated HC1 to a pH of 0.5 and extracted with three

portions of 200 ml ether. The combined ether layers were washed with 50 ml of

1 N HC1, reduced by evaporation under vacuo to a volume of 150 ml and then

dried on MgSi3,. After evaporation under vacuo, the crude product (yield 50 %)

was obtained as a white solid, which was dried in vacuo over P20s and then

recrystallized at least two times from n-hexane.

The mono-sodium salts were obtained by neutralization of the acid with

0.95 eq. of sodium ethanolate in anhydrous ethanol. The white precipitate was filtered off, extensively washed with ether to remove any acid present, and

finally dried in vacuo over P,Os. The di-sodium salts were prepared

analogously using 2 eq. of sodium ethanolate. After filtration, the product

was washed with dilute ethanolate, anhydrous ethanol, and ether, respectively,

and the salt was finally dried in vacuo over P,O,. The allcylphosphates with Zo = 1.5 were prepared by mixing equimolar amounts of the mono- and di-sodium

salts.

Sodium n-decylsulfate (Merck) was used as received. The presence of trace amounts of n-decylalcohol cannot be excluded67. Didodecylphosphoric acid (mp

59.1 - 60.2 OC, lit.239 58 - 59 OC, Alpha Chemicals) was converted into the

sodium salt by addition of sodium ethanolate in ethanol, followed by removal of the ethanol by evaporation and drying in vacuo over P20s. PPO (weight-averaged mw 1,000, Aldrich) was used as received. The purification of PVME and PEO has been described in sections 3.5 and 2.6, respectively. The

water used in all experiments was demineralized and distilled twice in an

all-quartz distillation unit.

Cmc measurements. Conductivity measurements were performed as described in

section 3.5. Cmc determinations based on pH measurements were performed as

follows. A surfactant stock solution (up to 3 ml, in water or in an aqueous

polymer solution) was continuously injected into 5 ml of a thermostated,

stirred polymer solution (same concentration of polymer) or into water under a

constant flow of nitrogen. The pH was recorded continuously using a glass electroae ~ 0 ~ e C t e d to a Coming 130 pH meter. An abrupt increase of the pH was clearly observed in all cases and the cmc was taken as the intersection

point of the tangents drawn before and after the transition. These cmc values were reproducible to within 3 7%.

Cloud point measurements. See section 4.5.

3 1 NMR measurements. P spectra were recorded at Larmor frequencies of 120 mHz and 80 MHz on a Varian VXR-300 and Nicolet NT-200 spectrometer, respectively,

at a temperature of 25 OC. The TI values were measured using the inversion

recovery method, under 'H decoupling. For the TI measurements samples under a

N, atmosphere were used. These were obtained by at least ten cycles of evacuation and N, pressure, while the solution in the NMR tube was emerged in

an ultrasonic bath.

Preparation of vesicles. DDP vesicles were prepared by injecting 80 pl of a solution of 10 mg of DDP in 100 pl of ethanol with a pre-heated microsyringe

into 1 rnl of 5 rnM Hepes15 mM sodium acetate buffer (pH 7.4), thermostated at

55 OC under vigorous stining. After cooling, 0.5 rnl of the vesicle solution

was diluted with 1 ml of buffer solution, and 0.5 ml was diluted with 1 ml of

a buffered PVME solution to a final PVME concentration of 0.5 g .d~- l .

Electron microscopy. For negative staining with 1 % (wlv) uranyl acetate or

1 90 (w/v) ammonium molybdate, the two droplet method was used. Carbon-coated

Formvar grids, pretreated by glow discharge in air, were used as supporting

matrix. The samples were examined using a Philips EM300 electron microscope,

operating at 80 kV. All electron micrographs were recorded by Mr. Tino

Fonteijn, to whom I am much indebted.

CHAPTER 6

THE EFFECT OF HEADGROUP CHARGE ON POLYMER-MICELLE INTERACTION:

SPHERICAL AND RODLIKE MICELLES FORMED FROM 2-ALKYLMALONATE SALTS

6.1 Introduction

We have already mentioned (Chapter 5) that the number of articles on

mono-n-alkylphosphates is scanty compared to those on SDS. The amount of

published r e ~ e a r c h ~ ' ~ ~ ~ ~ ~ ' on 2-alkylmalonates, is even less. In principle,

the alkylmalonates are valuable surfactants for studies of the effect of

charge variation on polymer-micelle interaction, since the (structural)

headgroup charge can be varied from Zo = 1 to Zo = 2.

COOH COO- / K a /

n - C n H , n + ~ C H F ~ - c , H ~ ~ + I c H \ \ coo- coo-

The pKa value of the (second) dissociation step is 5.69 for malonic acid in

watera2. The pK value of the 2-alkylmalonic acid will be slightly different

and depends on the aggregation state of the molecule (section 5.1). It seems

likely that both the mono-anionic and the di-anionic surfactant form micelles.

However, micellization has hitherto only been reported for the

di-anion 215,240,241 . To the best of our knowledge, there are no literature

data on micellization of the mono-anionic molecule. This lack of information

on the mono-anion of 2-alkylmalonates probably stems from the high Krafft

temperatures of the common alkali-metal salts (section 6.2).

shinoda215 has studied the cmc values of a homologous series of

di-potassium 2-akylmalonates (n = 8 - 18), and found that the free energy

change upon micellization per methylene group is 1.08 kT. This value equals

that for the fatty acid soaps. The decrease of the cmc due to the addition of

salt is twice as strong for the di-potassium 2-alkylmalonates, compared to

that of the corresponding fatty acid soaps215.

Vikingstad et al.240 found that the change in partial molal volume during

micelle formation for di-sodium 2-alkylmalonates (n = 8 - 12) is almost equal

to that of the corresponding sodium alkanoates. The change in compressibility

during micelle formation was found to be somewhat smaller for the

2-alkylmalonates than for the akylcarboxylates, indicating a small negative

contribution from the surfactant headgroup to this quantity"0. Kvamme and

Hoiland et a ~ . * ~ ' observed that the partial molar volume of some primary

alcohols (I-n-propanol to 1-n-heptanol) in micellar solutions of sodium

decanoate and di-sodium dodecylmalonate are the same. Thus, it is quite clear

from the available data, that the double-charged malonate surfactants behave

similarly to the usual mono-charged surfactants, such as the alkanoates.

In this chapter, the aggregation behavior of the mono- and di-salts of

2-alkylmalonic acids (n = 10, 12), in the absence and presence of polymers,

will be discussed. In case of the mono-salts, the tetramethylammonium salts

(me,') have been used to avoid the high Krafft temperatures, which we

encountered for the alkali-metal salts. Interestingly, viscoelastic properties

are observed for the mono-NM~,' salt of 2-dodecylmalonic acid, even at very

low concentrations (1.5 mM). This curious behavior has been studied using 'H NMR spectroscopy. The visually observed viscoelastic properties disappear upon

addition of PEO or PVME. Finally, the effect of the surfactants on the cloud

point of PVME will be presented.

6.2 The aggregation behavior of mono- and di-salts of 2-alkylrnalonic acids in

aqueous solution

Krafft temperatures for the mono-potassium, -cesium, and

-tetramethylammonium salts of 2-alkylmalonic acids (n = 10, 12) are listed in

Table 6.1. The Krafft temperature is the temperature at which the solubility

of a surfactant equals the cmc. The decrease in Krafft temperature on going

from K+ to Cs+ is in accord with the decrease in the cmc of fatty acid soaps

Table 6.1 Krafft temperatures for mono salts of 2-alkylmalonic acids.

surfac tant Krafft temperature, OC

and alkylsulfates in the sequence ~ i + to CS+ 243, but reflects also that the

lattice energy of the solid surfactant is diminished in this sequence. For the + mono-NMe, salts the Krafft temperature is reduced to a temperature below

20 OC, most likely as a result of the reduced cation-anion Coulomb

interactions in the solid, but also because the slightly hydrophobic NM~,' ion 244245 will bind more strongly to anionic micelles and, hence, lower the cmc .

Cmc values for a series of mono- and di-salts of 2-alkylmalonic acids and

for m e , + myristate are summarized in Table 6.2. Most cmc values were

measured using the pinacyanol chloride absorption methodz1', because this

method gives good results also in the presence of polymers. Conductivity

measurements did not provide accurate cmc's (except for the di-potassium

salts) in the presence of polymers, since no clear breaks in the conductivity

vs. concentration plots were obtained. This may be explained in terms of the

smaller aggregation numbers of the polymer-bound micelles3, the concomitant

lower degree of counterion binding and the reduced cooperativity of surfactant

aggregation (see also section 5.2). Cmc measurements based on the

concentration dependence of the pH, which proved to be a useful alternative in

the case of alkyl phosphates (section 5.2). were prohibited by precipitation

of the mono-potassium salt at the liquid junction.

The di-sodium and di-potassium salts of Zalkylmalonic acids are known to + form rnicelles of the spheroid The behavior of the di-We, salts

is quite comparable, for example, the cmc's of the di-K and di-NM~,' salts of

Table 6.2 Cmc values (mM) for 2-alkylmalonate micelles in water and in the

presence of polymersa.

surfac tant Hzo PEO 1Ok PEO 20k PVME

a) Polymer concentration: 0.5 g.d~-', temperature: 25 OC. b) Pinacyanol

chloride absorption method. c) No accurate cmc values could be obtained.

d) Conductometric method. e) pH method (section 5.6). f) From reference 215.

2-dodecylmalonic acid are similar (Table 6.2).

However, the aggregation of the mono-NM~,+ salt of 2-dodecylmalonic acid

is completely different. In this case viscoelastic solutions are formed,

indicative of the presence of flexible, rodlike aggregates. The

viscoelasticity can be observed visually as the recoil of air bubbles when a

swirling motion of the solution is abruptly stopped132b. This viscoelasticity

is observed in the concentration range from the cmc (ca. 1.4 rnM) up to

approximately 13 rnM. In separate experiments it was shown that addition of up

to 0.2 equivalent of the di-NM~,' salt to the mono-me,' salt of

2-dodecylmalonic acid has only minor effects on the viscoelasticity and its

dependence on surfactant concentration . This implies that the viscoelasticity

is not critically dependent on the state of protonation of the surfactant in

the rnicelle. At present we have no unequivocal explanation for the

disappearance of the viscoelasticity at higher concentrations. It may result

either from a reduction of the length of the rods or from a decrease of the

structural relaxation time of the networkl3lS6. Both effects may be

associated with a concentration dependence of the degree of protonation of the

malonate in the surfactant assembly.

The formation of rodlike rnicelles is confirmed by 'H NMR spectral data.

In the same concentration range in which viscoelasticity is observed, the

proton resonances of the alkyl chain are severely broadened (Figure 6.1). This

is characteristic for rodlike rnicelles and the effect has been attributed to

increased T values142.

Although mono-NM~,+ 2-dodecylmalonate dissolves spontaneously in water

at room temperature fonning clear solutions, precipitation occurs upon

standing overnight, particularly in the concentration range for viscoelastic

behavior. Precipitation is greatly accelerated by stirring of the solutions,

indicating a shear-promoted growth or aggregation of the rods. Indeed, it has

been suggested that the rodlike rnicelles may be embryos of the hydrated

I - 3 -1 [surfactant , l o rnol.kg

1 Figure 6.1 H NMR line width at half height of the alkyl chain methylene

protons of the mono-NM~,' salt of 2-dodecylmalonic acid at 30 OC, (o) in H,O; (0) in H,O containing 0.25 g . d ~ - ' of PEO 10k.

crystalline phase, formed through secondary aggregation of

micelles 133.137247 . Since only about half of the carboxylate groups are

ionized, intermicellar interactions may certainly be thermodynamically

favorable249. Nevertheless, it was quite feasible to measure reproducible cmc

values as long as vigorous stirring was avoided.

It is interesting to note the structural resemblance (Figure 6.2) between

mono-NM~,+ 2-dodecylmalonate and cetyltrimethylammonium salicylate (CTASal) 135.138.142.146 , the arche-type of a swfactant forming rodlike

micelles (see also section 3.1.3). Cetyltrimethylamrnonium salts with m- or

p-hydroxybenzoates counterions do not form rodlike micelles, indicating that

the relative orientation of the OH and CO; groups is of decisive importance'38

(section 3.1.3). It is perhaps surprising that a surfactant molecule with a

large headgroup like mono-me,+ 2-dodecylmalonate does form rods in view of

Israelachvili's theory220, which relates the morphology of the aggregate to

the shape of the surfactant molecule. However, the headgroups in aggregates + formed from mono-NMe, 2-dodecylmalonate are probably drawn together by

inter-headgroup hydrogen-bonding, thereby producing a less wedge-shaped

surfactant. Similar interaction between -COO- and -COOH headgroups has also

been reported for fatty acid Interheadgroup association of

n-dodecyldimethylarnine oxide at half-ionization is also known to result in the

formation of large aggregates 120,183,190249 (section 4.1.2). The cmc of m e , ' myristate, a surfactant which lacks the second carboxylic

function, has been measured for comparison. This cmc (4.8 rnM) is definitely

larger than that for mono-me,+ 2-dodecylmalonate, and this difference

illustrates the stabilizing effect of the additional COOH moiety in the

Figure 6.2 Structural resemblance between CTASal and mono-NMe, + dodecylmalonate.

mono-anionic malonate, which is about the same as that of two methylene groups (compare with the cmc of mono-NM~,+ 2-decylmalonate). It is likely that the

stabilizing effect benefits from interheadgroup hydrogen bonding since

introduction of a COOH group in the R group of CH,(CH,),,N+M~,R leads to a

smaller decrease of the cmcZO. In the latter type of surfactants C0,H-C0,-

hydrogen bonding will certainly be less efficient. Surprisingly,

~ o z ~ c k a - ~ o s z a k ~ ~ ' found that the influence of the ester moiety, -COO-, in

(cH,),N+CH,COOC HheICl- on the omc also equals that of two methylene

groups. Finally we note that neither mono-me,+ 2-decylmalonate nor NMe, +

myristate are able to form viscoelastic solutions. The difference in aptitude to form rods between the mono-NM~,' 2-decyl and 2-dodecylmalonate illustrates

the dependence on alkyl chain length222. An increasing tendency to form rods

at increasing chain length was also found by ~ o f f m a n n ~ ~ for a series of n-allcyltrimethylarnrnonium salicylates.

The relation between viscoelastic behavior and chain length is peculiar

in view of the theory of Israelachvili, since the shape of the surfactant does

not change (see also section 5.2). The formation of aggregates of smaller size in case of the shorter chain surfactants is preferred for entropic reasons.

The smaller aggregation number of spherical micelles formed from shorter

surfactants causes the difference in behavior. This is expressed in the

'ladder model' of h4isselZ3.

The absence of viscoelastic behavior of NM~,+ myristate is difficult to understand in terms of Israelachvili's theory2u). The smaller headgroup of the

myristate compared to that of mono-NM~,+ 2-dodecylmalonate wodd seem to

make rod formation more favorable. Several reasons may lie at the origin of

the difference in aggregation behavior of this malonate and myristate: (i) The

aggregation number of the spherical micelles formed from m e , + myristate is

lower than for the 2-dodecylmalonate. According to the 'ladder m~del'"~, this

would prevent rod formation. (ii) The COOH and COO- groups together serve as a kind of bidentate ligand for m e , + , which is not possible in the myristate.

(iii) Interheadgroup interactions will be much less pronounced for the

myristate than for the mono-NM~,+ 2-dodecylmalonate.

6.3 Aggregation of mono- and di-salts of 2-alkylmalonic acids in polymer

solutions

Cmc values in the absence and presence of PEO 10k, PEO 20k, and PVME are

listed in Table 6.2 for several mono- and di-salts of 2-akylmalonic acids and

for m e , + myristate. For the double charged surfactants the cmc values are

considerably reduced in the presence of PEO and PVME, which points to

polymer-micelle interaction. As described in Chapters 4 and 5, the

stabilization of the rnicelles by interaction with polymers may be quantified

by calculating AGO. - AGO. (eq. 5.1, section 5.2). This equation is only mm-pol m1c

valid, if the polymer has no effect on the activity of the surfactant

monomers. For di-NM~,+ 2-dodecylmalonatePE0 A G ~ ~ - ~ , - AG:~~ ranges from

-0.5 kl.mol-' to -1.6 kl.mol-' depending on the molecular weight of PEO. This

dependence on PEO molecular weight was also found for the phosphate

surfactants (section 5.2). For sodium dodecylsulfate/PEO, A G ~ , ~ , - A G O . m ~ : is

-1.0 k.J.mo1-' both for PEO 10k6* and PEO 20k. The alkyl chain length, which is

known to affect polymer-rnicelle interaction significantly3, is comparable for

both surfactants, since there is only one additional methine moiety in + di-NMe, 2-dodecylmalonate. Thus we conclude that rnicelles formed from

d i -me ,+ 2-dodecylmalonate interact as strongly with PEO as SDS rnicelles do,

despite the double charged and bulky malonate headgroup. This result is

unexpected in view of ~ a ~ a r a j a n ' s ~ l and ~uckenstein's" theories, which hinge

on the idea that a bulky headgroup hampers the binding of a polymer at the

micellar surface (section 3.1.2). However, the surface of micelles formed from

d i -me ,+ 2-dodecylmalonate has structurally much in common with

poly(carboxy1ates) like poly(methacry1ic acid) (PMAA) and poly(acry1ic acid)

(PAA), which are known 103-105 to interact strongly with PEO. These

interactions presumably involve (cooperative) hydrogen-bonding, but

hydrophobic interactions may also play a role, since PMAA has a much greater

complexation tendency than PAA"~- '~~. The cooperative nature of the

complexation is favorable for the binding process, because the loss in entropy

per bound molecule or monomeric unit will be less. Nonionic surfactants of the

poly(oxyethy1ene) ether type also interact with poly(carboxy1ic acidsf9. The

carboxylate groups in rnicelles formed from di-K and d i -me ,+ 2-dodecylmalonate

should be protonated to some extent since electrostatic headgroup repulsions and the low local polarity will tend to increase their pKa.

For micelles formed from d i - ~ ~ e , ' Zdodecylmalonate and NM~,' myristate

the interaction with PVME is much stronger than with PEO (Table 6.2). This is

in line with previous results3, which revealed that polymer-micelle

interaction becomes stronger with increasing hydrophobicity of the polymer

(see also sections 2.5, 3.2, 4.2, 4.3, and 5.2.). Two effects may account for

this observation, (i) the transfer of the more hydrophobic polymer segments to

the micellar surface will be more favorable and (ii) the micellar core will be more effectively shielded from water by the more hydrophobic polymer.

The binding of micelles formed from m e , + myristate to PEO and PVME is

weaker than that of the corresponding 2-dodecylmalonate, which suggests the importance of specific interactions, most likely hydrogen bonding, between the

partially protonated carboxylate groups and the ether oxygen atoms of the

polymers.

Interestingly, the cmc of mono-NM~,' Zdodecylrnalonate is increased in the presence of PVME. This suggests a stabilization of the surfactant monomer

by the polymer. Clouding point measurements support this conclusion (section

6.4). An alternative explanation in terms of a destabilization of the

polymer-bound micelles is unlikely, since in that case the system will

preferentially form unperturbed rnicelles. The most important observation,

however, is the complete disappearance of the viscoelasticity of the aqueous + micellar solutions of mono-NMe, 2-dodecylmalonate in the presence of PVME or

PEO. In accord with the results described in Chapter 3, the disappearance of viscoelasticity is interpreted in term of a polymer-induced transition from rodlike to spherical micelles. This is supported by the observation that the 1 H NMR line broadening of the alkyl chain methylene protons is absent after

addition of the polymers (Figure 6.1). As in the case of CTATs and CT'ASal (Chapter 3), spherical polymer-bound micelles are formed in favor of rodlike

micelles, because the surface-to-volume ratio is higher for the spherical

micelles. A comparatively large surface area of the micelles will result in a

reduction of both headgroup-headgroup and headgroup-adsorbed polymer

repulsion. The extra core-water contact of the spherical micelles is

stabilized through binding of the polymer. Interestingly, ~ a ~ a r a j a n ~ " has

theoretically predicted that rodlike micelles of an anionic and a nonionic

surfactant will become ellipsoid upon interaction with polymer.

The above results have definitely confirmed interaction between micelles

formed from mono- and di-NM~,' 2-dodecylmalonates and the polymers PEO and

PVME. In view of the effect of the polymers on the respective cmc's, it is

tempting to conclude that the double-charged swfactant micelle exhibits the

stronger polymer-micelle interaction. This result would contrast with the

results for n-alkyl phosphate surfactants where the polymer-micelle

interaction becomes weaker upon increasing the structural charge of the

surfactant (Chapter 5). However, we emphasize that direct comparison of the

changes of the cmc for the mono- and di-salts of the 2-n-dodecylmalonic acid

is tricky for at least two reasons. First, in the case of the mono salt, the

polymers also interact with the monomeric surfactant. Second, the morphology

of the initial surfactant aggregate is different: rodlike micelles for the

single-charged surfactant and spherical micelles for the double charge

surfactant. Although the rodlike micelles are transferred into spherical

micelles upon addition of the polymers, no cmc is known for the hypothetical

formation of unperturbed spherical micelles for this particular surfactant.

It may well be that no general rules can be formulated regarding the

effect of surfactant charge on the strength of polymer-rnicelle interaction. In the case of the not fully charged surfactants, there will be competition

between interheadgroup hydrogen-bonding and hydrogen-bonding interactions

between the headgroups and the polymer. Furthermore, hydrophobic interactions

and hydration shell overlap effects will play a role and the overall gain in

free energy upon polymer-micelle complexation will be a compromise between a

variety of not necessarily coupled interaction forces.

6.4 Clouding behavior of PVME

The binding of micelles formed from mono- and d i - ~ ~ e , ' 2-dodecyl-

malonate, and NM~,+ myristate to PVME is also apparent from the raise of the

clouding temperature of the polymer (Figure 6.3, see also sections 4.4 and

5.3). The presence of mono-NM~,+ 2-decylmalonate does not induce such a raise

Temp. 'C

0 5 10 15 20 2 5

- 3 [surfartant] .10 mot. kg"

Figure 6.3 Clouding temperatures of an aqueous solution of PVME (0.5 g .d~- ' )

in the presence of surfactants: (o) mono-me,+ decylmalonate; (0)

mono-me,+ dodecylmalonate; ( 0 ) di-NM~,+ dodecylmalonate; (A)

m e , ' myristate.

in clouding temperature of a PVME solution. By contrast, this surfactant

causes a decrease in clouding temperature, particularly below its cmc of

4.9 mM. Also for mono-NM~,+ 2-dodecylmalonate an initial decrease in clouding temperature is o b s e ~ e d until the cmc is reached.

This minimum in clouding temperature cannot be the result of the ionic strength of a surfactant solution below the cmc, because in that case, it

would certainly also occur in the presence of the di -me,+ 2-dodecylmalonate

and NM~,' myristate (and other surfactants, section 5.3). The minimum most

likely originates from preferential solubilization of the surfactant monomers in the polymer-rich Above the cmc of mono-NM~,+ 2-dodecylmalonate,

the PVME solution slowly regains its original clouding temperature upon

increasing the surfactant concentration. It is usually found that upon

increasing the concentration of a surfactant above its cmc value, the

concentration of free monomers gradually decreases95Y4. This is due to the

slowly increasing ionic strength of the solution, which preferentially

stabilizes the micellar form. This decrease in monomer concentration may

explain the eventual recovery of the clouding temperature to its original

value. Obviously, micelles formed from mono-NMe,' 2-decylmalonate do not bind to PVlbE, since otherwise a substantial increase of the clouding temperature

beyond that of the surfactant-free PVME solution, would have been found above

the cmc, just as is the case with mono-me,+ 2-dodecylmalonate. The effect of

surfactant chain length on polymer-micelle formation has been discussed in

section 5.2.

The conclusions regarding the effect of these surfactants on the clouding

behavior of PVME agree well with those based on the cmc values. The binding of

surfactant monomers to PVME in the case of m o n o - ~ e , + 2-alkylmalonate has not

been encountered before. Neither the phosphate surfactants (Chapter 5) nor the

arnine oxide surfactants (Chapter 4) bind to PVME below the cmc . P

6.5 Experimental section

Materials. Diethyl 2-alkylmalonates were prepared from diethyl malonate and

the corresponding alkyl bromide using a standard procedure"5. In a 500 ml three-necked flask, equipped with a sealed stirrer, 4.6 g of sodium (0.2 mol)

in small pieces is added ,at room temperature, to 100 ml of anhydrous ethanol.

When all sodium has reacted to form sodium ethanolate, 33 g of diethyl

malonate (0.2 rnol) was added dropwise. If necessary, extra ethanol was added

to dissolve all sodium diethyl malonate. Then 50 g of n-dodecyl bromide (0.2

mol) or 44.2 g of n-decyl bromide (0.2 mol) was added dropwise, and

subsequently the reaction mixture was refluxed for 0.5 hours. After cooling,

the solution was decanted and ethanol was evaporated under reduced pressure.

After stirring the residue with 80 rnl of water, the organic fraction is

extracted three times with ether. The combined ether layers were washed with a

small amount of water and dried on anhydrous MgSO,. After evaporation of the

ether under reduced pressure, the product was distilled

(bp 145 - 146 OC/ 0.05 mm Hg for the diethyl 2-dodecylmalonate;

bp 123 OCI 0.01 mm Hg for the diethyl2-decylmalonate). The yield was 64 % for

the dodecyl compound and 75 % for the decyl compound. Hydrolysis was camed

out as follows. To a cooled solution of the diethyl 2-alkylmalonate (0.1 mol)

in ethanol (250 mL) was added 250 rnL of 2N NaOH. The mixture was stirred

overnight, cooled to 5 OC and acidified (Ha) . Extraction with ether followed by removal of the ether in vacuo afforded the crude product. Cold

crystallization (room temperature to -50 OC) from acetone gave 2-decylmalonic

acid (mp 117 - 118 OC) and 2-dodecylmalonic acid (mp 121.5 - 122 OC, lit.215

121 - 121.5 OC) in 80 % yields. Myristic acid (mp 55.1 OC, lit.58 242, Ega)

was used as received. The mono- and di-salts of the 2-alkylmalonic acids were

obtained by addition of the appropriate amount of an alcoholic solution of the

corresponding hydroxide. The mono-tetramethylammonium salts were crystallized

in the cold (room temperature to -50 OC) from acetone. The di-cesium and

di-tetramethylammonium salts were crystallized from acetone-THF.

Mono-tetramethylammonium 2-decylmalonate: 'H NMR (CDCI,): 0.8 (t, 3H), 1.1 - 1.3 (br s, 16H), 1.75 (m, 2H), 2.8 (t, lH), 3.3 (s, 12H) ppm. 13C NMR (D20):

7.5, 16.3, 21.4, 23.2, 23.3, 23.4, 23.5, 23.7, 25.7, 48.5, 49 (t, J,, = 3.2 Hz), 170.0 ppm.

1 Mono-tetramethylammonium 2-dodecylmalonate: H NMR (D20): 0.66 (t, 3H),

1.0-1.2 (br s, 20H), 1.61 (m, 2H). 3.0 (s, 12H) ppm. 13c NMR (D20): 7.5, 16.4,

21.5, 23.3, 23.4, 23.5, 23.6, 23.7, 25.7, 42.6, 49.0 (t, J,, = 3.2 Hz), 169.9

PPm. Di-tetramethylammonium 2-dodecylmalonate: 'H NMR (D,O): 0.68 (t, 3H), 1.0-1.2

(br s, 20H), 1.5 (m, 2H), 2.85 (t, lH), 3.0 (s, 24H) ppm. 13C NMR (D20): 7.7,

16.3, 17.4, 21.9, 23.1, 23.4, 23.5, 24.4, 25.6, 49.1 (t, J,, = 3.2 Hz), 52.7,

173.0 ppm.

PEO 10k (weight-averaged mw 10,000, Fluka), PEO 20k (mw 20,000, Sigma), and

PVME (50% (wlw) aqueous solution, inherent viscosity 0.57, Aldrich) were

purified as described in sections 3.5 and 2.6 respectively. All solutions were

made up with deionized, double-distilled water.

Cmc measurements. Spectrophotometric measurements of the cmc were performed

by determining the absorption of pinacyanol chloride215 at 615 nm, 570 nm and

495 nm at a probe concentration of ca. M using a Perkin-Elmer h5 spectrophotometer. At the cmc the absorption at 615 nm and 570 nm increases

whereas the 495 nm absorption decreases. Conductivity measurements were

performed as described in section 3.5 and the pH measurements for the cmc

determination as described in section 5.6.

Krafft temperatures and clouding points. Krafft temperatures were determined

by recording the transmission at 400 nm of a vigorously stirred dispersion at

increasing temperatures, using a Perkin-Elmer h5 spectrophotometer. The Krafft

temperature is taken as the onset of the sudden increase in transmission of a

100 mM surfactant dispersion. The clouding point is taken as the temperature

at which the transmission at 400 nm is 50 %. At the lowest concentration of

the rnono-NM~,' salts of 2-decyl- and 2-dodecylmalonic acid and at the highest

concentration of NM~,' myristate, the clouding of PVME occurs over a large

(ca. 10 OC) :emperature range. This implies that a slight haziness may be

observed at temperatures below the clouding points reported in this work.

NMR measurements. Line widths at peak half-height (Avln ) were calculated

from 'H NMR spectra of the surfactant aggregate in D,O solutions The spectra

were recorded on a Bruker WH-90-DS spectrometer (90 MHz) operating in the FT mode at 30 OC.

CHAPTER 7

SDS-INDUCED ENHANCEMENT OF THE VISCOSITY AND VISCOELASTICITY OF AQUEOUS SOLUTIONS OF PEO

7.1 Introduction

The preceding chapters deal with surfactant/polymer combinations that

have hitherto not been studied. The incentive of those investigations was to

reveal the relation between headgroup structure and charge, and the propensity

of rnicelles for interaction with polymers. This chapter, in contrast, is

devoted to the most widely studied3 surfactant/polymer combination, namely sodium dodecylsulfate (SDS) and poly(ethy1ene oxide) (PEO), and the investigation is focussed on the rheological aspects of this system. The

system PEOISDS certainly has its advantages. Both compounds are easily

available and cheap, and PEO may be purchased in a large variety of molecular

weights. Furthermore, it is the pronounced hydrophilicity of PEO, which makes

interaction with micelles formed from SDS (or alkylphosphates, Chapter 5) so intriguing, since hydrophobic interaction seems to play such a crucial role in

the association. The binding of polymer segments onto the hydrophobic

core-water interface of the micelles favors the micellization process, but is

expected to be accompanied by an unfavorable transfer of the PEO segments from the aqueous phase to the surface of the micelle. Still, PEO-bound micelles of

SDS are formed at a lower cmc than for unperturbed micelles, thus the

stabilization of the micelle more than compensates for this unfavorable free

energy for transfer of polymer segments. The influence of PEO on micellar properties358'122, like aggregation

number M-66254, cmc3, counterion bindingM, and solubilizing power4s has

been studied in considerable detail. It is also known that above a molecular

weight of 4000, PEOISDS association is independent of molecular weight3. Furthermore, ~ a ~ a r a j a n ~ ' and ~uckenstein~' have both proposed quantitative

models for polymer-micelle association, that describe the case of PEOISDS

interaction. Surprisingly, the influence of SDS on the properties of the

polymer has remained rather unexplored, despite the wealth of research on PEO

Several authors have reported rheological studies on the influence of SDS

(among other surfactants) on the viscosity of aqueous solutions of pE0457,61.257258 or PVP~"~'~~. However nearly all these studies have

been performed using capillary (Ubbelohde) v i s ~ o m e t r y ~ ~ ~ ~ ' ~ ~ ~ ~ , which is

best suited for the measurement of the viscosity of fluids under Newtonian

flow. As a consequence little information has been obtained about the changes

in viscoelasticity, which is a property of non-Newtonian fluids'67 (see also

sections 3.3 and 3.4 for a brief introduction into the terminology of

rheology). There are two exceptions, as far as we are aware. One is a study by

~ a n c e - ~ o m e z ~ ~ ~ , who used stress relaxation after cessation of steady-state

flow to investigate the viscoelastic properties of PEO in aqueous solutions in

the presence of salt or surfactant. He found, depending on the molecular

weight fraction of the polymer, an increase or a decrease in the stress

relaxation (ar a shear rate of 5 s ) , upon addition of an alkyl

benzenesulfonate. The other exception is a study by Uhl and ~rud'homme~'~, who

observed an increase in viscosity and viscoelasticity of a PEO solution at

sufficiently high SDS concentrations, but no detailed analysis of the data was

presented.

In 1957 saito4 reported the increase in viscosity, measured by capillary

viscometry, of a PVP solution upon addition of surfactant. He proposed

adsorption of mutually repelling surfactant molecules, individually bound onto

the polymer chain, to explain the results. As discussed in Chapter 1, the

model has been modified and, instead, micelles are considered to bind to the

polymer5. An increase in the viscosity upon SDS addition has also been 437.61 observed with the use of capillary viscometry for PEO solutions .

Several authors have studied the influence of SDS on the rheology of

cellulose derivatives like methybellulose5' and ethyl(hydoxyethy1)-

cellulose77. In these cases the changes in viscosity are more complicated due

to polymer aggregation in aqueous solution.

The present chapter describes a study of the rheology of an aqueous

solution of 0.25 g d ~ - ' of PEO of high molecular weight (5 x lo6) at various

SDS concentrations using cone-and-plate rheometry. The polymer concentration

is chosen with care to be well below the overlap concentration at which PEO coils start to 'feel' each other (see also section 7.4). and to ensure also a

reasonabe concentration range for polymer-micelle interaction6'. Our equipment

allowed the measurement of not only the shear rate dependence of the viscosity

but also the viscoelasticity. This is a great advantage over capillary

viscometry. In addition to the known increase in viscosity upon SDS addition,

we find a concomitant increase in viscoelasticity. A power-law model proved

adequate to describe the shear rate dependence of the viscosity. Furthermore

the viscoelasticity data revealed a partial breakdown of the polymer-micelle

complexes above a critical shear stress, which was not apparent from the

corresponding viscosity data.

7.2 The influence of SDS on the viscosity of a PEO solution

The effect of SDS on the apparent viscosity (see section 3.3) of the

0.25 g . d ~ - ' aqueous solution of PEO (mw 5 x lo6) at two fixed shear rates is

depicted in Figure 7.1. The curves clearly show three distinct regions, 5759,61 similar to previous results obtained by using capillary viscometry ,

both at low (168.3 s-') or high (2689.4 s-') shear rate. In region 1. below

the critical concentration for formation of polymer-bound rnicelles

(cmc 5.4 d7) the viscosity changes only slightly upon addition of SDS. P

Besides polymer coils, only free surfactant ions are present in the solution.

Above the cmc in region 11, the viscosity increases considerably as P'

electrostatic repulsion between the anionic micelles bound to the polymer

causes the coils to expand. This increase of the viscosity can also be

observed visually when swirling the solutions gently. In region 111, above

20 rnM SDS the viscosity levels off and a small decrease is found upon further

addition of SDS. This concentration corresponds to the saturation

concentration (cat), at which the maximum number of SDS micelles is bound to

the polymer. This concentration is in excellent agreement with cat determined

from conductivity measurements by witte6"', who found that c is 40 mh4 for Bbt

SDS in the presence of 0.5 g .d~- ' of PEO. Further addition of SDS results in

Figure 7.1 Apparent viscosity as a function of the SDS concentration at

different shear rates: (u) 168.3 s-'; (0) 2689.4 s-'.

the formation of free micelles. The slight decrease in viscosity was explained

by Fran~ois et al.57 in terms of a contraction of the extended coils, due to a decrease in electrostatic repulsion between the micelles as a result of the higher ionic strength.

The apparent viscosity of the PEO solution at a fixed SDS concentration

drops with increasing shear rate, especially at SDS concentrations above the

cmc . This is indicative of non-Newtonian behavior'67. The data set, which P

consists of 11 shear ratelviscosity combinations at each SDS concentration,

was analyzed according to the following simple power-law model (eq 7.1)'~~, in

which 'C represents shear stress and y shear rate, and in which K and n are

fitting parameters. The data produce a good fit to the model (Table 7.1). It

Table 7.1 Non-Newtonian parameters K and n, and the correlation

coefficient r of an aqueous PEO solution (0.25 g.d~- ') at

different SDS concentrations at 2 5 ' ~ .

should be noted that K and n exhibit extremes at 6 and 20 rnM of SDS,

corresponding to the cmc and can,, respectively. The dependence of K on SDS P

concentration is similar to the concentration dependence of the apparent

viscosity at fixed shear rate. The parameter n, which has a value of one for

Newtonian liquids, decreases even further below one above the cmc . It is P

evident that binding of SDS micelles onto the PEO polymer induces increasingly

non-Newtonian behavior.

7.3 The influence of SDS on the viscoelasticity of a PEO solution

An aqueous PEO solution not only exhibits viscous flow but

also viscoelasticity. The latter property is of great industrial importance,

since it is used to reduce 'drag' in pumping fluids through pipelines43.

Transport of fluids is greatly facilitated if a compound is added, which

renders the liquid viscoelastic.

Viscoelasticity may be quantified as a first normal stress difference,

using a cone-and-plate measuring device for the viscometer. Figure 7.2 depicts

the first normal stress difference for the 0.25 g . d ~ ' PEO solution

(mw 5 x lo6) as a function of the shear rate for various SDS concentrations.

Log ( f i rs t normal

stress difference, P a )

log (shear ra te .s - '1

Figure 7.2 Logarithm of the first normal stress difference, indicating

viscoelasticity, as a function of the logarithm of the shear

rate. For clarity, each curve is shifted upward 0.2 with respect

to the previous curve at lower SDS concentration. SDS

concentrations: (a) 0 mM, (H) 2.2 rnM; (A) 3.9 mM; (A) 5.2 mM, (0) 8.1 mM, (+) 14.4 mM, (0) 20.2 mM, ( 0 ) 29.5 mM, (V) 39.5 mM, (V) 45.9 rnM.

The enormous increase in first normal stress difference induced by SDS is even

more obvious from Figure 7.3, which shows the first normal stress difference

at fixed shear rate. The same three regions as apparent from Figure 7.1 can be

distinguished. Obviously the viscoelasticity, as inferred from the first

normal stress difference, is greatly enhanced when SDS micelles bind to the

polymer. Another feature of Figure 7.2 is the almost constant first normal

stress difference at the highest shear rates in the PEO solutions containing

SDS concentrations near or above csat. From a plot of the first normal stress

difference against shear stress (Figure 7.4) it is apparent that this leveling

off starts at about the same shear stress of 25 Pa. This phenomenon can be

interpreted as a partial breakdown of the polymer-rnicelle complex, mediated by

First normal

stress diffence , Pa

Figure7.3 Viscoelasticity as indicated by the first nonnal stress

difference as a function of the SDS concentration at a shear rate

of 952 s-'.

First normal

stress difference. Pa

shear stress. Pa

Figure 7.4 The relation between first normal stress difference and shear

stress at various SDS concentrations. For clarity each curve is

shifted upward 1000 Pa with respect to the previous curve at

lower SDS concentration. SDS concentrations: (n) 0 mM, (m)

2.2 mM, (A) 3.9 mM; (A)5.2 mM; (0) 8.1 mM, (+) 14.4 mM, (0)

20.2 mM, ( 0 ) 29.5 mM; (V) 39.5 mM; (V) 45.9 mM.

the shear stress or, more precisely, the hydrodynamic drag force16'. If the

hydrodynamic drag force exceeds the force that keeps the micelles bound to the

polymer, the micelles will be ripped off. This phenomenon most likely induces

the leveling off of the first normal stress differences. It must be emphasized

that the binding of the micelles onto the polymer segments becomes weaker as

the SDS concentration reaches c~:~'. The reason is that the intermicellar

electrostatic repulsion will increase, whereas the stabilization of the

hydrophobic core-water interface remains constant. At first sight it seems

surprising that the viscosity is not influenced in this region of

concentration and shear rate. However, it is known that viscoelasticity is

much more sensitive to shape and flexibility of polymer coils than

viscosity167.

In summary, some novel aspects of the rheology of PEO/SDS have been

elucidated. The shear rate dependence of the apparent viscosity may be

described by the power-law model even in the case when micelles are bound to

the polymer. Furthermore, the viscoelasticity is enhanced by binding of SDS

micelles, and on the basis of the data on the first normal stress difference,

break-down of the polymer-micelle complex at a critical shear stress is

observed. The increase in viscoelasticity may be of commercial interest, since

the possibility of enhanced drag reduction is combined with the possibility to

solubilize apolar compounds in aqueous solutions.

7.4 Experimental section

Materials. SDS (BDH, especially pure) and PEO (weight-averaged mw 5 x lo6,

Aldrich) were used as received. Water was deionized and distilled twice.

Rheological measurements. Solutions were prepared several hours before the

measurements by adding appropriate amounts of SDS to a 0.25 g.dL.l aqueous

solution of PEO. The overlap concentration (c*) of PEO of this molecular

weight is 0.4 g . d ~ - l and thus well above the employed concentrations2.

Furthermore, during the two weeks that are needed to ensure complete

dissolution of the polymer, some degradation of PEO will be unavoidable4''.

Therefore, the actual molecular weight will be lower and, hence, the actual

overlap concentration, which is calculated as (110 x d5)*', will be even

higher. Rheological measurements were performed on a Brabender Rheotron

rheometer with cone-and-plate geometry, equipped with a Normal F-sensor, which

allows the measurement of first normal stress differences. Although some

destruction of the polymer chains was observed at higher shear rates, the corresponding effects on the viscometric data are negligible compared to the

overall effects of SDS addition. Every PEOISDS solution was only used once.

All measurements were performed at 25 OC. The rheological measurements were

all canied out at the Department of Chemical Technology of the University of

Groningen, with the help of Dr. J.P. Sek and Prof.Dr.Ir. L.P.B.M. Janssen.

CHAPTER 8

AN ATTEMPT TO MODEL POLYMER-MICELLE INTERACTION QUANTITATIVELY

8.1 Introduction

During this study, many data have been squired on new polymer/micelle combinations. This stimulated us to develop a model that quantitatively

describes a broad range of polymer/micelle systems. In the literature, several quantitative treatments of polymer-micelle

interaction may be found. Those of Gilanyi and wolfram6', and of ~ a l 1 ~ have

been developed to provide a basis for the interpretation of thermodynamic information, obtained from techniques such as electromotive-force measurements

and dialysis equilibria. Gilanyi and wolfram63 have applied their model to the

systems PVA/SDS, PVPISDS, and PEOISDS for the analysis of potentiometric data

on the activity of n-dodecylsulfate. The model of s all^, published in 1985, did not contain experimental data, and no appropriate data were available in

the literature for him to test his model . In 1989, however, Takisawa and Hall

with several coworker^'^ published a chemical relaxation and equilibrium study

on the binding of sodium n-octyl- and n-decylsulfate to PVP and PEO combining

theory and experimental data. One of the main concl~sions'~ was that "the equilibrium between the bound and free surfactant is complex in the sense that no simple expression can be derived to explain this process at all surfactant

concentrations. .. . I q .

Both ~ a ~ a r a j a n ~ l and ~uckenstein~' have made attempts to relate the cmc's

and aggregation numbers of the polymer-bound micelles to properties of the

surfactant molecules and polymers. For the surfactant molecule, properties

such as headgroup area, molecular volume, and length of the alkyl chain, are

particularly considered, since they determine the packing constraints in the aggregate. For the polymer, ~ a ~ a r a j a n ~ l introduced a parameter, a that

PI'

indicates the surface area of a micelle that may be shielded by the

macromolecule. ~uckenstein~l uses an experimentally accessible parameter for

the polymer, namely the reduction in interfacial tension between water and

n-octane, upon addition of the polymer to water. Nagarajan61 and ~uckenstein~l

claim to be able to predict whether or not interaction between a surfactant

micelle and a polymer will occur and to predict the values of the cmc and aggregation number of the polymer-bound micelles. ~agara jan~l tested his model

on PEO/SDS and on the combinations sodium n-decyl-, n-dodecyl-, and n-tetradecylsulfate with PVP, and on certain nonionic swfactants with

polymer. ~uckenstein" tested his model on PEO in the presence of SDS, Triton-X100, and n-dodecyltrimethylammonium chloride. The models of

~ a ~ a r a j a n ~ ' and ~uckenstein~l will be discussed in the next section, since in

principle these models may be thought to be applicable also to our

polymer-micelle systems. We attempted to apply the above models to our polymer-micelle systems.

However, since this attempt was in vain, we have made an endeavor to subject the data to the 'dressed micelle' model of Evans and ~ i n h a r n ~ * ' ~ ~ . This model

hinges on the nonlinear Poisson-Boltzmann equation and has mainly been

developed to understand counterion binding. The approach is quite different

from that of Nagarajan and Ruckenstein. However, this model is applicable only

to monovalent surfactants. We have adopted the Evans-Ninham model for the

description of polymer-bound rnicelles on the assumption that the same balancing between repulsive (electrostatic) interactions and attractive (hydrophobic) interactions determines the size and stability of both

polymer-bound rnicelles and unperturbed micelles. The results will be presented

in section 8.3.2. In section 8.4 a comparison will be made between the models

of Nagarajan and Ruckenstein on the one hand and the 'dressed micelle' model on the other.

8.2 The models for polymer-micelle interaction developed by Nagarajan and Ruckenstein

The models of ~ a ~ a r a j a n ~ l and ~uckenstein'l are set up along similar

lines and may thus be discussed together. The models center upon the

optimaiization of the aggregation number (n). The equilibrium size distribution of the aggregates may be obtained from

in which Xn refers to the mole fraction of aggregates of size n, XI is the

mole fraction of the dissolved surfactant monomers, and s pi is the difference

in standard free energy between the surfactant molecule in the aggregate of

size n (p i ) and a dissolved surfactant monomer in water (p:). If the micelle

is interpreted as a pseudophase, the optimal aggregate is defined by the

condition of minimization of the standard free energy per molecule of the micelle. This implies:

Then, by using the approximation that at the cmc Xn is small enough, equations

8.1 and 8.2 may be combined to yield an expression for the cmc (in mole fraction units),

ln cmc = AQ k~ a t n = n opt

(8.3)

which is the same as that used in Chapters 4 and 5. If the cmc for the

formation of free micelles is higher than that for polymer-bound micelles, the latter will be formed first.

Now it remains to fmd an expression for pi, both for the free micelles

and for the polymer-bound micelles. Only the expressions for the polymer-bound

micelles will be discussed, since the expressions for the free micelles

are then obvious enough, or may be found in the referen~e$"~'. ~ a ~ a r a j a n ~ '

uses equation 8.4.

In the above expression, the first term refers to the free energy change associated with the transfer of the akyl chain from water to a liquid

hydrocarbon phase. The interior of the micelle is not identical to that of a

liquid hydrocarbon because of the slight ordering of the chains inside the

micelle induced by the constraint on the polar headgroups to remain at the micellar surface. The second term accounts for the free energy corrections

associated with this ordering effect. The third term represents the free energy of formation of the micellar core-water interface. This free energy is

lower than that of free micelles because of the shielding provided by the

bound polymer segments. The parameter, a serves as a quantitative measure pol'

of the effectiveness of micellar binding to the nonionic polymer. Furthermore, a represents the surface area of the micellar core per surfactant molecule.

and a. the area per surfactant molecule shielded from contact with water by

the polar headgroup of the surfactant. The interfacial tension, o, is

considered to be the same as the macroscopic interfacial tension between

liquid hydrocarbons and water. The fourth term refers to steric repulsions between the headgroups and between the headgroups and polymer segments. The

area a is the cross-sectional area of the polar headgroup. The last term in P

eq. 8.4 reflects electrostatic repulsions between the headgroups. The

expression for this term results from the linearized Poisson-Boltzmann

equation and contains among other parameters, the counterion binding (q). This

q is used as a fitting parameter for obtaining approximately correct n and cmc

values through optimization of n. The value is kept invariant for free and polymer-bound micelles.

~uckensteins" treatment deviates from that of Nagarajan in the third

term, which describes the formation of the core-water interface. Ruckenstein

(and also ~ u b e r ~ ~ ) uses expression 8.5, in which a,, denotes the

cross-sectional area of the hydrocarbon chain, and Ao and Ao are the changes P

interfacial term = ( o - Ao) (a - a ) + a Ao if a < a, P P P P

(8.5)

in the interfacial tensions between the hydrocarbon core and water, and

between the headgroups and water, respectively, caused by the presence of the

polymer. The quantities Ao and Ao are taken to be equal in the calculations P

of Ruckenstein. The main parameter of his model is this ACJ, which is evaluated as the difference in interfacial tension between water and n-octane, and an

aqueous solution of polymer and n-octane. Also the fourth term, which describes steric repulsions, is different in

Ruckenstein's treatment compared to Nagarajan's. Since Ruckenstein does not

assume the micelles to be in direct contact with the polymer, the part a ,/a Po

is absent in Ruckenstein's fourth term.

The difficulty associated with both models is, that a variety of input parameters have to be used, which are rather tricky to estimate. Also the

counterion binding is used as a fitting parameter to yield more or less the

experimental values for n and cmc. The parameter a has to be guessed. In pol

contrast, Ao, may be measured, but only if the polymer does not dissolve in

the hydrocarbon phase. In Nagarajan's approach the free energy of transfer of

polymer segments to the micellar phase is accounted for in a In pol'

Ruckenstein's treatment this quantity is implicit in the method of measuring

Ao.

We have subjected the experimental data presented in Table 8.1 for

PPOISDS and PEOISDS for analysis in terms of both models. Instead of optimizing the aggregation number, we used the experimental value. Although

several values for a and Ao were tried, SDS/PPO could not be accomodated in pol

the model. Since we also wanted to fit n-decylphosphate (Zo = 1.0) and CTAB with various polymers (PEO, PVME, and PPO) even more parameters would have to be guessed or estimated. Therefore, we abandoned these approaches. Instead,

72.261 the 'dressed micelle' model of Evans and Ninham was employed, which only

needs the experimental aggregation number and cmc as input parameters and

yields information, inter alia, on the average interfacial tension between

micellar core and water. The problem is attacked, in fact, the other way

around. The experimental values of cmc and aggregation number are used to

reveal the basic contributions to rnicellization in order to understand better

the underlying principles of polymer-micelle interaction.

8.3 The 'dressed micelle' model of Evans and Ninham

8.3.1 Theory

The basic equation 8.3, which relates the cmc to Ap;, is also used by 72,261 Evans and Ninham . But both the expression for A ~ : and the procedure that

is used to apply the model to a surfactant system are different. Let us begin

with an examination of AJL;. This free energy difference is built up from (i)

the hydrophobic free energy of transfer of hydrocarbon tails from water to the

interior of a micelle (gk), and (ii) surface contributions (g). Thus:

The surface term, gs, involves as yet unquantified free energy

contributions due to steric repulsion between headgroups, hydration effects,

entropic terms, hydrocarbon chain packing, and electrostatics. These effects

are opposed by an attractive free energy due to the surface tension of the

hydrocarbon core-water interface. At a low level of approximation gs is

written as:

The term gel was calculated using the nonlinear Poisson-Boltzmann equation.

The surface area a per molecule in the micelle is calculated from geometric

factors using the relations 8.8,

4n R~ = n a and 4 n R 3 = n v 3

where v is the known volume and R is the length of the hydrocarbon tailz6'.

Evans and ~ i n h a m ~ l say about the yoa term: "It must be remarked at once that by lumping all unknowns through the principle of compensating errors into a

constant quantity yo, an 'effective' interfacial tension, we disguise a

multitude of sins". The authors validate their theory by testing it with

experimental data and conclude that the dressed micelle picture indeed has a 72,261 certain validity and appeal, firmly based in statistical mechanics .

Now the procedure is as follows. The authors consider an isolated

spherical rnicelle, in a neutralizing background of counterions, monomers (and

co-ions). The size of the micelles is determined by the known aggregation

numbers. Intermicellar interactions are ignored. Then, an analytical approximation for the double-layer free energy, go,, and for the adsorption excess of ions about the micelle, e.g. the counterion binding (q), is derived

from the Poisson-Boltzmann equation. Since in the equilibrium rnicelle the

attractive and repulsive surface forces must be exactly balanced, condition (8.9) will hold:

Since gel can be calculated explicitly by the non-linear Poisson-Boltzmann

equation, yo is determined and g may be calculated using also eq. 8.7. Then.

gtr may be obtained from equations 8.6 and 8.3 using the known cmc value. The

calculations are straightforward once one is aware that the authors have used

the old-fashioned electrostatic unit system (e.s.u.).

The precise expressions for yoa and gel include several parameters that have to be calculated first72. The expression for the Debye length, K, is

given by equation 8.10.

in which no is the bulk electrolyte concentration (the cmc in our case), e the

magnitude of unit charge, and & the dielectric constant. At 298 K, K is

obtained in cm-' as the square root of 1.0793 x l0I5 x cmc, if the cmc is

expressed in m01.k~". Three other parameters, s, z, and y, are introduced by equations 8.1 1, 8.12, and 8.13, respectively.

y = 2 arccosh (z) = 2 ln ( z + d(z2 - 1)) (8.13)

in which a denotes the rnicellar surface area per surfactant molecule and R the

radius of the hydrocarbon core. The parameter s is obtained in the same units

as K-' x a-' using 8.964 x x K-' x a-'. No further difficulties arising

from the use of the e.s.u. unit system are encountered. The expression for g and yoa are:

el

yo a = 2kT sinh ( ~ 1 2 ) s

Note that the bold plus sign (+) in equation 8.15 is, by mistake, a minus in 'T'

the original literat~re'~. It is rewarding to recognize that the second half

of eq. 8.14 (from 411cRs onwards), multiplied by 4, equals the second half in

eq. 8.15 (from 161~Rs onwards). Furthermore, the first term within brackets of

eq. 8.15 equals the counterion binding which is given by eq. 8.16.

Although all calculations may be carried out using a pocket calculator,

it is advisable to computerize them to avoid mistakes and save time.

8.3.2 The 'dressed micelle' model applied to various polymer-micelle

systems

The same electrostatic and other balances between opposing surface

forces, as described by the dressed micelle model, are at play in the

formation of polymer-bound micelles. Therefore, we have applied this theory to

polymer-micelle systems (at surfactant concentrations below c ). Recently, 881

Treiner and ~ ~ u ~ e n ~ ~ have succesfully applied this model to describe the

potentiometric behavior of Cu2+ in the systems Cu(DS),/PEO and

cu(DS),/PVP. Table 8.1 lists both the input data and the model parameters for the

surfactants SDS, (TAB, and n-decylphosphate (Z = 1.0) in aqueous solutions

and in solutions containing 0.5 g d ~ " of PEO, PVME, or PPO. The data include

the input quantities cmc and n, as well as the values of R and a, and the

quantities g , y a, ge, and gn. In addition, the relevant parameters such as el 0

the Debye length K" and the (calculated) counterion binding q are also

listed. As already shown previously by Evans and inh ham^^', the values for gn

are in good agreement with expectations. The term is comparable to the sum of

the first two terms in eq. 8.4 as employed in the models of Nagarajan and

Ruckenstein (section 8.2). Using ~uckenstein's~' treatment one obtains the

following values for +: -37.2 W .moK1 (n-decylphosphate). -43.5 W .mol-l

(SDS) and -56.0 ld.mol-' (CTAB), which agree closely with our (output) data.

The value of gn in the presence of PEO coincides closely with that in

Table 8.1 Input data and model parameters for various surfactants in the

absence and presence of polymers at 298 K.

input output

system cmc n R a lil ge: yo.": -gwa Y,b q mM A A2 A

SDS 8.1 58 16.9 62 33.8 12.9 6.89 19.8 42.2 18.5 0.87

SDSIPPO 4.0 29 13.4 78 48.1 12.7 7.69 20.4 44.0 16.3 0.84 SDSPBO 5.6 34 14.2 74 40.7 12.4 7.40 19.8 42.6 16.6 0.84

SDSIPVME 5 28 13.3 79 43.1 12.2 7.58 19.8 42.8 15.9 0.83

CTAB 0.95 69 19.6 70 98.8 17.1 7.58 24.7 51.8 18.0 0.91

CTABPPO 0.37 35 15.6 88 158.2 17.4 8.60 26.0 55.5 16.3 0.90

CTABIPVME 0.46 35 15.6 88 141.9 17.0 8.52 25.5 54.4 16.1 0.90

- -

a) In kJ.mol-' b) In dyne.cm-'c) DeP denotes n-decylphosphate at Z =1.0.

d) molecular weight 20 k.

aqueous solutions in the absence of polymer whereas in the presence of PVME or

PPO slightly higher absolute values are consistently found. These results

probably reflect that the transfer of segments of these hydrophobic polymers

from the aqueous to the micellar environment also contributes to the free

energy of micellization for the polymer-bound micelle, AF;. For PPO, the

increase in -+ ranges from 1.8 to 3.7 kJ.m01-~, which is less than the free

energy of transfer of one CH, group (3.10 kJ.mol-I 262 to 3.45 kJ.mol-I 294)

or one CH, group (8.78 kJ.mol- 1 294 to 9.41 kJ.mo1" 262) from water to a

hydrocarbon environment.

It is tempting to relate this change in the value of g& directly to

(partial) dehydration of, for instance, CH, moieties of PPO. However, the

drastic decrease in aggregation number of the micelles may also influence ga.

More CH, groups of the surfactant may be at the surface of the micelle in the

case of polymer-bound micelles. These methylene groups will be there in

contact with either water or polymer. In the former case the contribution of

the surfactant molecules to gtr will even be lower and concomitantly the

contribution to g& attributable to the polymer will be even higher.

The 'effective' interfacial tension, yo, is lower for the micelles in the

presence of polymers than in aqueous solution. This is in accord with the

qualitative view that a polymer may stabilize the hydrophobic core-water

interface. Furthermore, it is noteworthy that the interfacial tension in

micelles formed from SDS and CTAB is slightly more reduced by PVME

than by PPO, even though the latter polymer causes a stronger reduction of the

cmc. No relation has been found between the microscopic yo and the macroscopic

surface tension or interfacial tension with n-octane of an aqueous polymer

solution or of an aqueous polymerlsurfactant solution.

The reduction in yo does not result in a smaller value for the total free

energy term, yoa, because of the increase in surface area per molecule, a, of

the polymer-bound micelles compared to free micelles. The term yoa is

consistently higher for the polymer-bound micelles, but gel shows in most

cases a slight reduction. Two opposing factors play a role in determining gel:

the larger surface area per molecule of the polymer-bound micelles causes a

reduction in gel, whereas the lower cmc values cause an increase in gel via an increase in the reciprocal Debye length.

The terms yoa and gel can be combined to yield a surface free energy, g8,

for the polymer-bound micelles, which equals that of free micelles or is

slightly higher. It is interesting that these increases in surface free energy

are only found for the hydrophobic polymers PVME and PPO. This is in harmony

with our qualitative view that these polymers may unfavorably disturb the

Stem layer to a small extent, since that may be compensated by a larger

contribution in hydrophobic energy (gu in this model). This was very clearly

apparent in Chapter 2, in which the interaction between PPO and OTG micelles

has been described. For this nonionic surfactant the disturbance of the Stem

layer will be mainly due to steric repulsion between headgroup and polymer

segments. Since the cmc of OTG is not affected by the presence of PPO, the

disturbance of the Stern layer must be compensated by a favorable free energy

for transfer of the polymer.

In conclusion, since the surface free energy is unchanged or increased

upon binding of the micelles onto the polymer, the reduction in cmc stems from

the increased negative contribution from the free energy of transfer of

hydrocarbon chains from water to the micellar core combined with that of the

transfer of polymer segments to the rnicellar surface.

8.4 Comparison of the models

The most important practical advantage of the model of Evans and Ninham

compared to those of Nagarajan and Ruckenstein is the fact that the former may

be used for any micellar system for which the aggregation number and cmc are

known. The main restriction, forwarded by Evans and Ninham, is that the

surfactant molecules have to be monovalent. As shown in section 8.3.2, the

dressed micelle model can indeed be applied to a large variety of

polymer/micelle systems. Realistic output quantities are obtained, that are in

harmony with qualitative predictions. As mentioned in section 8.2, application

of the models of Nagarajan and Ruckenstein requires too many input parameters,

which have to be guessed.

The application of the model of Evans and Ninham for polymer/micelle

systems is not without pitfalls, but these are similar to those for the models

of Nagarajan and Ruckenstein. One of the major concerns is the choice of R. We

have followed other authors in estimating the radius of the hydrocarbon region

for both free and polymer-bound micelles. Even for free micelles, one might

argue that the dielectric boundary begins at the centre of the headgroup. 72,261 Evans and Ninham have discussed this matter and decided in favor of the

hydrocarbon region to define the radius. But how does a polymer affect the

radius of the micelle? If the polymer is really adsorbed on the hydrocarbon

surface, there are no problems. However, if the polymer is slightly embedded

in the micellar outer layer the actual radius will be larger than that

obtained from eq. 8.8, using the total volume of hydrocarbon chains.

Another problem is the dielectric constant, E. Usually the value for

water is used in calculations on rnicelles. Even for free micelles this is a

questionable simplification. However, experimental values for the

micropolarity in the Stern layer vary widely33 and it is still unclear whether

the relevant value for E is measured experimentally. The presence of a polymer

in the Stem region, which will almost certainly influence the dielectricum

between the headgroups, makes the choice even more difficult. Nevertheless,

Treiner and ~ ~ u ~ e n ~ ~ were able to describe the potentiometric behavior of cu2+ in the systems Cu(DS),/PEO and Cu(DS),/PVP using the 'dressed micelle' model

and the dielectric constant of water.

The final difficulty in the applied input parameters is, that the

analytical formulas for gel are most accurate if KR > 0 . 5 ~ ~ ' , which is not

always the case in our data set. Still, the outcome for gtr and yo is as

expected. Probably, the inaccuracies induced by the too small value of KR are

not dramatic. The uncertainties in the choice of R and & are also present in

the models of Nagarajan and Ruckenstein. The question of the KR value is also

likely to play a role in the application of the linearized Poisson-Boltzmann

equation, which is employed by Nagarajan and Ruckenstein, but no restrictions

in input data for the calculation of the electric term have been discussed by

them.

The counterion binding in the presence of polymers is the only parameter

for which no satisfactory value is obtained upon application of the 'dressed

micelle' model. The model predicts a one or two percent decrease in counterion

binding in the presence of polymers, whereas w i d 8 reports a 20 % decrease

in the experimental counterion binding of SDS micelles upon addition of PEO.

We do not know the origin of this disagreement. Nagarajan6' and ~uckenstein~'

keep the counterion binding constant for free and polymer-bound micelles and

even use it as a fitting parameter.

In conclusion, the model of Evans and Ninham provides a useful basis for

a quantitative analysis of polymer-micelle interaction. This novel application

of the model can be extended to a broad range of polymer-rnicelle systems when

the aggregation number and cmc are known. As long as yo and gtr cannot be

related to known parameters of the individual surfactant and polymer, the model cannot predict whether or not interaction will take place.

8.5 Experimental section

The cmc's and aggregation numbers of micelles of CI'AB in the absence and

presence of 0.5 g . d ~ - ' of PPO or PVME may be found in Tables 3.1 and 3.2 respectively. For all polymer-bound micelles the aggregation number at ca.

20 rnM above the cmc is chosen.

The cmc values of SDS and SDSPPO may be found in Table 5.2. The

aggregation numbers of SDS and SDS/PPO were taken from ~ i t t e ~ ~ . The cmc of

SDSPVME was determined by conductometry as described in section 3.5. The

aggregation number of SDSIPVME was determined by fluorescence quenching using tris-(4,4'-bipyridyl)ruthenium(II) perchlorateJ9-methylanthracene, in a similar way as described in section 3.5.

The cmc's of n-decylphosphate (Z = 1.0) in the absence and presence of

polymers may be found in Table 5.2. The aggregation numbers were measured for

the surfactant at Z = 1.1, because at Z = 1.0 the experimental temperature

(25 OC) is too close to the Krafft temperature to allow the addition of the

quencher Pmethylanthracene. We assume that the aggregation numbers will not

be much different for Z = 1.0 and Z = 1.1. Evans has shown that the outcome

of the calculations is not very dependent on the precise value of n263. The

same experimental method was used as for SDS/PVME.

All cmc values and aggregation numbers were obtained at 25 OC.

In the calculation of the volume of the hydrocarbon chain of the

surfactant molecule, we used 54.3 A' for the CH, group and 26.9 A3 for the CH,

CHAPTER 9

CONCLUDING REMARKS

9.1 Introduction

In this thesis, various aspects of polymer-micelle interactions have been investigated. This chapter contains several discussions on these aspects. For

the reader's convenience, however, the major conclusions from the preceding

chapters are outlined first, to serve as a basis for the following discussions.

9.2 Conclusions

General conclusions

GC (1) Polymer-micelle association may occur without a noticeable reduction of the cmc, particularly in the case of nonionic micelles.

GC (2) The occurence of polymer-micelle association can be revealed, inter alia, by a change in clouding behavior of the polymer, by a reduction

in Krafft temperature of the surfactant, by a decrease in aggregation number, and by AHrnk measurements.

GC (3) The stability of polymer-bound micelles relative to free micelles

increases with increasing hydrophobicity of the polymer. The rather

hydrophobic PPO causes a stronger reduction of the cmc of various

ionic surfactants than the isomeric, better water-soluble polymer PVME. Both polymers show a much stronger propensity to undergo

polymer-micelle interaction than the much more hydrophilic polymer

PEO. GC (4) Polymer-bound micelles favor a larger surface area-to-volume ratio

than the unperturbed micelles. This preference results in smaller

aggregation numbers, and in the breakdown of rodlike micelles upon

interaction with a polymer.

GC (5) Increasing the length of the alkyl chain of the surfactant

strengthens the interaction with polymers.

GC (6) The dressed micelle model of Evans and inh ham'*'^' is applicable to the quantitative description of a wide range of polymer-bound

micelles.

GC (7) The novel pH-method for determination of the cmc can be very useful,

especially for measurements in the presence of polymers, in which

case conductivity measurements, among other techniques fail.

Conclusions concerning specific systems

CSS (1) Nonionic micelles of OTG associate with PPO but not with PEO.

Microcalorimetric measurements revealed that this interaction is

endothermic.

CSS (2) Increasing the positive charge of micellized DDAO (0 + +1) causes

stronger association with PVME and PPO. Interaction between DDAO

micelles and PEO is absent at all stages of protonation.

CSS (3) Increasing the negative charge of micellized n-decylphosphate

(-1 -+ -2) appears to reduce association with PEO, PPO, and PVME.

CSS (4) Strong association occurs between micelles formed from the di-salt of

2-n-dodecylmalonate and PEO or PVME. However, no direct comparison

is possible with the propensity for interaction of the mono-salt with

these polymers, due to interaction between the monomeric mono-salt

and PEO or PVME.

CSS (5) Interaction of micelles formed from n-decylphosphate (Zo= 1.1) with

PVME leaves the chemical shift, coupling constants and longitudinal

relaxation of the 3 1 ~ - ~ ~ ~ resonances unaltered.

CSS (6) Binding of SDS micelles to PEO enhances the viscoelasticity of the

polymer solution.

CSS (7) The shear rate dependence of the apparent viscosity of an aqueous

solution containing PEO and SDS can be described by a power law model

for non-Newtonian behavior. Extremes in the model parameters occur at

the cmc and saturation concentration. P

CSS (8) SDS micelles are ripped from PEO above a critical shear stress.

CSS (9) Vesicles formed from sodium di-n-dodecylphosphate show interaction

with PVME, as revealed by the perturbed clouding behavior of PVME and

electron micrographs of the vesicles.

9.3 A criterion and a measure for polymer-micelle interaction

A reduction of the cmc due to the presence of polymer clearly fails to be the ultimate criterion for polymer-micelle attraction. It is certainly

decisive in one sense; that is, if a reduction takes place, it definitely points to polymer-micelle association. However, if the cmc is unperturbed,

additional data, for instance microcalorimetric data or aggregation numbers,

may still reveal such interaction. Therefore, the qualitative question whether

polymer-micelle association occurs at all, must be solved first. The degree of

micelle stabilization (cmc reduction) or the amount of micelles that may be

bound to the polymer may then serve, in principle, as quantitative

measures.

The measurement of the surfactant concentration at which the polymer

becomes saturated with micelles is a prerequisite for determining the number

of bound micelles. Unfortunately, the saturation concentration is difficult to

obtain, since the formation of free micelles may start before polymer

saturation is complete and the total amount of bound surfactant depends also on the number of free micelles present73. Therefore, the degree of

stabilization of micelles by binding to a polymer is still the most useful and

practical quantitative measure for a comparison of polymer-micelle

combinations. Apart from practical considerations, 'micelle stabilization'

obtained from the cmc values indeed reflects interaction of the polymer with

the micelle, whereas 'the number of bound micelles' depends largely on

intermicellar interactions between bound micelles.

Another quantity, which may be thought to provide a measure for the binding force between rnicelles and a polymer, is the critical shear stress at

which rnicelles are ripped from the polymer. This approach, however, suffers

from the same drawbacks as the 'number of bound micelles'. First, the approach

also greatly depends on intermicellar interactions. Second, it is not widely

applicable, since the necessary shear stresses are difficult to reach if the

viscosity of the polymer solution is not sufficiently high to start with and considerably increased upon binding of micelles. Furthermore, the polymer

solution must have a certain viscoelasticity. Finally, it is the hydrodynamic

drag force and not the shear stress directly, which is held responsible for

ripping the micelles from the polymer.

9.4 The driving force for polymer-micelle interaction

It goes without saying that the driving force for polymer-rnicelle

interaction is a reduction in free energy of the total system. Still, it is

worthwhile to note that both stabilization of the micelle proper and a

reduction in the free energy of the (hydrated) polymer may provide the major

contribution to the total free energy for the formation of polymer-bound

rnicelles.

Stabilization of the micelle upon binding onto a polymer can result from

(i) a reduction in interfacial tension between the hydrophobic core and water,

(ii) specific interactions between the polymer and surfactant headgroups, and

(iii) a decrease in electrostatic repulsion between charged headgroups due to a lower aggregation number. The micellization process may, however, also be

impaired to a certain extent by binding to a polymer, due to (i) an increase

in surface area per surfactant molecule in the micelle, associated with the

smaller aggregation number, (ii) steric (or other) repulsions between polymer

segments and surfactant headgroups, and (iii) an increase in electrostatic

repulsion related to the lower ionic strength which originates from a lower

cmc. The net effect of the polymer on the free energy of the surfactant

molecules in the polymer-bound micelle must be combined with the change in

free energy of the polymer upon the transfer from the aqueous surroundings to

the micellar surface. The free energy of transfer will be primarily related to the hydrophobicity of the polymer, but will also depend on steric requirements at the micellar surface and on the influence of the surfactant and counterions

on the hydration sheath of the polymer. Polymers formed from quite bulky

monomers are known to associate appreciably with all kinds of micelles, thus

the steric requirements do not seem to be very stringent. The perturbation of

the hydration sheath of the polymer, which is stronger for most anions than for the common cations and is sometimes used to explain the difference between

anionic and cationic surfactants in their interaction with polymers, is also

not likely to be decisive. A dominant role for this perturbation of the

hydration sheath, namely, would not be in accordance with the reduction in

association tendency upon increasing the charge of the phosphate surfactants,

because more highly charged phosphate salts exert a greater influence on, for

instance, the clouding temperatures of polymers. Binding of a polymer to a micelle is also enhanced when the length of the

alkyl chain of the surfactant is increased. This is a common feature for hydrophobic binding to micelles, but the origin of the effect is still not

well understood.

We conclude that for rather hydrophobic polymers, like PVME and PPO, the

nature of the micelles is not of decisive importance, because the free energy

of transfer of the polymer is the dominant contribution and may even

compensate an unfavorable influence of polymer-micelle interaction on the micellization process per se. For hydrophilic polymers like PEO (and PVA, PVP,

etc.) the matter is more delicate. The precise geometry and chemical structure of the surfactant become decisive and stabilization of the micelles presents the major contribution to the total free energy:

9.5 The role of the charge and structure of the surfactant headgroup

Micellar charge, whether positive or negative, definitely stimulates

micelle stabilization upon binding of polymers. It is not, however, a

prerequisite for association, as has long been propagated. Neither does

increasing the surfactant charge to higher values than unity necessarily

result in a stronger stabilization of the micelle. The major effects of surfactant charge on the stabilization of

polymer-bound micelles are a contribution from the reduction in electrostatic

repulsion due to the smaller size of the bound micelles, and the influence of

charged groups on the hydration sheaths of polymers. The former effect is

operative for both negatively and positively charged surfactants. The latter

effect does, in practice, depend on the sign of the charge, since only a

limited choice of charged groups can be used as headgroups of a surfactant.

The negatively charged headgroups like -0SO;. 0 ~ 0 t - a n d OP0,H' exert a

strong influence on the hydration of a polymer like PEO, as revealed by

clouding point and flocculation studies (see section 3.1.2). In contrast, the

positively charged headgroups, such as -NH,+ and -NM~,+ show only a weak

influence. The origin of this difference in behavior is not yet understood,

but it is clear that it is related to properties of the hydrated ionic

headgroups.

The headgroups of anionic surfactants invariably possess several oxygen

atoms, which may serve as hydrogen-bond acceptors towards water. It is

possible that, in case of the anionic surfactants, a hydrogen-bond network

mediated by water loosely links the polymer to the headgroups. The cationic

surfactants with a trimethylammonium headgroup cannot form hydrogen bonds with

water, but those having an ammonium or an N-hydroxydimethylamrnonium head-

group may serve as a hydrogen-bond donor. The hydration will therefore be

'reversed', compared to that of the anionic surfactants, but hydrogen bonding

is possible. Yet, also the ammonium and N-hydroxydimethylamrnonium surfactants

do not form polymer-bound micelles with a polymer like PEO. However, the

strength of the interaction of micelles of DDAO at P = 0.75 with PVME (-0.5

k.J.rnol-') or PPO (-1.1 kJ.mol-') is comparable to that of n-decylphosphate at Zo = 1.0 with PPO (-1.0 kl.rno1-'). Admittedly, a DDAO molecule contains two

more methylene units in the akyl chain, but on the other hand, the phosphate

has a higher charge.

Unfortunately, it is not yet possible to unravel the precise role of

hydrogen bonding between polymer segments and hydrated headgroups. The

importance of specific interactions, however, is definitely apparent from the

difference in interaction tendency between the sulfate and phosphate

surfactants and from the very strong interaction of the di-salt of

2-allcylmalonates with polymers.

The size of the headgroup has often been considered as the major factor

in determining polymer-rnicelle interaction. However, its importance must not

be overvalued in view of the following arguments, some of which were already

presented in section 3.1.2. (i) Polymers formed from quite bulky monomers

interact strongly with all kinds of micelles. Consider, for instance,

hydroxypropylcellulose/~~~53 and ethylhydroxyethylcellulose/n-dodecyl-

trimethylammonium bromide7'. (ii) Micelles formed from n-alkylammonium

surfactants do not interact with PEO, despite the small size of the headgroup.

(iii) Micelles formed from the di-salt of 2-n-dodecylmalonate interact very

strongly with PEO, notwithstanding the bulky headgroup. (iv) The size of the

headgroup of an alkylphosphate is smaller or comparable to that of an

alkylsulfate, but, nevertheless, micelles of the former surfactant are

considerably less stabilized by polymers.

In conclusion, polymer-micelle interaction depends on several properties

of the surfactant molecule, such as the chemical nature, geometry, and charge.

There are some restrictions in chemical nature and geometry of the surfactant

to provide a certain sign for the charge. For instance it is hard to find an

anionic surfactant without an oxygen-rich headgroup or a cationic surfactant

with many oxygen atoms and without a quarternary nitrogen atom. Therefore, it

is not yet possible to formulate general rules concerning the relative

importance of the properties, mentioned above. Cationic, anionic, and nonionic

surfactants have all been shown to undergo polymer-rnicelle interaction on the

premise that the polymer is sufficiently hydrophobic. The first nonionic and

cationic surfactant that interacts substantially with, for instance, PEO has

still to be reported. For cationic rnicelles, a betaine, like R-N(M~),+CH,COOH,

may be found to interact with PEO, in view of the favorable interaction

between the ether linkage and the COOH moiety.

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SUMMARY

The industrial importance of polymer-micelle complexes was recognized

long before a satisfying model for the morphology of these complexes was

developed. To date, the notion that surfactants may bind to polymers in the

form of micelles is f d y established. The limited choice of polymer-micelle

systems has, unfortunately, led to unwarranted generalizations. For instance,

nonionic micelles were considered to be indifferent towards poIymers, and

cationic surfactants, limited almost exclusively to alkyltrimethylarnmonium

salts, were recorded to be much less prone to interact with polymers than

anionic micelles, largely represented by sodium dodecylsulfate (SDS). This

study aims- at a better understanding of the role of charge of the surfactant

molecule on polymer-micelle interaction. To this end surfactants were

investigated for which the charge can be varied without a concomitant. drastic

change in structure. Furthermore, a rheological study on selected

polymer-micelle systems is presented, and finally an endeavor was made to

apply an existing micelle model for the quantitative description of

polymer-micelle complexes.

In Chapter 2, nonionic micelles formed from n-octyl-P-D-thioglucoside

(OTG) are shown to interact with poly(propy1ene oxide) (PPO), but not with the

more hydrophilic poly(ethy1ene oxide) (PEO). Interestingly, the cmc of OTG is

not affected by the association of the micelles with PPO, Microcalorimetric

data revealed that the interaction is endothermic. Furthermore, interaction

was clearly apparent from the perturbed clouding behavior of PPO and the

change in Krafft temperature of OTG.

In Chapter 3, current views on the role of surfactant charge on

polymer-micelle interaction are reviewed. Those views include (i) the idea

that the size of the headgroup is of decisive importance due to steric or

other repulsions among headgroups and polymer segments at the micetlar

surface, and (ii) the suggestion, that mainly the usual anionic headgroups

exert a great influence on the hydration sheaths of polymers, as apparent from

studies on clouding temperatures. The effect of PEO, PPO,

poly(vinylpyrro1idone) (PVP) and poly(vinylmethy1ether) (PVME) on the cmc and

aggregation number of cetyltrimethylarnmonium bromide (CTAB) is presented. The

non-interacting polymers PEO and PVP leave these quantities unchanged, whereas

PVME and PPO induce the formation of polymer-bound micelles, which are of

smaller size than free micelles. A novel and very extreme manifestation of the

reduction in miceliar size upon binding to polymers is presented by the

breakdown of rodlike rnicelles of cetyltrirnethylamrnonium salicylate (ClTASal)

and tosylate (CTATs) to spherical polymer-bound rnicelles in the presence of

PPO and PVME.

Chapter 4 contains a study on the interaction of micelles formed from n-dodecyldimethylarnine oxide (DDAO), at various degrees of protonation (P), with PVME, PPO and PEO. Interaction with the relatively hydrophobic polymers

PVME and PPO was revealed by a decrease in aggregation number of the micelles

at all p's and by the perturbed clouding behavior of the polymers. However, at

a low value of /3 the interaction is not accompanied by a reduction in cmc. The

stronger stabilization of the micelles at higher P is attributed to the

reduction in electrostatic repulsion between the headgroups upon formation of

the smaller polymer-bound micelles. No interaction takes place between PEO and

DDAO micelles at any P. Chapter 5 describes the interaction with polymers of micelles formed from

n-decylphosphate, of which the structural charge can be varied from -1 to -2.

A novel method to determine cmc's has been developed, based on the abrupt pH

change at the cmc. In contrast to expectations, the stabilization of the micelles by PVME, PPO and PEO is lower at increasing surfactant charge. The

strong hydration of the highly charged groups is held responsible for the

diminished interaction with polymers. The influence of charge is also apparent

from the effects on the clouding temperature of PVME. The characteristics of

the "P-NMR resonances of the phosphate surfactant at Z, = 1.1 remain

unaltered by interaction with PVME. Preliminary studies of the effect of PVME

on vesicles formed from di-n-dodecylphosphate reveal that association takes

place. The clouding behavior of PVME is disturbed very peculiarly and electron

micrographs show that the polymer-bound vesicles adopt more sausage-like

shapes.

Chapter 6 is concerned with the aggregation behavior of mono- and

di-salts of 2-alkylmalonates in the absence and presence of polymers. The mono NM~,' salt of 2-dodecylmalonate has a Krafft temperature below room

temperature, contrary to the alkali metal salts, and is the first anionic

surfactant that forms viscoelastic solutions at extremely low (2.5 mM) concentrations. The visually observable viscoelasticity and concomitant 'H-NMR

line broadening point to the presence of rods. Both phenomena disappear upon addition of PVME or PEO, due to the preferred formation of spherical polymer-bound rnicelles. Micelles formed from the di-salts of 2-n-dodecylmalonate interact strongly with PEO and PVME, as evidenced by a

severe reduction of the cmc. The strong interaction is thought to result from

hydrogen bonding interactions. A direct comparison of the mono- and di-salts

in their propensity for polymer-rnicelle interaction is impaired by the finding

that the mono-salt in its monomeric state also interacts with the polymers.

A rheological study on a 0.25 g.d~'' aqueous solution of high molecular

weight PEO (mol wt. 5 x lo6) at various SDS concentrations is presented in Chapter 7. The apparent viscosity as well as the first normal stress

difference, which is a , measure for viscoelasticity, are increased by binding

of SDS micelles to the polymer. These effects originate from coil expansion,

due to electrostatic repulsion between the polymer-bound micelles. The relation between shear rate and the apparent viscosity has been described succesfully using a power-law model for non-Newtonian behavior. A curious

effect is observed at SDS concentrations near or above the saturation concentration when the shear stress surmounts a critical value. At these

conditions, the first normal stress difference reaches a constant value.

independent of shear rate. This most likely indicates that the micelles are

ripped from the polymer by the hydrodynamic drag force associated with this

critical shear stress.

In Chapter 8, the dressed micelle model of Evans and Ninham is applied to polymer/monovalent surfactant systems, for which the necessary data have been

acquired during this study. The model makes use of the non-linear

Poisson-Boltzrnann equation and needs the cmc and aggregation number as input

data. Information is then obtained on the balance of attractive hydrophobic interactions and repulsive interactions at the micellar surface. The models advanced by Nagarajan and Ruckenstein are discussed and considered, but proved

to be of little practical use for our extensive data set, since too many input

parameters are required.

Some final discussions and a list of the major conclusions are presented

in Chapter 9. It is argued that the occurrence of polymer-micelle interaction

is not necessarily accompanied by a stabilization of the rnicelles. The latter

phenomenon, though, is very useful as a measure for the tendency of rnicelles

or polymers to interact. In the case of rather hydrophobic polymers, the driving force for the formation of polymer-bound rnicelles is mainly the free

energy change for the polymer segments upon transfer from the aqueous to the

micellar phase. For more hydrophilic polymers, like PEO, the matter is more delicate and the driving force for interaction is much more governed by the stabilization of the micelles per se. The stabilization of micelles by polymers depends on the precise structure of the surfactant. It is suggested,

that the difference in interaction tendency between cationic and anionic surfactants may, in part, be related to the differences in hydration between

the oxygen-rich anionic headgroups and the usual quaternary ammonium-type

headgroups of cationic surfactants.

SAMENVATTING

De invloed van wateroplosbare niet-ionogene polymeren op het

aggregatiegedrag van surfactanten heeft geleid tot allerlei industriele

toepassingen, ruim voordat een bevredigend model voor de morfologie van een polymeer-rnicel complex ontwikkeld was. Tegenwoordig is het idee, dat

surfactantmolekulen aan somrnige polymeren binden in de vorm van micellen,

algemeen geaccepteerd. Helaas heeft de beperkte keuze van polymeer-mice1

combinaties geleid tot ongeoorloofde generalisaties. Zo werd bijvoorbeeld beweerd, dat niet-ionogene micellen niet binden aan niet-ionogene polymeren en

dat kationogene surfactanten, waarvan eigenlijk alleen de alkyl-

trimethylamrnonium zouten zijn bestudeerd, een veel zwakkere neiging tot

binding hebben dan anionogene micellen, grotendeels vertegenwoordigd door natrium n-dodecylsulfaat (SDS). Het onderzoek beschreven in dit proefschrift heeft tot doe1 de invloed van de lading van het surfactantmolekuul op de

vorming van polymeer-gebonden micellen beter te begrijpen. Hiertoe zijn

surfactanten bestudeerd waarvan de lading kan worden gevarieerd, zonder tegelijk de structuur van de kopgroep drastisch te beinvloeden. Tevens wordt

een reologische onderzoek aan een polymeer-rnicelsysteem gepresenteerd en wordt

een bestaand micelmodel toegepast om de vorming van polymeer-gebonden micellen

kwantitatief te beschrijven.

In Hoofdstuk 2 wordt aangetoond, dat niet-ionogene micellen, opgebouwd uit n-octyl-P-D-thioglucoside, binden aan poly(propy1een oxide) (PPO), maar

niet aan het meer hydrofiele poly(ethy1een oxide) (PEO). Zeer interessant is

het feit dat de cmc van OTG niet beinvloed wordt door de interactie met PPO. Uit microcalorimetrische experimenten bleek dat de interactie endotherm is.

Ook worden het 'clouding' gedrag van PPO en de Krafft temperatuur van OTG

beinvloed.

In Hoofdstuk 3 worden de huidige inzichten betreffende de rol van lading

op polymeer-rnicelinteractie besproken, zoals (i) het idee dat de grootte van de kopgroep van doorslaggevend belang is, en (ii) de suggestie, gebaseerd op

clouding temperaturen, dat de gangbare anionogene kopgroepen een groter effect

zouden hebben op de hydratatieschil van polymeren. De invloed van PPO, PEO,

polyvinylpyrrolidon (PVP) en poly(vinylmethy1ether) (PVME) op de cmc en het aggregatiegetal van cetyltrimethylammoniumbromide (CTAB) is onderzocht. Het blijkt dat PEO en PVP geen mAB micellen binden en noch de cmc noch het

aggregatiegetal beinvloeden, terwijl PPO en PVME de vorming bewerkstelligen

van polymeer-gebonden micellen, die kleiner zijn clan de vrije micellen. Een

nieuw en extreem voorbeeld van deze afname in aggregatiegetal door binding aan polymeren is de complete afbraak van sraafiormige micellen van

cetyltrimethylarnmonium salicylaat (CTASal) en tosylaat (CTATs) tot bolvormige

polymeer-gebonden micellen in aanwezigheid van PPO en PVME.

Hoofdstuk 4 behandelt een studie naar de interactie van micellen van n-dodecyldimethylamine oxide (DDAO), bij verschillende protoneringsgraad (P), met PEO, PVME en PPO. Het optreden van polymeer-miceIbinding met de relatief

hydrofobe polymeren PVME en PPO bleek uit de afname van het aggregatiegetal

van de micellen bij alle p's en uit het veranderende 'clouding' gedrag van deze polymeren. Echter, bij een lage protoneringsgraad gaat de associatie niet gepaard met een afnarne in cmc. De sterkere stabilisatie van micellen bij een

hogere P wordt toegeschreven aan de afname in electrostatische afstoting

tussen de kopgroepen wanneer de kleinere polymeer-gebonden micellen gevormd

worden. Er vindt geen interactie plaats tussen DDAO micellen en PEO ongeacht

de waarde van p. Hoofdstuk 5 beschouwt de interactie van polymeren met micellen gevormd

door n-decylfosfaat, waarvan de lading tussen -1 en -2 kan worden gevarieerd. Aangezien de bestaande methoden om de cmc te bepalen faalden, is er een nieuwe

methode ontwikkeld, gebaseerd op de abrupte verandering in pH bij de cmc.

Tegen de verwachting in neemt de stabilizatie van de micellen door PVME, PPO en PEO af met toenemende surfactantlading. De sterke hydratatie van de

tweewaardig geladen kopgroepen ligt hieraan waarschijnlijk ten grondslag. De

invloed van lading blijkt ook uit de invloed op de 'clouding' temperatuur van

PVME. De karakteristieken van de 3 1 ~ - ~ ~ ~ resonantie van n-decylfosfaat met

Zo = - 1.1 blijven onveranderd na toevoeging van PVME. Inleidende experimenten

naar de invloed van PVME op vesicles van di-n-dodecylfosfaat laten zien dat er

associatie optreedt. Het 'clouding' gedrag van PVME wordt op een zeer

eigenaardige wijze verstoord door de vesicles, en electronen microscopie

opnamen laten zien dat de polymeer-gebonden micellen meer langgerekt van vorm

zijn.

In Hoofdstuk 6 wordt het aggregatiegedrag van mono- en dizouten van

2-alkylmalonaten bij am- en afwezigheid van polymeren bekeken. In

tegenstelling tot de gangbare alkalimetaalzouten heeft mono-NM~,' 2-n-dodecyl-

malonaat een Krafft temperatuur beneden kamertemperatuur, en het is het eerste

anionogene surfactant, dat a1 viscoelastische oplossingen vormt bij extreem lage concentraties (2.5 mM). Deze visueel waarneembare viscoelasticiteit en de 1 H-NMR lijnverbreding van het surfactant, wijzen op de aanwezigheid van

staafvormige micellen. Beide verschijnselen verdwijnen na toevoeging van PVME

of PEO, doordat er dan een voorkew bestaat voor de vonning van bolvormige

polymeer-gebonden micellen. Micellen gevormd door dizouten van

2-n-dodecylmalonaat binden sterk aan PEO en PVME, zoals blijkt uit een

drastische afname in cmc. De sterke interactie komt waarschijnlijk door

waterstofbrugvorming. Een directe vergelijking tussen de mono- en dizouten in

hun neiging om met polymeren te associeren is niet mogelijk omdat het

mono-zout ook in monomere vorm aan de polymeren bindt.

Een reologische studie aan een waterige oplossing van 0.25 g . d ~ l van

hoog-molekulair PEO (mw 5 x lo6) bij verschillende SDS concentraties wordt

beschreven in Hoofdstuk 7. De schijnbare viscositeit en het eerste normaal

spanningsverschil, dat een maat is voor de viscoelasticiteit, nemen toe

wanneer SDS micellen aan PEO binden. De reden hiemoor is dat electrostatische

afstoting tussen de polymeer-gebonden micellen ketenuitdijing bewerkstelligt.

De relatie tussen de afschuifsnelheid en de schijnbare viscositeit kan met

succes worden beschreven door een 'power-law' model voor niet-Newtoniaans

gedrag. Bij SDS concentraties nabij of boven de verzadigingsconcentratie, en

bij een afschuifspanning boven een bepaalde kritische waarde, bereikt het

eerste normaal spanningsverschil een constante waarde, onafhankelijk van de

afschuifsnelheid. Dit komt waarschijnlijk doordat de micellen van het polymeer

gerukt worden door de hydrodynamische 'drag' kracht, die geassocieerd is met

deze kritische afschuifspanning . In Hoofdstuk 8 wordt het 'dressed micelle' model van Evans en Ninham

toegepast op polymeer/monovalent surfactant systemen, waarvoor de

noodzakelijke gegevens tijdens dit onderzoek zijn verzameld. Het model maakt

gebruik van de niet-lineaire Poisson-Boltzmann vergelijking, en gebruikt de

cmc en het aggregatiegetal als invoergegevens. Vervolgens wordt informatie

verkregen over de balans van attractieve hydrofobe interacties en afstotende

interacties aan het miceloppervlak. De modellen, die voorgesteld zijn door

Nagarajan en Ruckenstein, worden ook beschouwd, maar bleken van weinig

praktische waarde, doordat teveel invoerparameters zijn vereist. Enkele afsluitende discussies en een opsomming van de belangrijkste

conclusies staan in Hoofdstuk 9. Het wordt beargumenteerd, dat het optreden

van polymeer-mice1 interacties 10s gezien moet worden van de stabilisatie van

de rnicellen door polymeren. Deze stabilisatie is echter we1 nuttig als maat

voor de neiging van rnicellen of polymeren om interactie te vertonen. De

drijvende kracht voor de vorming van polymeer-gebonden micellen is

voornamelijk de verandering in de vnje energie van de polymeersegmenten bij

het overbrengen van polymeer van de waterige naar de rnicellaire fase, in het

geval van relatief hydrofobe polymeren. Voor meer hydrofiele polymeren, zoals PEO, is de zaak meer gecompliceerd en speelt voornamelijk de stabilisatie van

het mice1 zelf een belangrijke rol. Deze micelstabilisatie hangt af van de

precieze structuur van het surfactant. Zeer waarschijnlijk is het verschil in

neiging tot interactie tussen kationogene en anionogene surfactanten

gedeeltelijk gerelateerd aan verschillen in hydratatie tussen de zuurstofnjke

anionogene kopgroepen en de gangbare quartenaire ammoniumkopgroepen van de

kationogene surfactanten.