COUNTER ION EFFECTS ON THE KRAFFT TEMPERATURE AND … · counter-ion effects on the krafft temperature and micelle formation of ionic surfactants in aqueous solution. a dissertation

COUNTER-ION EFFECTS ON THE KRAFFT TEMPERATURE AND MICELLE FORMATION OF IONIC SURFACTANTS IN

AQUEOUS SOLUTION

A DISSERTATION SUBMITTED TO THE DEPARTMENT OF CHEMISTRY IN THE PARTIAL FULFILLMENT OF THE REQUIREMENT FOR THE DEGREE OF MASTER

OF PHILOSOPHY IN CHEMISTRY (PHYSICAL-INORGANIC)

M.Phil Thesis

SUBMITTED BY

Komol Kanta Sharker

St. ID-0413033209F

Session-April, 2013

Department of Chemistry Bangladesh University of Engineering and Technology

Dhaka-1000, Bangladesh September, 2016

I

II

III

ABSTRACT

In this work the effect of some sodium and chloride salts on the Krafft temperature

(TK) and critical micelle concentration (CMC) of two classical ionic surfactants,

Octadecyltrimethylammonium bromide (OTAB) and Sodium dodecyl sulfate (SDS) have

been investigated by conductometric and tensiometric method. Sodium salts of different

monovalent and divalent anions belonging to the Hofmeister series found to decrease or

increase the TK of OTAB. In terms of decreasing the TK the propensity follows the order:

C7H5O3− > C7H5O2

− > C6H5SO3− > SO4

2− > Cl− > NO3− > F− > Br− > SCN− > I−. The results

show that hydrotropic and kosmotropic counter-ions decrease while chaotropic counter-ions

increase the TK of the surfactant. Chloride salts of monovalent cation such as Li+, Na+, Cs+,

K+ affect the solubility of SDS and hence TK of the surfactant. Some salts increase while

some decrease the TK of the system. In terms of deceasing the TK the ions follows the trend:

Li+ > Na+ > Cs+ > K+. Added counter-ions screen the charge of the micelle head group and

facilitate closer packing of the surfactant. Thus added salts always decrease the CMC of the

surfactant. Different salts interact differently with surfactant and thus decrease the CMC

differently. For SDS the effectiveness in lowering the CMC the ions follows the order: Cs+ >

K+ > Na+ > Li+. On the other hand, in terms of OTAB the ions follow the following trends in

decreasing the CMC: C6H5SO3− > C7H5O2

− > C7H5O3− > SO4

2− > NO3− > Br− > Cl− > F−.

Thermodynamic parameters (Gibbs free energy, enthalpy and entropy changes) of

micellization and adsorption were calculated from the specific conductance and surface

tension data. The negative value of free energy change indicates the process to be

spontaneous. The enthalpy and entropy terms are found to compensate each other for both

micellization and adsorption. For most of the cases surface excess concentration (Г) was

found to be higher in presence of salts than pure surfactant showing lower equilibrium

surface tension of the system. The solubilization behavior of a water insoluble dye, Sudan

Red B (SRB), in the micellar system was studied by the UV–visible spectrophotometric

technique. The solubilization of SRB in OTAB in the presence of Na2SO4 was found to be

about 1.33 times higher than that in pure water. In the case of SDS the value was found to be

1.07 times in the presence of NaCl. This indicates that the solubilization of SRB in the

surfactant micelles significantly increases in the presence of added salts.

IV

Dedicated

To My Ever

Loving Parents,

Sweet Brother

And Sisters

V

ACKNOWLEDGEMENTS

In extreme humbleness I bow my head before supreme personality of Godhead Vagaban Shree

Krishna who created mankind in a most splendid manner and bestowed upon him a distinguished

honor in the form of knowledge.

I venture to get inspiration from an adage that knowledge is an ornament amongst friends and

armor against enemy and adore the historical day when I joined the august institution to acquire

knowledge. I feel elated in having successfully to accomplish my studies with the keep support

and guidance of many personages to whom I owe a depth of gratitude.

I fumble for the appreciate words to offer thanks and pay gratitude to my respectable and worthy

supervisor Prof. Dr. Md. Nazrul Islam, Department of Chemistry who always exhibited

commendable alacrity in providing me proper guidance combined with educative discussions and

suggestions whereby I was encouraged to complete my research work confidently. I would like to

thank him for always keeping his door open for me.

My respectable faculty teachers deserve praise and thanks for their educative and constructive

corrective suggestions whenever I needed.

I also would like to extend my heartfelt thanks to the Board of Examiners: Dr. Md. Nazrul Islam

(Chairman), Dr. Md. Rafique Ullah (Member, Ex-Officio), Dr. Md. Shakhawat Hossain Firoz

(Member), Dr. Mahbub Kabir (Member, External) for their corrective suggestions.

I reciprocate the respect and regards shown to me by lab fellows, the technical staff and office

bearers of Department and the unforgettable cooperation exhibited by them during my research

work. I am grateful and thankful to my friends, roommates, nears and dears who extended all

possible moral support and encouragement during my strenuous study period and prayed for me.

Without you, I couldn’t have such a joyful life in BUET.

The biggest of all of my acknowledgements goes to my family for getting me here. Your

sacrifices and encouragement has allowed me to be who I am. Without your constant support this

arduous task would never have met the fateful and fruitful end. Therefore, I would like to thank

my parents Modhu Shudon Sharker and Srimotee Sharker and my brother Mithun Chandra

Sharker for their support, encouragement, unselfish love and faith. I love you and am glad to

forever have your support. K. K. Sharker

September, 2016

VI

CONTENTS

Title Page No. DECLARATION I

CERTIFICATION OF THESIS II

ABSTRACT III

DEDICATION IV

ACKNOWLEDGEMENTS V

TABLE OF CONTENTS VI

LAYOUT OF THIS PAPER XIII

CHAPTER ONE: INTRODUCTION

1.1 SURFACTANTS AND ITS BULK AND INTERFACIAL PHENOMENA 1

1.2 TYPES OF SURFACTANTS 4

1.2.1 Anionic Surfactants 4

1.2.2 Cationic Surfactants 5

1.2.3 Nonionic Surfactants 5

1.2.4 Zwitterionic Surfactants 5

1.3 PHYSICAL STATE 7

1.4 PROPERTIES OF SURFACTANTS 7

1.4.1 Adsorption of Surfactants 7

1.4.2 Micellization 9

1.4.2.1 Micelle 9

1.4.2.2 Micellization Process 10

1.4.2.3 Critical Micelle Concentration 11

1.4.2.4 Factors affecting CMC in aqueous solution 13

1.4.2.5 Cooperative association process in Surfactants 14

1.4.2.6 Thermodynamics of micellization 17

1.4.2.7 Micellar Solubilization 20

VII

1.4.2.8 Solubilization Theory 21

1.4.2.9 Factors affecting solubilization 22

1.4.2.10 Reasons for self-aggregation of surfactant molecules 24

1.5 SURFACTANT SOLUBILITY 27

1.5.1 The Krafft temperature 27

1.5.2 The Cloud point 28

1.6 APPLICATION OF SURFACTANTS 29

1.6.1 Consumer Products 29

1.6.1.1 Detergents and dishwashing 29

1.6.1.2 Cosmetics and Personal Care Products 29

1.6.2 Industrial Applications 30

1.6.2.1 Food products 30

1.6.2.2 Pharmaceutical industry 30

1.6.2.3 Insecticides and herbicides 30

1.6.2.4 Agriculture 30

1.6.2.5 Textiles and fibers 31

1.6.2.6 Chemical industry 31

1.6.2.7 Plastics industry 31

1.6.2.8 Paints and lacquers 31

1.6.2.9 Cellulose and paper 31

1.6.2.10 Leather and furs 32

1.6.2.11 Photographic industry 32

1.6.2.12 Metal processing 32

1.6.2.13 Electroplating 32

1.6.2.14 Adhesives 32

1.6.2.15 Road construction and building materials 32

1.6.2.16 Firefighting 33

1.6.2.17 Mining and flotation 33

1.6.2.18 Oilfield chemicals 33

VIII

1.6.2.19 Cleaning agents 33

1.6.2.20 Other: Surfactants in biological systems 33

1.7 THE SCOPE AND OBJECTIVES OF THE PRESENT THESIS 34

REFERENCES 37

CHAPTER TWO: THEORY AND EXPERIMENTS

2.1 MATERIALS 42

2.1.1 Surfactants 42

2.1.2 Salts 42

2.1.3 Dye 43

2.2 METHOD 44

2.2.1 Measurement of Krafft Temperature 44

2.2.2 Measurement of Critical Micelle Concentration 45

2.2.3 Solubilization 47

REFERENCES 50

CHAPTER THREE: RESULTS AND DISCUSSION

3.1 EFFECT OF ELECTROLYTES ON KRAFFT TEMPERATURE 51

3.2 EFFECT OF ADDED SALTS ON SURFACE ADSORPTION

AND MICELLIZATION 61

3.3 SURFACE EXCESS CONCENTRATION 74

3.4 THERMODYNAMICS OF MICELLIZATION 77

3.5 THERMODYNAMICS OF SURFACE ADSORPTION 81

3.6 SOLUBILIZATION STUDY OF SUDAN RED B (SRB) 87

REFERENCES 94

CONCLUSIONS 99

APPENDIX 101

LISTS OF PUBLISHED PAPER 124

IX

LIST OF FIGURES

No. Title Page No. Figure 1.1: Typical surfactant structure 1

Figure 1.2: Adsorption of amphiphiles at the air/water interface and

micelle as formed by self-assembly of the monomer units 3

Figure 1.3: Different phase structure of self association of

surfactant monomer 10

Figure 1.4: Changes in the concentration dependence of a wide

range of physico-chemical changes around the

critical micelle concentration (CMC) 12

Figure 1.5: Effect of "N" on fraction of added surfactant that goes

to micelle 17

Figure 1.6: Relation between the solubilized material and

concentration of surfactant 21

Figure 1.7: The chemical and physical solubilization (incorporation) of

drugs within micelle 22

Figure 1.8: The Krafft temperature (TK) is the point at which surfactant

solubility equals the critical micelle concentration. Above

TK, surfactant molecules form a dispersed phase; below

TK, hydrated crystals are formed 28

Figure 2.1: Hydrated crystal in the beaker (left side) and arrangement

for Krafft temperature measurement

(right side: EUTECH CON 510 conductivity meter and

Froilabo RE 5 refrigerated bath circulator) 44

Figure 2.2: Surface tension measurement: Surface tensiometer

(Kruss K9) and refrigerated bath circulator (JSRC-13C) 46

Figure 2.3: Shaking of the surfactant solution with dye (Top: Stuart

Orbital shakers, SSL1) and solution after shaking (Below) 48

X

Figure 2.4: Jenway UV-spectrophotometer, model 7315 (Top) and a

spectrophotogram of SRB (Below) 49

Figure 3.1: Specific conductance vs. temperature plots of SDS in pure

water and in the presence of different electrolytes

at 0.005 ionic strength. (i) Pure SDS, (ii) LiCl, (iii) KCl,

(iv) CsCl, (v) NaCl. The sharp break point in the plot indicates

the Krafft Temperature 52

Figure 3.2: Specific conductance vs. temperature plots of OTAB in pure

water and in the presence of different electrolytes

at 0.005 ionic strength. (i) Pure OTAB, (ii) Na2SO4, (iii) NaBr,

(iv) NaF, (v) C6H5SO3Na, (vi) C7H5O2Na, (vii) NaNO3,

(viii) C7H5O3Na, (ix) NaCl. The sharp break point in the plot

indicates the Krafft Temperature 53

Figure 3.3: Effect of ionic strength of electrolytes on the Krafft

Temperature of SDS. (i) LiCl, (ii) NaCl, (iii) CsCl, (iv) KCl 55

Figure 3.4: Effect of ionic strength of electrolytes on the Krafft

Temperature of OTAB. (i) C7H5O3Na, (ii) C7H5O2Na,

(iii) Na2SO4, (iv) C6H5SO3Na, (v) NaF, (vi) NaNO3, (vii) NaCl,

(viii) NaBr, (ix) NaSCN, (x) NaI 56

Figure 3.5: Conductometric determination of CMC of SDS in pure water

at 30°C 62

Figure 3.6: Conductometric determination of CMC of SDS in the

presence of 0.005M NaCl solution at 30°C 62

Figure 3.7: Conductance vs. surfactant concentration plot for OTAB

in aqueous solution at 40°C 63

Figure 3.8: Conductance vs. surfactant concentration plot for OTAB in

the presence of 0.005M NaCl solution at 40°C 63

Figure 3.9: Surface tensiometric determination of CMC of SDS

in pure water at 30°C 65

XI

Figure 3.10: Surface tensiometric determination of CMC of SDS in the

presence of 0.005M NaCl solution at 30°C 65

Figure 3.11: Surface tension vs. Log10C plot for OTAB in aqueous solution

at 40°C 66

Figure 3.12: Surface tension vs. Log10C plot for OTAB in the presence

of 0.005M NaCl solution at 40°C 66

Figure 3.13: surface excess concentration of SDS (i) in pure and

(ii) in 0.005M aqueous solution of NaCl 75

Figure 3.14: surface excess concentration of OTAB (i) in pure and

(ii) in 0.005M NaCl solution. 75

Figure 3.15: Enthalpy-Entropy compensation plot for (a) Micellization

(b) surface Adsorption of SDS in aqueous solution 84

Figure 3.16: Enthalpy-Entropy compensation plot for (a) Micellization

(b) surface Adsorption of OTAB in aqueous solution 85

Figure 3.17: Effect of surfactant concentration on the absorption spectra

of SRB: i 0.4, ii 0.6, iii 1.0, iv 1.5 and v 2.0 mM OTAB

solutions in pure water 88


of SRB: i 0.06, ii 0.1, iii 0.2, iv 0.4 and v 0.8 mM OTAB

solutions in 0.005 ionic strength Na2SO4 88


of SRB: i 8, ii 9, iii 10, iv 15, v 20 and vi 30 mM SDS solutions

in pure water 89


of SRB: i 6, ii 7, iii 8, iv 9, v 10 and vi 20 mM SDS solutions

in 0.005M NaCl 89

Figure 3.21: Solubilization of SRB in OTAB solution in (a) pure water and

(b) 0.005 ionic strength aqueous Na2SO4 solution. The break

point in the curve shows the CMC below which no significant

XII

absorbance was observed. This indicates the SRB solubilized

only when OTAB forms micelles 90

Figure 3.22: Solubilization of SRB in SDS solution in (a) pure water and

(b) 0.005M NaCl solution. The break point in the curve shows

the CMC below which no significant absorbance was

observed. This indicates the SRB solubilized only when SDS

forms micelles 91

LIST OF TABLES

No. Title Page No.

Table 1.1: Some representative examples of surfactant 6

Table 3.1: CMC values of OTAB at different temperatures in pure water

and in the presence of 0.005 ionic strength solutions of

several electrolytes 70

Table 3.2: CMC values of SDS at different temperatures in pure water

and in the presence of 0.005 ionic strength solutions of some

electrolytes 71

Table 3.3: Thermodynamic parameters of adsorption and micellization*

of the SDS surfactants solution. 78


of the SDS – 0.005M NaCl surfactants solution. 78


of the OTAB surfactants solution. 79


of the OTAB – 0.005M NaCl surfactants solution. 79

Table 3.7: Tc value for OTAB and SDS in water and 0.005M NaCl solution 86

Table 3.8: Molar Solubilization Ratio (MSR) values of SRB in SDS 93

Table 3.9: Molar Solubilization Ratio (MSR) values of SRB in OTAB 93

XIII

LAYOUT OF THIS DISSERTATION

This thesis paper has been divided into three chapters-

👉 Chapter one presents a general introduction. Here review of some earlier

research works is given for present investigation. Objectives of the present work

are also described in this chapter.

👉 Theory and experimental procedures are explained in chapter two.

👉 Experimental results and discussions are presented in chapter three. The

conclusions of this research work have also been discussed here.

👉 References are added at the end of the respective chapter.

👉 Appendix was also included at the end of this thesis paper.

👉 List of publications related to the present work have also been mentioned at the

very end of this thesis paper.

XIV

“Education is the most powerful weapon which you can use to

change the world”.

……. Nelson Mandela

Chapter One

Introduction

Introduction

1

1.1 SURFACTANTS AND ITS BULK AND INTERFACIAL

PHENOMENA

Surfactants are compounds that lower the surface tension of the liquid, the interfacial

tension between two liquids or interfacial tension between a liquid and solid. Surfactants

can act as wetting agents, emulsifiers, foaming agents and dispersants. For this reason

they are used in vast amounts in domestic and industrial applications such as in soaps,

detergents, paints, dyestuffs, paper coatings, inks, plastics and fibers, personal care and

cosmetics, agrochemicals, pharmaceuticals, food processing, oil industry, etc. [1-3].

They are amphiphilic molecules and carry in the same molecule two moieties of

completely different properties: one moiety is polar and hydrophilic; the other is nonpolar

and hydrophobic (Figure 1.1). Therefore, these molecules contain both a water soluble

and water insoluble (or oil soluble) component. Soap molecules made up of long

hydrocarbon chain (hydrophobic) ending with a carboxyl group (polar) is a good example

of an amphiphile molecule. Because of its dual affinity, an amphiphilic molecule does not

feel "at ease" in any solvent, be it polar or non-polar, since there is always one of the

groups which "does not like" the solvent environment. This is why amphiphilic

molecules exhibit a very strong tendency to migrate to interfaces or surfaces and to

orientate so that the polar group lies in water and the non-polar group is placed out of it,

and eventually at the air-water or oil-water interface [4-6].

(Head Group) (Tail Group)

Figure 1.1: Typical surfactant structure

Introduction

2

When surfactants are dissolved in water less work is required to bring a surfactant

molecule to the surface than a water molecule, as migration of the surfactant to

the surface is a spontaneous process. So these molecules are strongly attracted to

and accumulate (adsorb) at the air/water interface or the particle (assumed

hydrophobic)/water interface. As a result of their molecular structure, the molecules

orientate themselves with the hydrophilic part pointed toward water (polar) and the

hydrophobic part away from it. This results in the formation of an oriented monolayer of

the amphiphiles at the interface as shown in Figure 2. This strong tendency of the

amphiphiles to adsorb at an interface is termed surface activity and amphiphiles are also

known as surface active agents (SAA) or surfactants [5, 7]. However, high density

condensed phase formation in adsorbed monolayer sometimes becomes difficult due to

electrostatic repulsion, bulkiness as well as strong hydration of the polar head group. In

such a case, hydrophobic interactions among the alkyl chains make it more favorable to

remain in the bulk of the aqueous solution by forming colloidal sized clusters in solution,

known as micelles and the concentration of monomeric amphiphile at which micelles

appear is called the critical micelle concentration (CMC). The CMC is an important

characteristic of a surfactant [8]. Below this concentration surfactant molecules remain as

single molecule but above this concentration they aggregate as micelles [9]. Thus, the

CMC represents a phase separation between single molecules of surfactant and surfactant

aggregates in dynamic equilibrium [10]. Below the CMC micelles are not present and

adsorption is a dynamic equilibrium with surfactant molecules perpetually arriving at,

and leaving the surface. Above the CMC, the concentration of unaggregated surfactant

will stay constant and the number of micelles will increase as the total surfactant

concentration increases and the system then consists of an adsorbed monomolecular

layer, free monomers and micellised surfactant in the bulk, with all these three states in

equilibrium [11]. [Figure1.2]

A typical micelle in aqueous solution forms with the hydrophilic head regions in contact

with the water and the hydrophobic aliphatic tail regions buried in the inner portion of the

micelle [11]. It is believed that surfactant molecules or ions are associated in micelles

because the forces that act between polar water molecules exceed the forces that act

Introduction

3

between hydrocarbon chains and water. Therefore, the transfer of hydrocarbon chains

from water into a phase close to them in polarity is energetically favorable [12].

An immediate consequence of the adsorption of surfactant molecules at an interface is

that its interfacial energy is reduced. For a water surface covered with a monolayer of

surfactant molecules, its surface tension is very much lower than that of clean water

surface [13].

Surface tension occurs when water molecules on a surface bond very tightly to other

water molecules both next to and below them. When surfactants are dissolved in water

they form a monolayer upon spontaneous adsorption at the air-water interface [14] and do

not completely mix with water but are able to bond to water and prevent water molecules

from binding as tightly to one another, thus lowering the tension or strength of the

surface.

Figure 1.2: Adsorption of amphiphiles at the air/water interface and micelle as formed by self-assembly of the monomer units

Introduction

4

Below the CMC surfactants tend to accumulate at the interface, reducing surface tension.

At CMC, the surface tension of the solution does not change but remains constant,

as the gas-liquid interface is already fully packed with the surfactant molecules.

Above the CMC, most of the surfactant molecules are inside the bulk aggregating

into micelles. When this occurs, the addition of surfactants just increases the number of

micelles and the surface tension becomes independent of surfactant concentration [15].

1.2 TYPES OF SURFACTANTS

Surfactants may be classified according to their applications (emulsifiers, foaming agents,

wetting agents, dispersants etc.), some physical characteristics (water and oil solubility

and stability) and chemical structure of both the head and tail group of surfactants. The

head group can be charged or neutral, small and compact in size, or a polymeric chain.

The tail group is usually a single or double, straight or branched hydrocarbon chain, but

may also be a fluorocarbon, or a siloxane, or contain aromatic group(s).

Since the hydrophilic part normally achieves its solubility either by ionic interactions or

by hydrogen bonding, the simplest classification is based on surfactant head group type,

with further subgroups according to the nature of the lyophobic moiety. Four basic

classes therefore emerge as: Anionic, Cationic, Nonionic and Zwitterionic [16-19].

1.2.1 Anionic Surfactant

Anionic surfactants are dissociated in water into two oppositely charged species anion

(the surfactant ion) and cation (counter ion). Carboxylate, sulfate, sulfonate and

phosphate are the polar groups found in anionic surfactants. The counterions most

commonly used are sodium, potassium, ammonium, calcium and various protonated alkyl

amines. One main reason for their popularity is the ease and low cost of manufacture.

Anionics are used in most detergent formulations and the best detergency is obtained by

alkyl chains in the C12-C18 range. They are by far the largest surfactants class. They are

generally sensitive to hard water. Sensitivity decreases in the order carboxylate >

phosphate > sulfate ≅ sulfonate.

Introduction

5

1.2.2 Cationic Surfactant

Cationic surfactants are dissociated in water into an amphiphilic cation and an anion,

most often of the halogen type. A very large proportion of this class corresponds to

nitrogen compounds such as fatty amine salts and quaternary ammoniums, with one or

several long chain of the alkyl type, often coming from natural fatty acids. These

surfactants are in general more expensive than anionics and are only used in which there

is no cheaper substitute. They are the third largest surfactants class. They adsorb strongly

to most surfaces and their main uses are related to in situ surface modification.

1.2.3 Nonionic Surfactants

Nonionic surfactants do not ionize in aqueous solution, because their hydrophilic group is

of a non-dissociable type, such as alcohol, phenol, ether, ester, or amide. A large

proportion of these nonionic surfactants are made hydrophilic by the presence of a

polyethylene glycol chain, obtained by the polycondensation of ethylene oxide. They

are called polyethoxylated nonionics. The polycondensation of propylene oxide produce

a polyether which (in opposition to polyethylene oxide) is slightly hydrophobic. This

polyether chain is used as the lipophilic group in the so-called polyEOpolyPO block

copolymers, which are most often included in a different class, e.g. polymeric surfactants.

They are the second largest surfactant class. They are normally compatible with all other

types of surfactants. They are not sensitive to hard water. Their physicochemical

properties are not markedly affected by electrolytes. Contrary to ionic compounds they

become less water soluble-more hydrophobic.

1.2.4 Zwitterionic Surfactant

When the headgroup of a surfactant molecule contain both a negative and a positive

charge it is called amphoteric or zwitterionic. Whereas the positive charge is almost

invariably ammonium, the source of negative charge may vary, although carboxylate is

by far the most common. Some amphoteric surfactants are insensitive to pH, whereas

others are cationic at low pH and anionic at high pH, with an amphoteric behavior at

intermediate pH. Amphoteric surfactants are generally quite expensive, and consequently,

Introduction

6

their use is limited to very special applications such as cosmetics where their high

biological compatibility and low toxicity is of primary importance. They are the smallest

surfactant class. They are compatible with all other classes of surfactants. They are not

sensitive to hard water. Most types show very low eye and skin irritation. They are

therefore well suited for shampoos and other personal care products.

The past two decades have seen the introduction of a new class of surface active

substance, so-called polymeric surfactants or surface active polymers, which result from

the association of one or several macromolecular structures exhibiting hydrophilic and

lipophilic characters, either as separated blocks or as grafts. They are now very

commonly used in formulating products as different as cosmetics, paints, foodstuffs, and

petroleum production additives. Recently, there has been considerable research interest in

certain dimeric surfactants, containing two hydrphobic tails and two head groups known

as gemini surfactants, which have efficiency in lowering surface tension and very low

CMC. Some representative surfactants along with their chemical formulae are listed in

Table 1.1.

Table 1.1: Some representative examples of surfactant

Class Examples Molecular structure

Anionic

Sodium stearate CH3(CH2)16 - COO‾Na+

Sodium dodecyl sulfate CH3(CH2)11 - SO4‾Na+

Sodium dodecyl benzene sulphonate CH3(CH2)10C6H4 - SO3‾Na+

Cationic Laurylamine hydrochloride CH3(CH2)11NH3+Cl‾

Hexadecyltrimethylammonium bromide CH3(CH2)15N+(CH3)3Cl‾

Tetradecyltrimethylammonium bromide CH3(CH2)13N+(CH3)3Cl‾

Non-ionic Polyoxyethylene(4)dodecanol CH3(CH2)11-O-(CH2CH2O)4H

Polyoxyethylene(9)hexadecanol CH3(CH2)15-O-(CH2CH2O)9H

Zwitterionic Dodecyl betaine C12H25N+(CH3)2CH2COO‾

Dodecyldimethylammonium acetate CH3(CH2)11(CH3)2N+CH2COO‾

Gemini Bis (quaternary ammonium bromide) C12H25N+(CH3)2-(CH2)8-

N+(CH3)2C12H25 2Br‾

Introduction

7

1.3 PHYSICAL STATE

Ionic surfactants are generally amorphous or crystalline solids and nonionic surfactants

are liquid or solid. Crystalline surfactants can be prepared relatively purely. They can be

polymorphic, if their structures have different unit cell, or polytypic if their structures

have one dimensional polymorphism. Amorphous solids are surfactants that have one or

more chiral centres and exist in multiple optical isomers. Liquid crystalline surfactants

exhibit properties common to crystalline and to liquid physical state. Liquid surfactants

are fundamentally amorphous with no long range order and are typically isotropics.

1.4 PROPERTIES OF SURFACTANTS

Surfactants distort water structure and raise free energy of solution. The system, however,

has natural tendency to minimize its free energy. To satisfy this natural desire the system

may undergo- (A) Adsorption (B) Micellization

1.4.1 Adsorption of Surfactants

Adsorption is an entropically driven process by which molecules diffuse preferentially

from a bulk phase to an interface. Due to the affinity that a surfactant molecule

encounters towards both polar and non-polar phases, thermodynamic stability (i.e. a

minimum in free energy or maximum in entropy of the system) occurs when these

surfactants are adsorbed at a polar/non-polar (e.g. oil/water or air/water) interface. Due to

its amphiphilic structure, the surfactant can adsorb onto interfaces and lower the tension

(γ) of the interfaces. The adsorption dynamics, i.e. the time-dependent adsorption process

of surfactant molecules onto interfaces, is of significant importance in lots of applications

including foaming, emulsifying and coating processes, in which bubbles, drops or films

are rapidly formed [20-22]. The surfactant adsorption process from the bulk to the

air/water interface can be divided into two: the motion of the surfactant molecules from

the bulk to the sub-surface and the transfer of molecules from the sub-surface to the

air/water interface [23-25]

Due to the different environment of molecules located at an interface compared to those

from either bulk phase, an interface is associated with a surface free energy. At the air-

Introduction

8

water surface for example, water molecules are subjected to unequal short-range

attractive forces and thus, undergo a net inward pull to the bulk phase. Minimisation of

the contact area with the gas phase is therefore a spontaneous process, explaining why

drops and bubbles are round. The surface free energy per unit area, defined as the surface

tension (γo), is then the minimum amount of work (Wmin) required to create new unit area

of that interface (∆A), so Wmin= γo × ∆A. Another, but less intuitive, definition of surface

tension is given as the force acting normal to the liquid-gas interface per unit length of

the resulting thin film on the surface [15].

A surface-active agent is therefore a substance that at low concentrations adsorbs thereby

changing the amount of work required to expand that interface. In particular surfactants

can significantly reduce interfacial tension due to their dual chemical nature. Considering

the air-water boundary, the force driving adsorption is unfavourable hydrophobic

interactions within the bulk phase. There, water molecules interact with one another

through hydrogen bonding, so the presence of hydrocarbon groups in dissolved

amphiphilic molecules causes distortion of the solvent structure apparently increasing the

free energy of the system. This is known as the hydrophobic effect [26].

Less work is required to bring a surfactant molecule to the surface than a water molecule,

so that migration of the surfactant to the surface is a spontaneous process. At the gas-

liquid interface, the result is the formation of an oriented suractant monolayer with the

hydrophobic tails pointing out of, and the head group inside, the water phase. The

balance against the tendency of the surface to contract under normal surface tension

forces causes an increase in the surface (or expanding) pressure π, and therefore a

decrease in surface tension γ of the solution. The surface pressure is defined as π = γo − γ,

where γo is the surface tension of a clean air-water surface.

Depending on the surfactant molecular structure, adsorption takes place over various

concentration ranges and rates, but typically, above a well-defined concentration – the

critical micelle concentration (CMC) – micellisation or aggregation takes place. At the

CMC, the interface is at (near) maximum coverage and to minimise further free energy,

molecules begin to aggregate in the bulk phase. Above the CMC, the system then consists

Introduction

9

of an adsorbed mono-molecular layer, free monomers and micellised surfactant

molecules in the bulk, with all these three states in equilibrium.

1.4.2 Micellization

1.4.2.1 Micelle

The solubility pattern with respect to solvent properties of a non-polar compound like

alkane is in sharp contrast to that of a charged or otherwise strongly polar chemical

species. If these two features occur simultaneously in the same chemical entity, an

interesting phenomenon is observed. For aqueous solutions, one well known situation is

that the polar group is located in the solution while the nonpolar part seeks to avoid the

aqueous environment by stretching into the gas phase or into an adjacent non-polar liquid

phase. Except for this adsorption at gas –liquid, liquid-liquid or liquid-solid interfaces

there is an alternative possibility to avoid the unfavorable contact between non-polar

groups and water and between polar groups and non-polar solvent, i.e. by self-association

into various types of aggregates (Figure 1.3). The term micelle is introduced by the

pioneer in the field J.W. McBain in 1913 to describe the formation of colloidal properties

by detergents and soaps [27].

The word “micelle” has also been used in biology and in colloid chemistry for other

phenomena. Important features of the micelle are the high aggregation number and

effective separation of hydrophilic and hydrophobic part. It was established at an early

stage that micelle formation displays peculiar concentration dependence. Thus at low

concentration an aqueous ionic surfactant solution behaves essentially as a strong

electrolyte. On the other hand, an increased amphiphile concentration leads to a

corresponding increase in the amount of micelles while the monomer concentration stays

roughly independent of the total amphiphile concentration. Under these circumstances,

pronounced changes in the concentration dependence of a large number of properties

occur at the CMC.

The existence of micelles in a solution is an important parameter due to a number of

important interfacial phenomena, such as detergency and solubilization. Furthermore,

micelles have become a subject of great interest in the fields of organic chemistry and the

Introduction

10

biochemistry because of their unusual catalysis of organic reactions and their similarity to

biological membranes and globular proteins. Aggregation is not, however, just limited to

aqueous solution; it is sometimes observed in non-aqueous polar solvents such as

ethylene glycol and non-polar solvents such as hexane [15].

Figure 1.3: Different phase structure of self association of surfactant monomer

1.4.2.2 Micellization Process:

The aggregation phenomenon of amphiphilic molecules involves contributions from both

repulsive and attractive interactions. Especially, in ionic surfactants, the repulsive forces

originated primarily from electrostatic repulsion between the polar head groups [28],

whereas attractive interactions have generally been attributed to hydrophobic interactions

between the non-polar tails of the surfactant monomers [29]. However, in this context a

considerable emphasis has been given to the London dispersion interactions [30-31].

These interactions depend on various factors such as temperature, dielectric constant of

the medium, length of the alkyl chain, presence of additives and relative size and charge

of the headgroup [32-33]. The formation of micelles and its dependence on different

factors such as temperature, additives, dielectric constant of the medium, the extent

of counter-ion binding (for ionic surfactants), solubilization etc. are important

Introduction

11

physicochemical aspects that need detailed and intensive attention for both fundamental

understanding and industrial applications. The dominance of the favorable interaction

between alkyl chains of the surfactant favors micellization and lead CMC to lower

values by stabilizing micelles while the opposing repulsive interaction between the

polar/charged head groups disfavor micellization and leads CMC to higher values [32].

To differentiate among these different kinds of intractions, the surfactant solution

properties, such as critical micellar concentration (CMC), micelle shape and size,

solubility and Krafft temperature have been considerably important [34]. Micelles are

known to have an anisotropic water distribution within their structure. In other words, the

water concentration decreases from the bulk towards the interior of the micelle, with a

completely hydrophobic-like interior. Thus, micellar solution consists of special medium

in which hydrophobic organic compounds can be solubilized in aqueous surfactant

solution, which are otherwise insoluble in water [35-36]. At low concentration in water,

surfactants exist mostly as monomers [37]. At higher concentrations, the surfactants

molecules grouped together in a manner that their hydrophobic tails (usally an n-alkyl

hydrocarbon chain containing 8 to 18 methylene groups) tend to coaggregate to form

more or less spherical micelles with hydrocarbon chains forming a core and the polar

hydrophilic heads on the surface providing protection. A major source of stability of

micelle is the existence of an electric charge on their surface. On account of this charge,

ions of opposite charge tend to cluster nearby, and an ionic atmosphere is formed.

1.4.2.3 Critical Micelle Concentration

The change in surface properties as the concentration of an aqueous solution of a

surfactant rises is characteristic of most surface active molecules. During earlier studies

of the solution properties of surfactants, it was recognized that the bulk solution

properties of these materials were unusual and could change abruptly over a very small

concentration range, indicating the presence of colloidal particles in the solution [39].

Equivalent conductance of any ionic surfactant, plotted against the square root of its

concentration gives a curve instead of smooth curve characteristic of ionic electrolyte

[Figure 1.4]. This sharp break in the conductivity of the solution indicates a sharp

increase in the mass per unit charge of material in solution. That is interpreted as

Introduction

12

evidence of the formation of micelles from the monomeric surfactant molecules with part

of the charge of the micelle neutralized by associated counter ions. The threshold

concentration at which micellization begins is known as the critical concentration.

Similar behavior in almost all measurable physical properties is observed by all types of

surface active materials (anionic, cationic, nonionic, zwitterionic) which depend on size

or number of particles in solution [Figure 1.4].

Phillips [38] had used that CMC is the concentration at which the properties of the

surfactant solution changes in the most abrupt manner, i.e

𝑑3𝜑

𝑑𝑐3 = 0

where φ is any additive property which varies linearly with the concentration of

micellized end of unassociated surfactant. The discovery of this discontinuity in physical

properties and reasons for it were first described by McBain [39] in 1920s and there has

been a considerable volume of work on the subject since then.

Figure 1.4: Changes in the concentration dependence of a wide range of physico-chemical changes around the critical micelle concentration (CMC)

Introduction

13

1.4.2.4 Factors affecting CMC in aqueous solution

(i) The Hydrophobic Group

The length of the hydrocarbon chain is a major factor determining the CMC. For a

homologous series of linear single-chain surfactants the CMC decreases logarithmically

with carbon number. Interestingly, for straight-chain dialkyl sulfosuccinates the value is

double than that for the single chain compounds. Alkyl chain branching and double

bonds, aromatic groups or some other polar character in the hydrophobic part produce

noticeable changes in the CMC. In hydrocarbon surfactants, chain branching gives a

higher CMC than a comparable straight chain surfactant [15], and introduction of a

benzene ring in the chain is equivalent to about 3.5 carbon atoms [5].

(ii) The Hydrophilic Group

For surfactants with the same hydrocarbon chain, varying the hydrophile nature (i.e.,

from ionic to non-ionic) has an important effect on the CMC values. Ionic surfactants

have much higher CMC than nonionic surfactants containing equivalent hydrophobic

groups. For instance, for a C12 hydrocarbon the CMC with an ionic headgroup lies in the

range of 1 ×10-3mol dm-3, while a C12 non-ionic material exhibits a CMC in the range of

1 ×10-4mol dm-3.

(iii) Temperature

The effect of temperature on the CMC of surfactants in aqueous medium is complex.

Rosen [15] pointed out that the value appearing first to decrease with the temperature to

some minimum and then to increase with further increase in temperature. The increase of

the temperature causes decrease of the hydration of the hydrophilic group, which favors

the micellization. However temperature increase also causes disruption of the structured

water surrounding of the hydrophobic group, an effect that disfavors micellization. The

relative magnitude of these two opposing effects, therefore, determines whether the CMC

increases or decreases over a particular temperature range. From the data available in the

literature, the minimum in the CMC temperature curve appears to be around 25oC for

ionic surfactants [40] and around 50oC for nonionic [41].

Introduction

14

(iv) Salts

Addition of neutral salts to an aqueous solution of surfactant usually decreases the CMC

of ionic surfactants. This effect is less pronounced when the surfactants is nonionic. Salts

tend to screen electrostatic repulsions between headgroups and make the surfactant

effectively more hydrophobic. This increases hydrophobic interactions among the

surfactants cause them to aggregate at a lower concentration, thereby the CMC decreases

[42].

1.4.2.5 Cooperative association process in Surfactants

When surfactants associate into micelles, they form a liquid like aggregate. As there is no

specific mechanism related to specific aggregation number, the association of monomers

into micelles is described as stepwise addition of a monomer, S to the aggregate, Sn-1 as

in

S + Sn-1 ⇋ Sn (1)

By neglecting additional interactions between aggregates and between monomers, the

equilibrium would be

Kn = [Sn ]

S [Sn−1] (2)

This equation gives description of any stepwise association process in dilute solution. In

the case of aggregation, n of order 100, there would be a number of intractable

equilibrium constants Kn. However, because it is almost impossible to specify all the Kn

equilibrium steps, approximate model of micellization are being used.

(i) Isodesmic model:

In this model it is assumed that Kn is independent of n where regardless of either the total

concentration or of K, [S] K < 1. The aggregation distribution function

Introduction

15

f(n) = [Sn ]

[Sn ]∞n =1

(3)

decays exponentially with [S1] > [S2] > [Sn].

In this model, aggregation is a continuous process that does not show the abrupt onset in

a narrow concentration range, which typifies micelle formation. Isodesmic model

describe the association of dyes in aqueous solution quite well but it is less successful as

a description of the formation of micelles because the model does not predict a CMC. Its

basic shortcoming lies in making Kn independent of n and thus depriving the process of

cooperativity.

(ii) Phase separation model

In this model aggregation is approximated as a phase separation process in which the

activity of the monomer remains constant above the CMC. Micelle formation having

several features in common with the formation of a separate liquid phase provides basis

for this model in which micelles formally constitute a separate phase. In terms of

association described in equation (1), the phase separation model assumes that aggregates

with large n, dominate all others except the monomer. This assumption implies strong

cooperativity because, once aggregation has started, and it becomes more and more

favorable to add another monomer until a large aggregation number is reached. In the

pseudo separate phase, the surfactant possesses a certain chemical potential µ° (micelle)

in the aggregates when monomers and aggregates coexist in equilibrium

µ°(micelle) = µθ(solvent) + RT ln[S] (4)

[S] is the CMC (neglecting dimers and oligomers). The standard free energy of micelle

formation ∆G°mic represents the standard free energy difference between a monomer in

the micelle and the standard chemical potential in dilute solutions and

∆G°mic= µ°(micelle) - µθ(solvent) = RT lnCMC (5)

This equation provides a useful approximation for obtaining ∆ G°mic. This phase

separation model captures several but not all essential features of micelle formation. It

Introduction

16

describes the start mechanism of the self-assembly process but does not describe the stop

mechanism [43].

(iii) Closed association model

This model describes both start and stop features of micellization. It is assumed that

aggregation number dominates with only monomers and N-aggregates

NS ⇋ SN (6)

KN = [SN ]

[S]N (7)

The total surfactant concentration in terms of model of monomers is

[S]T = N[SN] + [S] = NKN[S]N + [S] (8)

This KN can be related to other equilibrium constant in equation (2) as

KN = KnN2 (9)

Fraction of added surfactants enters into an aggregate is given by derivative

∂{N SN }

∂[S]T (10)

Figure 1.5 shows three curves with varying values of N, the larger the N value, the more

abruptly the derivative 𝜕 𝑁 𝑆𝑁 /𝜕[𝑆]𝑇 changes from a low concentration value of zero

to the high concentration value of unity. When N → ∞, discontinuity in the derivative at

CMC is regained. As the aggregation number, N increases, the fraction of added

surfactant that goes to the micelle varies more and more steeply with total concentration

[S]T. In the limiting case in which the aggregation number becomes infinite the transition

becomes a step function that unambiguously defines the CMC while small aggregation

numbers to less defined values of CMC. In the case of ionic surfactant an equilibrium

between surfactant monomers, S, counterions, C+ and micelles, SN is written as

(N−P)C+ + N𝑆− ⇋ 𝑆𝑁−𝑃 (11)

Introduction

17

for which

KN = [SN−P ]

[𝑆−]N [C+]N−P (12)

1.4.2.6 Thermodynamics of micellization

To evaluate the thermodynamic parameters of micellization two approaches are generally

used: the phase separation model [44] and mass action model or the equilibrium model

[45]. If, however the aggregation number of the micelle is small, the mass action model is

used, while if the aggregation number is large, the phase separation model is applied.

According to the mass action model, the micelles and monomeric species are considered

to be in a kind of chemical equilibrium, while in phase separation model, the micelles are

considered to constitute a new phase formed in the system at and above the critical

micelle concentration. In each case classical thermodynamic approaches are used to

describe the overall process of micellization. Analysis of both approaches produces the

same general results in terms of the energetic of micelle formation.

Figure 1.5: Effect of "N" on fraction of added surfactant that goes to micelle

2

30 3

Introduction

18

In the case of ionic surfactants the equilibrium model is preferable because it is possible

to take into consideration, in an explicit way, the effect of the counterion dissociation.

The equilibrium model considers that the micellization process can be described by

equilibrium between monomers, counterions, and monodisperse micelles. In the case of a

cationic surfactant this equilibrium can be represented by

nS+ + (n−p)C− ⇋ Mp+ (13)

The corresponding equilibrium constant can be written as

K = [M𝑝+]

[S+]𝑛 [C−]𝑛−𝑝

or, lnK = ln[M𝑝+] − 𝑛ln S+ − (𝑛 − 𝑝)ln[C−]

or, RT lnK = RT (ln[M𝑝+] − 𝑛ln S+ − (𝑛 − 𝑝)ln[C−])

where S+ represents the surfactant cations, C− the corresponding counterions, and Mp+ the

micelle formed by n monomers with an effective charge of p. The standard free energy

of micellization per mole of surfactant, ∆G°mic, is given by

or, ∆G°mic = RT − 1

𝑛ln 𝑎M𝑃+ + ln𝑎S+ + 1 −

𝑝

𝑛 ln𝑎C− [∆G° = −RT lnK] (14)

where a is the activity of the respective species. For large n values the first term of the

parenthesis is negligible and both aS+ and aC− can be replaced by the activity at the

CMC.

Moreover, since the micellar formation occurs in dilute solutions, the activity can be

replaced by the surfactant concentration (expressed in mole fraction) at the CMC.

Considering these approximations, Eq. (14) can be expressed as [46]

∆G°mic = (2 – 𝛽) RT ln XCMC (15)

Where 𝛽 = 𝑃

𝑛 is the degree of dissociation of the counterion binding. For a completely

ionized micelle, β = 1 and for neutral 𝛽 = 0.

Introduction

19

Counterions drawn into the regions of charged head groups reduce the repulsive

electrostatic interactions between them, and this is the heuristic physical basis for the

model of counterion binding. In the case of ionic surfactants the relative contribution of

enthalpy and entropy determines the temperature dependence of the CMC. Since the

thermodynamic parameters are related by the Gibbs-Helmholtz equation, ∆𝐺°mic can be

separated into its enthalpic and entropic components

∆𝐺°mic = ∆𝐻°mic – T∆𝑆°mic (16)

For the cases when the aggregation number and the degree of ionization are temperature

dependent. In classical thermodynamics, ∆𝐻°mic is also given by the relation

∆𝐻°mic = – RT2 2 − 𝛽 ∂ ln XCMC

∂T .p− ln XCMC

∂𝛽

∂T .p (17)

If the change in β with temperature is small, Eq. (17) yields

∆𝐻°mic = − 2 − 𝛽 RT2 ∂ ln XCMC

∂T .p (18)

In this way, the enthalpy of micellization can be evaluated from the slope of a tangent to

a plot of ln XCMC versus T at a particular temperature. Once ∆𝐺°mic and ∆𝐻°mic have been

obtained, the entropy of micellization can be estimated from Eq. (16).

T∆𝑆°mic = ∆𝐻°mic − ∆𝐺°mic

The micellization process is governed primarily by the entropy gain associated with it

and the driving force for the process is the tendency of the lyophobic group of the

surfactant to transfer from the solvent environment to the interior of the micelle [47].

The increased freedom of the hydrophobic chain in the nonpolar interior of the micelle

compared to the aqueous environment plays an important role in entropy of micellization.

Any structuranl or environmental factors that may affect solvent-lyophobic group

interactions or interactions between the lyophobic groups in the interior of the micelle,

therefore, affect ∆𝐺°mic and consequently the value of the CMC.

Introduction

20

1.4.2.7 Micellar Solubilization

An important property of micelles is their ability to increase the solubility of sparingly

soluble or insoluble substances in water. Solubilization, as defined by McBain and

Hutchinson [48, 49], is a particular mode of bringing into solution of substances that are

otherwise insoluble in a given medium, involving the previous presence of colloidal

solution whose particles take up and incorporate within or upon themselves the

otherwise insoluble material. Solubilization by micelles is of importance in many

industrial processes such as detergency, micellar catalysis and extraction, emulsion,

polymerization, oil recovery, etc. [50] and in a variety of fundamental research oriented

studies like micellar modeling of biological membrane [11].

Below the CMC surfactant molecules exist as monomers and have only little or no

influence on the solubility of water-insoluble compounds but above this concentration

solubility increases sharply with surfactant concentration. If the solubility of a normally

solvent-insoluble materials is plotted against the concentration of the surfactant solution,

the solubility is very limited at concentrations below the CMC of the surfactant but rise

abruptly, once the CMC has been reached as shown in Figure 1.6. This indicates that

solubilization is a micellar phenomenon.

In solubilization, the solubilized material is in the same phase as the solubilizing solution,

and the system is consequently thermodynamically stable. The extent of solubilization

depends on many factors such as the structure of the surfactant, aggregation number,

micellar geometry, and temperature, ionic strength of the medium and the nature of the

solubilizate. The locus of solubilization of poorly water-soluble compounds in micellar

systems depends on the polarity of solubilizate. Non-polar molecules are solubilized in

the micelle core and substances with intermediate polarity are distributed along surfactant

molecules in certain intermediate position [50]. An increase in surfactant concentration in

solution increases the extent of solubilization of hydrophobic solutes because of an

increase in the number of micelles in the bulk. The solubilizing capacity of a surfactant is

usually expressed quantitatively by molar solubilization ratio (MSR). The MSR can be

expressed as the number of moles of the substance solubilized per mole of the surfactant

in solution [51].

Introduction

21

Figure 1.6: Relation between the solubilized material and concentration of surfactant

1.4.2.8 Solubilization Theory

The formation of additive-surfactant aggregates in the micellar solution can also be

explained based on solubilization theory [52]. The stepwise association between an

additive (D) molecule and the micelle (M) gives rise to the following equilibria

M+D ⇋ MD1

MD1 + D ⇋ MD2

MDm-1 +D ⇋ MDm

Where MD1 is the micelle associated with 1(one) molecules of the dye and K1 is the

stepwise association constant between MD1 and D. Assuming that the additive molecules

that solubilize within micelles obey a position distribution, the first stepwise association

constant, K1, can be obtained from the relation-

D1 −[D]

[D] = K1 [M1]

K1

K2

K3

Introduction

22

Here [M1] is total micelle concentration, [D1] is the total equivalent concentration of the

dye and [D] is the average number of additive incorporated into a single micelle

[D] = D1 −[D]

[M1]

Figure 1.7: The chemical and physical solubilization (incorporation) of drugs within micelle

1.4.2.9 Factors affecting solubilization

(i) Effect of structure of solubilizer

There are a number of factors regarding the structure of solubilizer such as chain length,

substitutions in the chain and position of hydrophilic group, which effect the

solubilization. The amount of material solubilized generally increases with increasing the

size of the micelles. The factors that cause an increase in either the diameter of the

micelle or its aggregation number can be expected to produce increased solubilization.

Introduction

23

An increase in the chain length of the hydrophobic portion of the surfactant generally

results in an increased solubilization capacity for hydrocarbons in the interior of the

micelle in aqueous media. Bivalent counterions show greater solubilizing power than the

corresponding univalent [53]. Nonionic surfactants, because of low CMC, are better

solubilizing agents than ionic surfactants in dilute solutions. In general, the solubilizing

power for hydrocarbons and polar compounds having same hydrophobic chain length

follows the order: [54] nonionics > cationics > anionics.

(ii) Effect of structure of the solubilizate

For polar solubilizates, the structure of the solubilizate shows variation in the depth of

penetration into the palisade layer of the micelle. In the case of more or less spherical

micelle, the polar compounds are solubilized close to the micelle-water interface, to a

greater extent than nonpolar solubilizates that are located in the inner core. Usually the

molecules having longer alkyl chain length and less polarity in nature show the smaller

degree of solubilization [55]. For condensed aromatic hydrocarbons the extent of

solubilization appears to decrease with an increase in the molecular size [56].

(iii) Effect of electrolytes

Neutral electrolytes in ionic surfactant solution decrease the repulsion between the

charged ionic surfactant headgroups, thereby decrease the CMC and increase the

aggregation number and volume of micelles. The increase in aggregation number of the

micelles presumably results in an increase in hydrocarbon solubilization in the inner core

of the micelle.

(iv) Effect of organic additives

The presence of solubilized hydrocarbons in the surfactant micelles generally increases

the solubility of polar compounds in these micelles. The solubilized hydrocarbon causes

the micelle to swell, and this may make it possible for the micelle to incorporate more

polar material in the palisade layer. The long chain polar compound which are less

capable of forming hydrogen bond, show the greater power to increase the solubilization

of hydrocarbons.

Introduction

24

(v) Effect of temperature

For ionic surfactants an increase in temperature generally results in an increase in the

extent of solubilization for both polar and nonpolar solubilizates, possibly because

increased thermal agitation increases the space available for solubilization in the micelle

[57]. For nonionic surfactants, the effect of temperature increase depends on the nature of

the solubilizate. Nonpolar materials, which are solubilized in the inner core of the

micelle, appear to show increased solubility as the temperature is raised. Increase in

temperature above 10°C, causes the increase in thermal agitation of the surfactant

molecules in the micelles which results in increased in solubilization. Further an increase

in temperature decreases the amount of material solubilized due to increased dehydration

and tighter coiling of the chains, decreasing the space available in the palisade layer.

1.4.2.10 Reasons for self-aggregation of surfactant molecules

(i) Hydrophobic Interaction

One of the important features that make water unique as a solvent is its response to a-

polar solutes. The tendency for a-polar molecules or molecular fragments to avoid

contact with water is said to be due to the hydrophobic interaction, which thus gives rise

to a thermodynamic force rather than a mechanical force. The hydrophobic interaction

and the mechanism of surfactant self-assembly has been studied extensively [58]. From a

thermodynamic point of view, surfactant self-assembly is entropy driven process [59].

When temperature is increased, entropy of water is increased due to the destruction of

structured water around the hydrophobic tail and entropy of surfactant is decreased a little

compared to the water. Even though it is an endothermic process, the free energy of the

whole process is negative which suggests micelle formation is a spontaneous process.

Generally, the water molecules are arranged in an ordered way around the monomeric

units of surfactants, which can be defined as „iceberg‟. During micellization, due to the

destruction of the iceberg a positive entropy change occurs. Despite this micellization-

favoring phenomenon, a negative entropy change can occur if the ordering of the

randomly oriented amphiphile molecules from the solvated form into a micelle structure

Introduction

25

is more pronounced than disordering effect due to the destruction of icebergs around the

alkyl chains. At the same time, the motion of the water molecules bound to the

hydrophilic heads become more restricted, contributing to the decrease in entropy [60].

(ii) Hydration

Due to its highly structured nature, water as a solvent displays a very complex behavior.

Thus in addition to direct ion-molecule interactions, the effect of a solute on the hydrogen

bonded network is of great importance. It is important to note that non-polar solutes have

particularly profound influences on water structure. Thus the alkyl groups markedly

reduce both the rotational and the translational mobility of the water molecules [61]. This

entropically unfavorable solution of nonpolar molecules or group in water is termed

“hydrophobic hydration”. X-ray diffraction studies have established their structure to be

of the clathrate type, with the solute surrounded by a layer of hydrogen-bonded water

molecules forming, for example, pentagonal dodecahedra. Thus even if the detailed

structure is not presently established, it is assumed that alkyl chain of an amphipile

monomer in water is surrounded by a hydrogen-bonded organized water layer. The polar

heads of the monomer interact with water in away similar to simple polar solutes and

electrolytes through hydrogen-bond, dipole-dipole and ion-dipole interactions. But when

the amphiphiles are in micelles these hydration features get affected. The nature and the

extent of this effect are interesting for both fundamental understanding and applied

aspects. Very few studies have been done on the hydration of non-ionic surfactants

because of the sensitive effects of temperature and concentration on their micellar size

and shape. There are also various spectroscopic methods for the study of amphiphile

hydration. Deuteron quadruple splitting studies may provide information on the number

of water molecules influenced in their orientation by the amphiphile aggregates in liquid

crystals [62]. For the lamellar phase of the systems alkali octanoate-decanol-water, for

example, at most about 5 water molecules per octanate are appreciably oriented [63].

Introduction

26

(iii) Counter-ion Binding

A counter ion is the ion that accompanies an ionic species in order to maintain electric

neutrality. In table salt (NaCl), the sodium cation is the counter ion for the chlorine anion

and vice versa. In a charged transition metal complex, a (i.e. non-coordnated) ion

accompanying the complex is termed the counterion. Counterions have a large influence

on the aggregation of the surfactant molecules in solution mainly through changes in the

ionic strength of the solution [64]. In addition, the valency of the counterion also

influences the CMC to a larger extent. The degree of the counterion binding is due to the

balance between the electrostatic forces which pull the counterion towards the oppositely

charged head group of micelles and the hydration forces which tends to inhibit the

binding [65]. The CMC value normally decreases as counterion binding increases.

Counterions or ions with opposite charge to that of the surface active moiety of the

surfactant are known to have an additional specific effect. For example, sodium bromide

was found to induce the growth of micelles of the cationic surfactant cetylpyridinium

bromide whereas sodium chloride did not [66]. Aromatic counterions like benzoate,

tosylate, salicylate, because of their strong binding at the micellar surface lower the CMC

while increasing the counterion binding [67]. Salicylate in particular is effective in

inducing micellar growth. The counterion binding also increases with increasing

counterion hydrophobicity enhancing the micelle formation [68]. Hydrophobic

counterions are interesting as charge carrier or quencher in biomembranes and membrane

photochemistry [69]. Addition of cationic surfactant to SDS is a special case of

hydrophobic counterion interaction. The CMC of a mixture of anionic and cationic

surfactant in aqueous solution is considerably lower than that of the individual

surfactants due to the synergistic interaction between the surfactant molecules and they

exhibit properties superior to their constituent single surfactant in many surfactant

applications [70].

Introduction

27

1.5 SURFACTANT SOLUBILITY

In aqueous solution, when all available interfaces are saturated, the overall energy

reduction may continue through other mechanisms. Depending on the system

composition, a surfactant molecule can play different roles in terms of aggregation

(formation of micelles, liquid crystal phases, bilayers or vesicles, etc). The physical

manifestation of one such mechanism is crystallisation or precipitation of surfactant from

solution – that is, bulkphase separation. While most common surfactants have a

substantial solubility in water, this can change significantly with variations in

hydrophobic tail length, head group nature, counterion valence, solution environment,

and most importantly, temperature.

1.5.1 The Krafft temperature

As for most solutes in water, increasing temperature produces an increase in solubility.

However, for ionic surfactants, which are initially insoluble, there is often a temperature

at which the solubility suddenly increases very dramatically. This is known as the Krafft

point or Krafft temperature, TK, which varies for each surfactant and is defined as the

intersection of the solubility and the CMC curves, i.e., it is the temperature at which the

solubility of the monomeric surfactant is equivalent to its CMC at the same temperature.

This is illustrated in Figure 1.8. At the TK an equilibrium exists between solid hydrated

surfactant, micelles and monomers. Below TK, surfactant monomers only exist in

equilibrium with the hydrated crystalline phase, and above TK, micelles are formed

providing much greater surfactant solubility. Above the TK maximum reduction in surface

or interfacial tension occurs at the CMC because the CMC then determines the surfactant

monomer concentration. The TK of ionic surfactants is found to vary with counterion

[71], alkyl chain length and chain structure. The knowledge of the TK is crucial in many

applications since below the TK the surfactant does not perform efficiently; hence typical

characteristics such as maximum surface tension lowering and micelle formation cannot

be achieved. The development of surfactants with a lower TK but still being very efficient

at lowering surface tension (i.e., long chain compounds) is usually achieved by

Introduction

28

introducing chain branching, multiple bonds in the alkyl chain or bulkier hydrophilic

groups thereby reducing intermolecular interactions that would tend to promote

crystallisation.

Figure 1.8: The Krafft temperature TK is the point at which surfactant solubility equals the

critical micelle concentration. Above TK, surfactant molecules form a dispersed phase; below

TK, hydrated crystals are formed.

1.5.2 The Cloud point

Nonionic surfactants do not exhibit krafft points. Instead, the solubility of nonionic

surfactants decreases with increasing temperature, and these surfactants may begin to lose

their surface active properties above a transition temperature referred to as the cloud

point. Above the cloud point, the system consists of an almost micelle-free dilute solution

at a concentration equal to its CMC at that temperature, and a surfactant-rich micellar

phase.

This separation is caused by a sharp increase in aggregation number and a decrease in

intermicellar repulsions [72] that produces a difference in density of the micelle-rich and

Introduction

29

micelle-poor phases. Since much larger particles are formed, the solution becomes visibly

turbid with large micelles efficiently scattering light. As with TK, the cloud point depends

on chemical structure. For polyoxyethylene (PEO) non-ionics, the cloud point increases

with increasing EO content for a given hydrophobic group, and at constant EO content it

may be lowered by decreasing the hydrophobe size, broadening the PEO chain-length

distribution, and branching in the hydrophobic group [73].

1.6 APPLICATIONS OF SURFACTANTS

In all processes that take place at interfaces, surfactants can become effective. By

application of surfactants, work processes may be simplified, accelerated, or economized.

Also, the quality, as well as the efficiency of much differing products, may be optimized.

An overview of the manifold application areas is given below:

1.6.1 Consumer Products

An important field of application for surfactants is consumer products. These products

are detergents, dishwashing agents, cleaning agents and personal products.

1.6.1.1 Detergents and dishwashing: The primary traditional application for surfactants

is their use as soaps and detergents for a wide variety of cleaning processes. Soap has

been used in personal hygiene for well over 2000 years with little change in the basic

chemistry of their production and use. New products with pleasant colors, odors, and

deodorant and antiperspirant activity have crept in to the market since the early twentieth

century. The soaps and detergents are used mainly in washing our clothes, dishes, houses,

and so on to remove unwanted dirt, oils, and other pollutants from the substrate.

1.6.1.2 Cosmetics and Personal Care Products: Cosmetics and personal care products

make up a vast multi-billion-dollar market worldwide, continues to grow as a result of

improved overall living standard. Such products are formulated mainly from surfactants

and other amphiphilic materials. It is probably safe to say that few, if any, cosmetic

Introduction

30

products known to women (or men, for that matter) are formulated without at least a

small amount of a surfactant or surface-active component. That includes not only the

more or less obvious creams and emulsions but also such decorative products as lipstick;

rouge; mascara; and hair dyes, tints, and rinses. An important aspect of such products is

that it may produce an adverse reaction in some cases. Unfortunately, our understanding

of the chemical reactions or interactions among surfactants, biological membranes, and

other components and structures is not sufficiently advanced to allow the formulator to

say with sufficient certainty what reaction an individual will have when in contact with a

surfactant. The possible adverse effects of surfactants in cosmetics and personal care

products, of course, be studied in depth for obvious safety reasons.

1.6.2 Industrial Applications

1.6.2.1 Food products: The food industry utilizes surfactants as cleaners and emulsifiers

[74]. Through application of natural or synthetic emulsifiers, O/W emulsions (milk

substitutes, ice cream, mayonnaise, sauces, etc.) and W/O emulsions (e.g., margarine) can

be improved in their consistency.

1.6.2.2 Pharmaceutical industry: The primary application of surfactants in the

pharmaceutical industry is as emulsifiers for creams and salves, but they are also used as

dispersing agents in tablets or as synergists for active ingredients. The most important

criterion for a specific application is the pharmacological or toxicological product safety.

1.6.2.3 Insecticides and herbicides: Active substances for the protection of growing

plants [75] are offered as powder or liquid concentrates, which are diluted to so-called

spray liquors for application. Surfactants are used here as aids for preparing satisfactorily

dispersed spray liquors for adequate wetting of the target, as well as for promoting

penetration of active substances into the plant.

1.6.2.4 Agriculture: In agriculture, surface active polymeric carboxylic acids or short

chain alkane sulfonates effect hydrophilizing of heavy soils. To prevent caking of

fertilizers in mixers and to achieve uniform distribution of fertilizers in the soil, dilute

Introduction

31

solutions of fatty alcohol polyglycol ethers, alkyl benzene sulfonates or cationic

surfactants are advantageous.

1.6.2.5 Textiles and fibers: In the manufacture and further processing of textiles,

surfactants have a role as auxiliaries in a number of process steps. In pretreating of textile

material, natural fibers are freed of accompanying substances (waxes, fats, pectines, seed

hulls and other impurities). The detergents and wetting agents needed for this are

primarily mixtures of different surfactant types. In the manufacture of textiles, surfactants

are applied to optimize individual processing steps (drawing, spinning, twisting,

texturizing, coning, weaving, knitting, etc.)

1.6.2.6 Chemical industry: The wetting and dispersing power of surfactants is being

utilized in chemical processes to aid processing. In systems containing immiscible

components, the reaction speed may be increased by the emulsification effect of

surfactants, e.g., in splitting of fats by the Twitchell process, in hydrolytic splitting of

wool wax and in hydrolysis of polyvinyl acetate. Also worth mentioning is phenol

manufacture by the cumene process, the preparation of ethylene carbamates, as well as

chlorination reactions. Surfactants may also be applied to increase the yield in extraction

processes.

1.6.2.7 Plastics industry: The application for surfactants in the plastics industry is in the

preparation of plastics dispersions (emulsion polymerization), pearl polymerizates,

polyurethane foams, mold release agents and in micro encapsulation processes etc.

1.6.2.8 Paints and laquers: Surfactants are also of great importance in the manufacture

of coating materials, paints, varnishes, lacquers, dyestuff pigments, binding materials,

and binders. Paints and lacquers are, for the most part, dispersed systems of dyestuff

pigments, binding materials and solvents. Therefore, surface active substances can speed

up the preparation of dispersions, and improve the dispersion degree and stability.

1.6.2.9 Cellulose and paper: Surfactants are employed in the pulp and paper industry for

the following purposes: rosin removal in pulp and paper manufacture, foam inhibition

and pigment dispersion in the manufacture of paper, emulsifying in paper sizing and

finishing processes, cleaning machinery, and regeneration of waste paper. In the

Introduction

32

regeneration of waste paper (deinking flotation process), wetting agents are used to

improve removal of substances adhering to the paper.

1.6.2.10 Leather and furs: The broad spectrum of the raw goods occurring in the leather

and fur industry [76] necessitates various wet treatment processes in which surfactants

and emulsifiers play a big role. In the finishing surface treatment (trimming) of the dry

leather, polymer films are applied to the leather surface, whereby the quality is improved.

The coatings can consist of polyacrylate-polyurethane-or polybutadiene dispersions.

1.6.2.11 Photographic industry: Surfactants are utilized in the photographic industry as

wetting agents in casting solutions and lubricants, as aids in the preparation of dye

emulsions and as additives in processing baths. In the application of antihalation layers,

filter layers, or other supercoats to photographic films various surfactants have proven

useful.

1.6.2.12 Metal processing: Surfactants do find broad application in the various processes

employed in the metal processing industry. In addition to the specific cleaning processes,

application in cooling lubricants, tempering oils, hydraulic emulsions, anticorrosion

agents, polishing pastes, mold separating agents, and metal drying agents is especially

noteworthy.

1.6.2.13 Electroplating: Surface active substances are applied in electrochemical

processes for removal of soil and grease from substrate surfaces prior to the actual

electrolytic process.

1.6.2.14 Adhesives: Surfactants are added to adhesives to effect a fast spreading on the

respective surfaces by lowering of interfacial tension between the substrate surfaces and

the adhesives. As a rule, surfactants find application only in aqueous adhesive

formulations, since organic solvents have inherently low interfacial tensions.

1.6.2.15 Road construction and building materials: Surfactants are applied in road

construction, in construction and building materials, in the preparation of bitumen

emulsions, as dispersants in the cement industry, in the utilization of polymer dispersions,

as additives to plasters and cement coatings and in wood impregnation.

Introduction

33

1.6.2.16 Firefighting: In fighting fires where water cannot penetrate toward the inside of

the fire source such as in fires of cotton, paper bales, wood flour, forest floors, etc., water

with wetting agent containing however may be fought more effectively. For fires of

storage tanks, in mines, on ships, in warehouses with combustible solid or liquid

materials, on airport runways, etc., heavy foams are better suited.

1.6.2.17 Mining and flotation: For prevention of coal dust explosions and as dust

binding agents for mineral dust in mining operations, calcium chloride pastes which are

brushed onto the rock surface, are being used with surfactants to improve the wettability

of the pastes. In the separation of raw material minerals, differences in surface properties

of individual mineral species are being utilized. Following suspension of finely milled

ore in water, air is sparged into the suspension. Minerals of value should float upwards by

attachment to the air bubbles and thus be separated from the accompanying burden. The

surface of the valuable mineral particles has to be hydrophobic to affect their attachment

to the air bubbles and surfactant works here actively.

1.6.2.18 Oilfield chemicals: Surfactants find manifold applications in crude oil

extraction activities [77]. In drilling operations, the properties of drilling fluids can be

improved. The application of drilling fluids, i.e. the continuous flushing of the bore hole,

has as its purpose to lubricate and cool the drilling tool, to flush the drilled out rock

particles upwards, to support the wall of the bore hole, and to prevent the sudden eruption

of oil or gas after penetration of the deposit. The basis of most drilling mud is bentonite.

Additionally used are heavy spar, protective colloids and thickeners.

1.6.2.19 Cleaning agents: The cleanliness of homes, work places, and public facilities, is

of great importance for reasons of hygiene, esthetics and value maintenance. Although

highly developed machines are available for the cleaning of both textiles and tableware,

the mechanical cleaning of fixed hard surfaces is only feasible on the large surface areas

found in the commercial sector. Hence, to a large extent hard surfaces have to be cleaned

by manual procedures. To simplify this work, cleaning agents are extensively utilized.

1.6.2.20 Other: Surfactants in biological systems: The understanding of the pulmonary

surfactant system, although discovered in 1929, has only been applied clinically since

Introduction

34

about 1990 for the treatment of respiratory distress syndrome. Surfactant replacement

therapy may also be used in treating other forms of lung disease, such as meconium

aspiration syndrome, neonatal pneumonia and congenital diaphragmatic hernia.

1.7 THE SCOPE AND OBJECTIVES OF THE PRESENT STUDY

Due to their immense application potential, surfactant-based systems are a topic of major

research interest in both academia and industry. They are one of the most important

groups of organic chemicals, and are used in vast amounts in domestic and industrial

applications. They are designed to remove dirt, sweat, sebum, and oils from the skin and

other surfaces. The main characteristic of these compounds is to decrease the surface

tension of solvent, and resulting many properties such as contact angle, foam properties

etc .and forming colloidal sized clusters known as micelle in solution [78]. These clusters

or aggregates of different morphologies endow the surfactant solutions with useful

properties. Such unique properties encouraged their applications in various field of study,

such as microbiology, pharmaceuticals industry, food industry, personal care, cosmetics,

catalytic reaction, oil recovery and polymerization, etc. [79]. However to initiate

aggregates or micelle the solution must attain a certain concentration level known as the

critical micelle concentration (CMC). Below the CMC surfactant molecule exist as

monomers and cannot show its activity. Many of its properties changes upon the

formation of micelles. Micellar solutions have the special characteristics of solubilizing

the hydrophobic organic compounds [80]. An increase in surfactant concentration in

solution increases the extent of solubilization of hydrophobic solutes because of an

increase in the number of micelles in the bulk. Studies of the solubilization of poorly

water-soluble compounds in non-aqueous and aqueous system have revealed a lot of

application in the practical fields such as drug formulations and drug carrier, drug

solubilization, separation, toxic waste removal etc. [81-82]. Therefore, it is a matter of

great research interest is to reduce the CMC to a lower value for wider application of

surfactants. In this study such an attempt has been taken to tune the CMC to a lower

value. A major area of concern nowadays is the micelle formation in the presence of

additives, among which surfactant- inorganic salts interactions are of great interest. Net

Introduction

35

charge, either on the molecules of one component or on both [81], determines the nature

of surfactant-salt interactions. When a salt is present in any aqueous surfactant system it

decreases the electrostatic repulsion between the charged head groups which causes a

decrease in the (CMC). For example, the CMC of cetyltrimethylammonium bromide

(CTAB) decreases from 0.92 to 0.56 mM when the NaCl concentration is 0.01 M [83].

Other major factors, which are playing an important role, are the length of the surfactant

hydrophobic tail, and temperature. The CMC values increase with temperature. The

temperature effect on the process of micellization of surfactants in water has usually been

analyzed in terms of two opposing factors. With an increase in temperature, the degree of

hydration of hydrophilic group decreases and this process is in favour of micellization.

On the other hand, it also breaks the water structure surrounding the hydrophobic groups

and is unfavourable for micellization [84] of the surfactant. The predominated one thus

determines CMC formation in aqueous surfactant solution. Increasing the number of

carbon atoms in the hydrophobic alkyl chain, decreased the (CMC). Longer chain length

of HTAB than that of TTAB increases the surface area of the micelle and, thus, reduces

the electrostatic repulsions [85]. The opposing repulsive interaction between the

polar/charged head groups disfavor micellization and leads CMC to higher values. So it is

a delicate balance between the interaction between hydrophobic alkyl chain and between

opposing repulsive head groups.

Another interesting characteristic feature shown by the ionic surfactant is their limited in

solubility below a certain critical temperature but above this temperature they are fully

soluble. This temperature is known as Krafft Temperature (TK). Below this temperature

the surfactant molecules remain as crystalline hydrated solids. At this state surfactant

solution loses many of its activities. The TK can also be termed as the melting

temperature of the hydrated solid surfactant [86]. The monomer solubility is essential for

the formation of micelle. At TK the surfactant monomers become soluble enough for the

formation of micellar aggregates and the solubility of an ionic surfactant becomes equal

to the CMC and there is an establishment of equilibrium state between crystalline

hydrated solid and micelle formation [15]. Above this temperature equilibrium state

shifted towards micelle formation and the solubility of the surfactant monomer increases

and micellar formation become thermodynamically favored [5]. For surfactants being

Introduction

36

used below TK, show lower effectiveness in reducing surface tension than similar

materials that are being used above their TK. The maximum reduction in surface tension

is determined by the concentration of surfactant at solution saturation [15]. The TK

increases with increase in the number of carbon atoms in the hydrophobic group and

decreases with branching or unsaturation in that group in a homologous series of ionic

surfactants [87]. Oxyethylenation of alkyl sulfates decreases their TK; oxypropylenation

decreases them even further. Alkane sulfonates have higher TK than their corresponding

alkyl sulfates. The substitution of triethyl for trimethyl in the head groups of cationic

alkyl trimethylammonium bromides leads to significant reduction in their TK values [88].

This probably explains why traditional surfactants bear a hydrocarbon chain usually

shorter than C18 [15]. On the other hand, increase in the number of head group in the

surfactant molecule increases the solubility of the surfactant in water and increases its

surface activity. When the surfactant contains two hydrophilic groups, however, its

solubility in water increases compared to conventional surfactants and shows much lower

Krafft points and at this stage the molecule can accommodate more carbon atoms in the

hydrophobic groups without becoming water-insoluble. The solubility of surfactant also

increases with the increasing the size of the head group. The concept of TK is very

important as below the TK surfactant cannot show their detergency, dispersing and

emulsifying properties as well as their characteristic properties of maximum lowering of

surface tension, formation of micelle thus solubilization of water insoluble organic

compounds. Therefore, it is essential to lower the TK of surfactants below room

temperature for their wider industrial applications. In many commercial formulations, the

solution contains a certain amount of dissolved salt, in addition to the surfactant ions and

their counterions [89, 90]. Usually added salts lower the critical micelle concentration

(CMC), increase the viscosity and surface activity of surfactants, which is favorable for

their industrial applications. Unfortunately, added salts elevate the TK of surfactants

which limits their industrial applications. The TK values of a number of ionic surfactants

have been measured in the presence of added electrolytes [91, 92]. These studies have

revealed that the TK increases with increasing the concentration of the added electrolyte.

At present, it has been the subject of many research to use surfactant with lower CMC

and depressed TK in comparison with pure surfactant.

Introduction

37

In the present work, we attempted to study the effect of some electrolytes on the TK and

micellar behavior of Octadecyltrimethylammonium Bromide (OTAB) and Sodium

Dodecyl Sulfate (SDS) in aqueous solution. Here we will show an important point that

was clothed for a long time of specific ion effect on TK of ionic surfactant. It is

engrossing to note here that the TK can increase or decrease depending on the nature of

electrolytes and the CMC can be depressed stunningly upon addition of electrolytes to the

surfactant solution. It is important to note here that except for Br− (common ion), SCN−

and I−, the rest of the anions used in this study are effective in lowering the TK and all the

anions are effectual to lower the CMC of the OTAB. Only Li+ is found to be effective in

lowering the TK while all cations used in this study are effective in lowering the CMC of

SDS. Moreover, a water insoluble dye, Sudan Red B (SRB) was solubilized in aqueous

micellar solution of OTAB and SDS in pure water and in aqueous salt solution. Since

many of the applications of surfactants lie in their capacity to form micelles, it can be

expected that the depression of the TK and lowering of the CMC in the presence of the

added electrolytes will favor wider industrial applications of OTAB and SDS.

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[83] Roy, J. C.; Islam, M. N.; Akhtaruzzaman, G.; J Surfact Deterg. 17, 231, 2013

[84] Varade, D.; Joshi, T.; Aswal, V. K.; Goyal, P. S.; Hassan, P. A.; Bahadur, P.;

Colloid Surf A. 259, 95, 2005

[85] Banipal, T. S.; Kaur, H.; Banipal, P. K.; Sood, A.K.; J Surfact Deterg17, 1181,

2014

[86] Tsuji, K.; Mino, J.; J. Phys. Chem. 82, 1610, 1978

[87] Gu, T.; Zhu, B-Y.; Rupprecht, H.; Prog. Colloid Polym. Sci. 88, 74,1992

[88] Davey, T. M.; Ducker, W. A.; Hayman, A. R.; Simpson, J.; Langmuir.14, 3210,

1998

[89] Diamant H.; Andelman, D.; J. Phys. Chem. 100, 13732, 1996

[90] Vijayan, S.; Ramachandran, C.; Shah, D. O.; J. Am. Oil Chem. Soc. 58, 566, 1981

[91] Carolina, V. G.; Bales, B. L.; J. Phys. Chem. B. 107, 5398, 2003

[92] Bakshi, MS.; Sood, R.; Colloids Surf A. 233, 203, 2004

Chapter Two

Experimental

section

Experimental

42

2.1 MATERIALS

2.1.1 Surfactants

1. Octadecyltrimethylammonium Bromide (OTAB)

Linear formula: CH3(CH2)17N(Br)(CH3)3 Structure:

2. Sodium Dodecyl Sulfate (SDS)

Linear formula: CH3(CH2)11OSO3Na Structure:

2.1.2 Salts

1. Sodium Fluoride (NaF)

2. Sodium Chloride (NaCl)

3. Sodium Bromide (NaBr)

4. Sodium Iodide (NaI)

5. Sodium Thiocyanate (NaSCN)

6. Sodium Nitrate (NaNO3)

7. Sodium Sulfate (Na2SO4)

8. Sodium Benzoate (C7H5O2Na)

Experimental

43

9. Sodium Salicylate (C7H5O3Na)

10. Sodium Benzene Sulfonate (C6H5SO3Na)

11. Lithium Chloride (LiCl)

12. Potassium Chloride (KCl)

13. Cesium Chloride (CsCl)

2.1.3 Dye

1-({3-methyl-4-[(3-methylphenyl)diazenyl]phenyl}diazenyl)naphthalen-2-ol

Linear formula: C24H20N4O Structure:

The cationic surfactant Octadecyltrimethylammonium Bromide (OTAB) was supplied by

Sigma-Aldrich, with a purity of > 99 % and was used without any further purification.

The anionic surfactants Sodium Dodecyl Sulfate was collected from MERCK and was

highly pure samples and was used as received. Some salts were obtained from BDH and

some from MERCK and Sigma-Aldrich with a purity > 99 % and were used as received.

The dye SRB was obtained from MERCK. Triple-distilled water from all-Pyrex glass

apparatus was used for the preparation of solutions. All the measurements were carried

out two or three times until reproducible data was obtained and when the data were found

to agree within ±1%, then the results were confirmed.

Experimental

44

2.2 METHOD

2.2.1 Measurement of Krafft Temperature

To determine TK, clear aqueous solutions of surfactant, SDS and OTAB in pure water

and in the presence of salt of counter-ion were prepared and placed in a refrigerator at

about 2°C for at least 24h, where the precipitation of surfactant hydrated crystals

occurred. The system was then taken out of the refrigerator when precipitation of the

hydrated surfactant occurred and then the temperature of the precipitated system was

raised gradually under constant stirring with a glass rod, and its conductance was

measured with the help of a EUTECH CON 510 conductivity meter.

Figure 2.1: Hydrated crystal in the beaker (left side) and arrangement for Krafft temperature measurement (right side: EUTECH CON 510 conductivity meter and Froilabo RE 5 refrigerated bath circulator)

Experimental

45

At each temperature, the conductance reading was checked every 2 min until it reach a

steady value. The temperature was measured using a sensor combined with conductivity

meter (precision of ±0.01) immersed in the investigated system. The Krafft temperature

was taken as the temperature where the conductance versus temperature plots showed an

abrupt change in slope. Operationally, TK values were determined from plots of the

second derivative of the data. This temperature was the same as that required to

completely dissolve the hydrated solid surfactant, judged visually to be the point of

complete clarification of the system. The reproducibility of TK measurements on a single

sample (typically ± 0.05°C) was superior to the reproducibility in samples presumably

prepared identically (averages about ±0.1°C). Details of the experimental procedure are

to be found elsewhere [1].

2.2.2 Measurement of Critical Micelle Concentration

Conductometric method: Conductivity measurements were carried out by using a

EUTECH CON 510 conductivity meter. Experiments were started with a dilute solution

and the subsequent concentrated solutions were obtained by adding a previously prepared

stock solution into a 100-mL beaker. The solution was stirred with glass rod after each

addition and the conductance of the solution was measured. The CMC was then taken

from the sharp break in the conductance vs concentration plot. The temperature of the

solution was kept constant by using a circulating water bath (Froilabo RE 5 refrigerated

bath circulator) with a precision of ±0.1°C. To observe the effect of electrolytes on the

CMC, surfactant solutions were prepared in various electrolytes solutions of desired

concentrations [2].

Surfacetensiometric method: To measure CMC, the surface tensions of the aqueous

surfactant solutions of different concentrations were measured by a surface tensiometer

(Kruss K9) furnished with a platinum plate. Before each measurement, the plate was

thoroughly washed by red heat. The solution was transferred into a vessel that was

thermostated by circulating water at the desired temperature. Before measurement, the

surface tension of the double distilled deionized water was confirmed to be in the range

Experimental

46

±0.3 mN/m at the respective temperature. Two readings were acquired under all

experimental conditions and standard deviations were 0.4 mN/m. The surface tension

measurements were started with a dilute solution and the subsequent concentrated

solutions were made by adding a previously prepared stock solution into the vessel. Care

was taken that the platinum plate was properly wetted with the solution. The

establishment of equilibrium was checked by repeated measurements at 5-min intervals

until the surface tension readings stabilized; this generally required 30–45 min. Details of

the experimental procedure are to be found elsewhere [2].

Figure 2.2: Surface tension measurement: Surface tensiometer (Kruss K9) and refrigerated bath circulator (JSRC-13C)

Experimental

47

2.2.3 Solubilization

Solubilization studies of Sudan Red B (SRB) in OTAB and SDS solution in pure water

and in the presence of Na2SO4 and NaCl respectively were conducted under the condition

of maximum solubilization at a temperature of 30±1°C for SDS and associated salt,

NaCl and 38±1°C for OTAB and with Na2SO4. The temperature for each system was

chosen above the Krafft temperature to ensure the micelle formation of the surfactants in

aqueous solution. 50-mL reagent bottles were used for this study. Surfactant solutions of

different concentrations were poured separately into some reagent bottles, where the

surfactant concentrations in the first few were below the CMC and the last few were

above the CMC. A fixed amount of the solubilizate (SRB) was added to maintain excess

product at least three times its solubility limits for achieving solubilization equilibrium.

To equilibrate the solution the bottles were continuously agitated using a shaker (Stuart

Orbital shakers, SSL1) at 250 rpm for 24 hours held in a horizontal position. The

solutions were then filtered in order to separate the non- solubilized excess of dye from

the solution using Whatman 41 Ashless Quantitative Filter Paper 2.5µm and filtrate was

then analyzed by using the UV–visible spectrophotometer (Jenway Spectrophotometer-

7315). The absorbance of each solution was measured by using a quartz cell of path

length 1 cm. The concentration of SRB in surfactant micelles was calculated from a

calibration curve obtained from the absorption spectra of known concentrations of SRB

in OTAB and SDS against a blank. The strong absorbance at λmax = 517 nm for OTAB

and λmax = 524 for SDS gave a satisfactory Beer’s law plot.

Experimental

48

Figure 2.3: Shaking of the surfactant solution with dye (Top: Stuart Orbital shakers, SSL1) and solution after shaking (Below)

Experimental

49

Figure 2.4: Jenway UV-spectrophotometer, model 7315 (Top) and a spectrophotogram of SRB (Below)

Experimental

50

REFERENCES

[1] Islam, M. N.; Sharker, K. K.; Sarker, K. C.; J Surfact Deterg.18, 651, 2015

[2] Roy, J. C.; Islam, M. N.; Aktaruzzaman, G.; J Surfact Deterg. 17, 231, 2014

http://link.springer.com/article/10.1007%2Fs11743-015-1696-4#/page-1

Chapter Three

Results and

discussion

Results and Discussion

51

3.1 EFFECT OF ELECTROLYTES ON KRAFFT TEMPERATURE

Specific conductance (κ) versus temperature curve of the solutions of aqueous Sodium

Dodecyl Sulfate (SDS), aqueous Octadecyltrimethylammonium Bromide (OTAB), and

their mixtures with several salts of 0.005 ionic strength are shown in Figures 3.1 and 3.2,

respectively. An important characteristic feature of ionic surfactants is their tendency to

precipitate from aqueous solutions as solid hydrates. At low temperature the solubility is

very limited until a certain temperature is reached commonly referred to as the Krafft

temperature (TK). But at this or higher this temperature surfactant is fully soluble in water

[1]. This temperature can be achieved by measuring the conductance of the surfactant

solution at different temperature. This is based on the fact that at low temperature

surfactants molecules remain hydrated. Therefore, at this temperature the conductivity of

the surfactant solution is somewhat lower due to low solubility of the surfactant in water.

While temperature is increased gradually, the surfactants molecules from hydrated state

start to ionize and thus increase the conductivity of the solution [2, 3]. The micelles are

spontaneously formed at the TK as the concentration of surfactant monomers becomes

equal to the CMC. Below the TK the surfactant solubility increases slowly with increasing

temperature because the surfactant exists as monomers. At the TK the surfactant

monomers form micelles showing a dramatic increase in solubility with increasing

temperature [1, 4, 5]. The SDS and OTAB solubility slowly increases below the TK. It is

only around the TK that a significant rise in conductivity can be seen, indicating a sharp

increase in the solubility of the surfactant. Beyond the TK the conductivity remains

almost steady. The TK was then taken when the sharp break in the κ vs. T plot occurred.

Further increase in temperature produces a small increase in conductivity, which can be

attributed to an increase in the thermal motion of the charged species [6]. Near the TK of

a surfactant, a network or worm-like micelles form. However, if the temperature is far

above the TK, worm-like micelles will transform to spherical micelles. Therefore, near the

TK the viscosity of a surfactant will be a maximum, which decreases if the temperature is

far above or below the TK [7]. Hence above the TK further increasing in conductivity is

governed by a delicate balance between the viscosity of the solution and the thermal

motion of the charged species. The effects of different electrolytes on the TK values of


52

SDS and OTAB are shown in Figure 3.1 and Figure 3.2, respectively. The TK values of

SDS and OTAB in pure water are found to be 14.45 and 36.74 οC respectively and are in

good agreement with the literature value [8, 9]. This variation in the TK is due to longer

carbon chain length of OTAB than that of SDS [10]. The TK varies differently for the two

surfactants [11]. Surfactants with dissimilar structures and counter-ions, a larger

depression in TK are detected [12]. Previously, it has been shown that the TK of ionic

surfactants can be changed by varying the counter-ion [13], or by increasing the degree of

unsaturation [14], or branching [12] in the hydrocarbon chain.

In the present work, we have investigated the influence of added electrolytes on the TK

and the CMC of two ionic surfactants in aqueous solution. It has been reported that the

Figure 3.1: Specific conductance vs. temperature plots of SDS in pure water and in the presence of different electrolytes at0.005 ionic strength. (i) Pure SDS, (ii) LiCl, (iii) KCl, (iv) CsCl, (v) NaCl. The sharp break point in the plot indicates the Krafft Temperature.

3 6 9 12 15 18 21 24 27 30 33 36

400

5001000

1100

1200

1300

1400

1500

v

iv

iii

ii

i

Sp

ecif

ic C

on

du

cta

nce (

µS

cm

-1)

Temperature (OC)


53

Figure 3.2: Specific conductance vs. temperature plots of OTAB in pure water and in the

presence of different electrolytes at 0.005 ionic strength. (i) Pure OTAB, (ii) Na2SO4, (iii) NaBr,

(iv) NaF, (v) C6H5SO3Na, (vi) C7H5O2Na, (vii) NaNO3, (viii) C7H5O3Na, (ix) NaCl. The sharp break

point in the plot indicates the Krafft Temperature.

inorganic additives do not always elevate the TK and that the added salts also lower TK

[15]. Islam, M. N. et al. showed that the krafft temperature can be tuned to lower value or

higher value by adding salts [16]. For example, the TK of Cetylpyridinium Chloride

(20.1°C in pure water) in presence of 0.005M NaCl is 21.4°C while in presence of

0.005M NaNO3 it is 14.3°C. TK depression of SDS in presence of LiCl was observed in

our study. At the time of preparing SDS solution with LiCl, NaCl and CsCl the solution

remained lucid at room temperature. But when KCl is introduced the system turned

cloudy and observed instant precipitation of the solution. This observation indicates that

10 15 20 25 30 35 40 450

50

100

500

600

700

800

900

ix

viii

vii

vi

v

iv

iii

ii

i

Sp

ecif

ic C

on

du

cta

nce (

µS

cm

-1)

Temperature (OC)


54

the mode of interaction of these ions with SDS in aqueous solution is governed by the

size and charge density of added counter-ions, affecting the solubility and thereby the TK

of the surfactant. Ions with a high charge to radius ratio (e.g., Li+) is called kosmotropes

or structure making ions, since it induce a more organized structure of the water around

the hydration sphere. On the other hand, ions with a low charge to radius ratio, such as

K+, Cs+ are termed as chaotropes or structure-breaking ions. Because of the more intense

electrostatic field, kosmotropes are more strongly hydrated than the chaotropes.

Consequently, the extent of distortion in the structure of free water surrounding the

hydrated kosmotropes is much less than that in the case of weakly hydrated chaotropes.

Thus Li+ being a kosmotrope remains in the bulk of the solution with a consequent

increase in solubility of SDS and thereby decrease the TK. Due to the common ion effect

Na+ increases the TK of the solution. This is due to the fact that when a solution contains a

salt in equilibrium with its ions, an increase in the concentration of one of the ions will

cause a corresponding decrease in the concentration of the other ion to maintain the

constancy of the solubility product of the ions present in solution [17]. Thus, to keep the

solubility product constant, the solubility of SDS in the presence of Na+ decreases,

showing an increase in the TK of the surfactant.

Vlachy et al. reported that alkyl sulphates in aqueous solution behave like a chaotrope

[18]. Being chaotrope Cs+ and K+ form contact ion pair with dodecyl sulfate ion and

decrease the solubility of the system and thus increase the TK of the system. The effect of

concentration of these salts on the TK of SDS is shown in Figure 3.3. Here, it is clear

from the graph that the TK of SDS decreases in the presence of Li+ and increases in the

presence of Na+, K+, and Cs+. An unusual behavior was observed for CsCl of 0.0025M.

The TK of CsCl for 0.0025M is lower than that of pure SDS. It has been reported that at

lower CsCl concentration ion-water interaction dominates over the ion-ion interaction

[19]. These ions disturb the highly structured liquid: the natural hydrogen bond network

is disrupted. Therefore, it can be anticipated that the free water molecules formed by the

presence of these ions should promote hydration of the surfactant. As a consequence, the

solubility of the surfactant increases resulting in a decrease in the TK. It is clear from

Figure 3.3 that Li+ has the greatest ability to lower the TK and the propensity follows the

order Li+ > Na+ > Cs+ > K+.


55

Figure 3.3: Effect of ionic strength of electrolytes on the Krafft Temperature of SDS. (i) LiCl, (ii) NaCl, (iii) CsCl, (iv) KCl

The TK of OTAB increases in presence of Br−, SCN−, I− and decreases in presence of

SO42−, NO3

−, F−, Cl−, C7H5O3−, C7H5O2

−, and C6H5SO3−. Added salts affect the solubility

thus TK of the system. Due to the common ion effect [20] Br− decreases the solubility of

the system with a consequent increase in TK. In accordance with the solubility-product

principle in the presence of an added electrolyte containing a common ion the solubility

decreases [21]. This is also true for OTAB where Br− is the common ion to that of the

surfactant solution. Thus, the presence of Br− results in an increase in the TK of OTAB

while the presence of NO3−, F−, Cl−, SO4

2−, C7H5O3−, C7H5O2

−, and C6H5SO3− lowers the

TK of the same surfactant as found in the present work. The effect of concentration of

these salts on the TK of OTAB is shown in Figure 3.4.

0.000 0.002 0.004 0.006 0.008 0.0105

10

15

20

25

30

35

iv

iii

ii

i

Kra

fft

Tem

pera

ture

(OC

)

Ionic Strength


56

Figure 3.4: Effect of ionic strength of electrolytes on the Krafft Temperature of OTAB. (i) C7H5O3Na, (ii) C7H5O2Na, (iii) Na2SO4, (iv) C6H5SO3Na, (v) NaF, (vi) NaNO3, (vii) NaCl, (viii) NaBr, (ix) NaSCN, (x) NaI.

Here, it is clear from Figure 3.4 that the TK of OTAB decreases in the presence of NO3−,

F−, Cl−, SO42− and increases in the presence of Br−, I−, and SCN−. Although both NO3

−

and Br− are chaotropes or structure breakers, one increases and another one decreases the

TK of OTAB. Hence the concept of structure breaking properties of ions cannot

satisfactorily explain the dependence of the TK of OTAB in the presence of added

electrolytes. To explain the influence of these salts on the TK, we have to consider the

concept of salting-in/out behavior of the ions along with the common ion effect on the

solubility of OTAB simultaneously. It is evident from Figure 3.4 that contrary to the

usual trend of the Hofmeister series, more chaotropic ions, SCN− and I− present in the

0.0000 0.0025 0.0050 0.0075 0.0100 0.0125

24

30

36

42

48

54

60

66

72

x

ix

viii

vii

viv

iviii ii i

Kra

fft

Tem

pera

ture

(oC

)

Ionic Strength


57

extreme right side in the series cause an increase in the TK (data were taken by visual

method) due to the salting out effect. On the other hand, less chaotropic Cl−, and NO3− as

well as kosmotropic SO42−, and to some extent F− lower the TK of the surfactant. In the

presence of NO3−, F−, Cl− and SO4

2− the TK decreases gradually with increasing the

concentration of the surfactant ions and then shows almost steady values with further

increases in concentration. This is in line with the observation of a previous study which

demonstrated that the TK varies with the concentration of the added electrolyte [22].

Islam, M. N. et al. [16] reported that SO42− has the highest ability to increase the surface

tension relative to pure water and this tendency decreases in the line with Hofmeister

series [23, 24]. It has been reported that large negatively charged anions with low charge

density tend to be pushed toward the water surface while those with high charge density

being extensively hydrated remain in the bulk [25, 26]. SO42− ion being kosmotropic in

nature with high charge density remain strongly hydrated in the bulk of the liquid. Thus,

it preferentially shows higher negative adsorption behavior in accordance with the Gibbs

adsorption equation [25]. Thus, for entropic reasons, the interface seeks to minimize its

volume, and this result in an increase in surface tension [27]. Water molecules at the air-

water interface possess a well-organized pattern with the negative oxygen atom pointed

towards the gaseous phase [28]. As a result, an electrical double layer is established

having a negative outermost surface while the positive innermost surface is directed to

the solvent side [28]. Therefore, it can be expected that chaotropes or structure breaking

ions such as NO3−, Cl−, Br−, I− and SCN− will preferentially accumulate near the

interface. Furthermore their extent of accumulation at the interface is also influenced by

their relative tendency of hydration in the bulk. Water molecules interact with individual

ions in charge density-dependent ways. These electrochemical processes dominate the

behavior of ions in the high dielectric regime of water and favor weakly hydrated anions

to adsorb preferentially at the air–water and low solvated hydrocarbon–water interface

[29]. The accumulation of these anions can interfere with hydrophobic hydration by

increasing the surface tension of the hydrocarbon–water interface and results in salting

out behavior of macromolecules [30]. Therefore, it is logical to expect that these ions will

be accumulated at the surface of the hydrophobic chain of the surfactant and directly


58

disturb the hydrophobic hydration, leading to salting out behavior with a consequent

increase in the TK.

The influence of the added electrolytes on the TK of the surfactants can also be explained

with the help of Collins “law of matching water affinities”. This theory explains the

interaction between oppositely charged ions based on their tendency for hydration [30,

31]. According to this concept the interaction between the ions in solution is associated

with the competition between the charge density dependent ion water interactions and the

hydrogen bond dominated water–water interactions. Large anions having low charge

density have the tendency to pair with large cations when their water affinities are similar

[30, 31]. These pairs with similar water affinities will be less hydrated and hence less

soluble than the separate ions. Applied to the present case, this means that SCN− and I−

ions will form contact ion pairs with the cationic part of the surfactant. Since Br−, SCN−

and I− are weakly hydrated chaotropes, their union with the octadecyltrimethylammonium

ion should lead to contact ion pairs with low solubility (compare: CsI is much less

soluble than LiI [32]). Differently stating, chaotropes form contact ion pairs with other

chaotropes and kosmotropes do other kosmotropes. Large difference in water affinities

keeps the chaotropes away from kosmotropes and weakly hydrated chaotropes cannot

break through the hydration shell of the strongly hydrated kosmotropes [31]. Nitrogen

based cations behave like chaotropes [33]. Therefore, it can be regarded that

octadecyltrimethylammoniumion ion should behave like a chaotrope and hence will be

weakly hydrated. Therefore, it can be expected that weakly hydrated chaotropes

I− and SCN− will readily form contact ion pairs with the weakly hydrated

octadecyltrimethylammonium ions. Such ion pairs will be much less hydrated than

separate ions and headgroups. Again the addition of electrolytes hampers polarizability of

water [34]. As a result, dielectric constant of water decreases in the presence of added

electrolytes [35]. Under such a condition, surfactant molecules feel stronger attraction for

added counter ions in solution and thus form contact ion pair with a consequent decrease

in the solubility of the surfactant. The electrostatic repulsion between surfactant

molecules is then decreased and promotes salting out behavior and this phenomenon

leads to an increase in TK. Such a salting out behavior of lysozyme in the presence of

strong chaotropes has been observed previously [36]. Ions with small radius have high


59

charge density and have the capability to bind the water molecules more tightly around

the hydration sphere. Therefore, less chaotropic Cl− and NO3− should exhibit higher

tendency for hydration than more chaotropic SCN− and I−. It has been reported that when

an ion is more strongly hydrated than its oppositely charged partner, their contact ion

pairing is not thermodynamically favorable because the dehydration of more strongly

hydrated ion costs more its energy than it can gain by forming a contact ion pair with the

more weakly hydrated ion [31]. Therefore, rather than forming contact ion pairs these

ions tend to stay away from one another being separated by the solvent molecules.

Furthermore, nuclear magnetic resonance experiments established that these ions increase

the activity of water molecules and that the water molecules adjacent to a chaotrope whirl

around more rapidly than in the bulk of the solution as expected for a water molecule

which is not held by its neighbors through hydrogen bonding [26]. Therefore it can be

expected that the presence of these ions should increase the concentration of free water

molecules and promote hydration of the surfactant. As a consequence, the solubility of

the surfactant increases in the presence of these ions, resulting in a decrease in the TK.

Similar behavior has also been observed for cetyltrimethylammonium bromide (CTAB)

and cetylpyridinium bromide (CPB) in the presence of NO3− and Cl− [2, 5]. Furthermore,

the salting-in effect phenomenon suggests that an added salt having no common ions

should increase the solubility of a sparingly soluble salt when the activity coefficient is

less than one [17]. The added salt increases the ionic strength of the medium and hence

the activity coefficient decreases. In order for the thermodynamic solubility product to

remain constant the solubility of the sparingly soluble salt increases. In the present study,

up to 0.01 M (0.01 ionic strength for Na2SO4 solution) salt solutions were used to

investigate the salt effect on the TK of OTAB where the activity coefficients of the salt

solutions remain below unity. Therefore, it is reasonable to expect that the solubility of

the surfactant should increase in the presence of the salts leading to a decrease in the TK.

Such a salting-in effect with a consequent decrease in the TK of CTAB in the

presence of added Cl− has been observed previously [5]. Both SO42− and F− are

strongly hydrated kosmotropes. They do not form a contact ion pair with the

octadecyltrimethylammoniumion ion due to large difference in water affinities. As

mentioned above, aqueous SO42− and F− solutions show much higher surface tensions


60

relative to pure water than chaotropic ions. In other words, the concentration of SO42− and

F− in the bulk is much higher than that at the air–water interface [3]. The increase in

surface tension of aqueous solution of inorganic ions can be explained by the Gibbs

adsorption equation in terms of negative adsorption [25]. Because of high charge density,

both SO42− and F− are extensively hydrated in the bulk. As a result, these strong

kosmotropes do not show any tendency to lose the hydration shell to form a contact ion

pair with the weakly hydrated octadecyltrimethylammonium ion. Hence, there exists a

significant electrostatic repulsion between the charged surfactant molecules, which favors

their dispersion in the aqueous solution leading to a decrease in the TK. From Figure 3.4

it appears that SO42− has higher ability and I− has the lowest ability to decrease the TK and

the propensity follows the order: SO42− > Cl− > NO3

− > F− > Br− > SCN− > I−. Thus it

appears that even though SO42− is a strong kosmotrope, its role in terms of lowering the

TK is more pronounced than chaotropic NO3−. Chen et al. [37] reported that SO4

2−appears

in its usual position in the direct Hofmeister series but migrates from its position and

behaves more like a chaotropic ion when the protein surface is positively charged. This

has been attributed to its lower tendency to lose the hydration shell and a stronger

charge–charge interaction than singly charged anions. Probably this argument holds in

the present case as the nitrogen present in OTAB is also positively charged. In previous

studies it has been shown that SO42− migrates from its usual position when it interacts

with a positively charged group as observed in the case of protein monolayers [36, 37].

Such a migration of SO42− has also been observed for micelle formation of n-dodecyl

-D-maltoside in aqueous solution [38].

In the present work, the effect of hydrotropes on the TK of OTAB has also been

investigated. Hodgdon et al. reported that hydrotropes with an amphiphilic molecular

structure possess the ability to increase the solubility (low value of TK) of sparingly

soluble organic molecules in water [39]. They increase the solubility due to weak

interaction with solute molecules [40]. Thus these hydrotropic molecules interact with

cationic part of surfactant via weak vander waals interactions such as π–π or attractive

dipole–dipole interaction and remain separated from this ion by hydrated layers of water

molecules. As a result, these hydrotropes do not show any tendency to lose their


61

hydration shell so that they can form contact ion pairs with the weakly hydrated

octadecyltrimethylammonium ion. Hence, there exists a significant electrostatic repulsion

between the charged surfactant molecules. This repulsion between the charged surfactant

molecules favors their dispersion in the aqueous solution leading to a decrease in the TK

[41]. The efficiency of a hydrotrope solubilization depends on the balance between

hydrophobic and hydrophilic part of hydrotrope [42]. The larger is the hydrophobic part

of an additive, the better is the hydrotropic efficiency; the presence of the charge on the

hydrophilic part is less significant [43]. This implies why sodium salicylate shows lower

TK value compared to the other two hydrotropes. Vlachy et al. reported that carboxylate

and sulfonate ions behave like a kosmotrope and the chaotrope, respectively [18]. As a

result, chaotropic C6H5SO3− ion cannot increase solubility of surfactant solution as much

as kosmotropic C7H5O2− can. So the tendency of decreasing the TK of three hydrotropes

follows the order: C7H5O3− > C7H5O2

− > C6H5SO3− and the overall propensity of

decreasing the TK of OTAB solution follows the order: C7H5O3− > C7H5O2

− > C6H5SO3−

> SO42− > Cl− > NO3

− > F− > Br− > SCN− > I−.

3.2 EFFECT OF ADDED SALTS ON SURFACE ADSORPTION AND

MICELLIZATION

At a certain surfactant concentration in a system when all interfaces and surfaces are

occupied by surfactant unimers, the surfactant unimers in the bulk start to aggregate into

micelles. This is due to the fact that surfactant molecules do not want their hydrophobic

tails to be in contact with water. To avoid the contact of water micelles are formed with

the hydrophobic tails pointing inwards and the hydrophilic head groups pointing

outwards, towards the water. The formation of micelles from the surfactant unimers is

mediated from the favorable interaction between the hydrophobic alkyl chains and

opposing repulsive interaction between the charged headgroups as well as the degree of

neutralization of micelle surface charge by the associated counter-ions [44]. The CMC of

SDS and OTAB in pure water and in the presence of added electrolytes at different

temperatures was measured by conductometric and tensiometric methods above the TK


62

Figure 3.5: Conductometric determination of CMC of SDS in pure water at 30°C

Figure 3.6: Conductometric determination of CMC of SDS in the presence of 0.005M NaCl solution at 30°C

0 2 4 6 8 10 12 14 160

100

200

300

400

500

600

700

Sp

ecif

ic C

on

du

cta

nce (

µS

cm

-1)

Concentration of SDS (mM)

0 2 4 6 8 10 12 14600

700

800

900

1000

1100

1200

Sp

ecif

ic C

on

du

cta

nce (

µS

cm

-1)

Concentration of SDS (mM)


63

Figure 3.7: Conductance vs. surfactant concentration plot for OTAB in aqueous solution at 40°C

Figure 3.8: Conductance vs. surfactant concentration plot for OTAB in the presence of 0.005M NaCl solution at 40°C

0.0 0.1 0.2 0.3 0.4 0.5 0.60

5

10

15

20

25

30

35

Sp

ecif

ic C

on

du

cta

nce (

µS

cm

-1)

Concentration of OTAB (mM)

0.00 0.03 0.06 0.09 0.12 0.15 0.18

597

600

603

606

609

612

Sp

ecif

ic C

on

du

cta

nce (

µS

cm

-1)

Concentration of OTAB (mM)


64

of each system. The studied temperature range for SDS was 20-35°C and for OTAB 30-

45°C and 0.005 ionic strength of added salts for both the surfactant. Below the TK we

could not measure the CMC because at this temperature precipitation of hydrated crystals

occurs and above 45οC the vaporization of water occurs which changes the solution

concentration and also the practical use of surfactants solution is rather limited above this

temperature. The TK can increase when the salt to surfactant concentration ratio is too

high [45]. At 0.005 ionic strength of the salt, the TK of the OTAB remains below 45οC in

the presence of the most electrolytes (studied in this work) except SCN− and I− ions.

Therefore, we could not measure the CMC of surfactant in the presence of SCN− and I−

ions. The CMC values were found to agree within 2-3% for all the calculated data.

Figures 3.5 and 3.6 show the specific conductance (κ) versus SDS concentration plots at

30οC for the surfactant in pure water and in the presence of Cl−, respectively. Figures 3.7

and 3.8 show the plots for OTAB at 40οC in pure water and in the presence of NaCl

respectively. In the plot the slope of the pre-micellar region is greater than that of the

post-micellar region. It is observed that the κ increases gradually with increasing the

concentration of the surfactant. This can be ascribed to an increase in the number of

surfactant monomers with increasing concentration. The break point in the conductance

versus concentration curve indicates a sharp increase in the mass per unit charge of the

surfactant system in solution and is explained as the evidence of micelle formation from

the surfactant unimers with part of the charge of the micelle neutralized by the associated

counter-ions. The intersection point between two slopes indicates the CMC of the

surfactant.

The gradual decrease in the surface tension (γ) with increasing the surfactant

concentration (C) is a consequence of spontaneous adsorption of surfactant molecules

from the bulk of the aqueous solution to the air–water interface. The surface tension (γ)

of the surfactants was measured over a range of concentrations above and below the

critical micelle concentration (CMC). Representative plots of γ versus logarithm of the

SDS concentration (log10C) in pure water and in the presence of NaCl are shown in

Figures 3.9 and 3.10 at 30°C respectively. Figures 3.11 and 3.12 illustrate surface

tension plot for OTAB in pure water and in the presence of NaCl at 40°C, respectively.


65

Figure 3.9: Surface tensiometric determination of CMC of SDS in pure water at 30°C

Figure 3.10: Surface tensiometric determination of CMC of SDS in the presence of 0.005M NaCl solution at 30°C.

-3.4 -3.2 -3.0 -2.8 -2.6 -2.4 -2.2 -2.0 -1.8 -1.635

40

45

50

55

60

65

Su

rface T

en

sio

n/ m

Nm

-1

log10

C

-3.4 -3.2 -3.0 -2.8 -2.6 -2.4 -2.2 -2.0 -1.8 -1.635

40

45

50

55

60

Su

rface T

en

sio

n/ m

Nm

-1

log10

C


66

Figure 3.11: Surface tension vs. Log10C plot for OTAB in aqueous solution at 40°C

Figure 3.12: Surface tension vs. Log10C plot for OTAB in the presence of 0.005M NaCl solution at 40°C

-4.6 -4.4 -4.2 -4.0 -3.8 -3.6 -3.4 -3.235

40

45

50

55

60

Su

rface T

en

sio

n/ m

Nm

-1

log10

C

-5.0 -4.8 -4.6 -4.4 -4.2 -4.0 -3.8

36

40

44

48

52

56

60

Su

rface T

en

sio

n/ m

Nm

-1

log10

C


67

A linear decrease in surface tension was observed with increasing the concentrations for

both the surfactants up to the CMC. After the CMC the surface tension values remained

almost constant due to saturation of the solution surface by the adsorbed molecules. After

reaching to saturation limit of the air-water interface by the surfactant unimers, every

addition of the stock solution contributes to form CMC. The CMC at a definite

temperature is demonstrated from the intersection point of the γ versus log10C plot.

Although CMC determined by different methods varies to some extent but an individual

method shows good reproducibility [46]. The CMC obtained from the conductometric

method is slightly higher than that obtained from the surface tensiometric method (Table

3.1 and 3.2). At saturation limit of the surfactant solution, surface tension reaches the

minimum constant equilibrium value commonly referred to as the equilibrium surface

tension (𝛾CMC). The 𝛾CMC for pure SDS was found to be higher than pure OTAB solution.

With increasing the chain length, the hydrophobicity of a surfactant molecule increases.

Longer carbon chain length of OTAB facilitates them to stay at air-water interface with

high concentration than SDS. This high concentration of OTAB reduces equilibrium

surface tension more strongly.

The addition of salt has a remarkable effect on the surface tension [47]. Tables 3.1 and

3.2 also present the surface tension at CMC (𝛾CMC) of SDS and OTAB, respectively in

pure water and in the presence of added salts. All the 𝛾CMC values in the presence of salts

were found to be lower than that of corresponding pure SDS solution and the magnitude

follows the order: Li+ > Na+ > K+ > Cs+. But for OTAB it can be seen that except for F−

and SO42− 𝛾CMC significantly decreases from that of pure OTAB in the presence of

different anions, and the magnitude of the 𝛾CMC values follows the order: F− > SO42− >

Cl− > C7H5O2− > Br− > NO3

− > C6H5SO3− > C7H5O3

−. Thus, it appears that F− is least

effective, while C7H5O3− is most effective in lowering the 𝛾CMC value. Added salts screen

the charge of the head group of the surfactant. As a result, the electrostatic repulsion

between the head-group is substantially minimized. Under this circumstance, the

adsorbed molecules attain a closer molecular packing showing a lower 𝛾CMC value

compared to the corresponding value in pure water. Chaotropic ions have a large

polarizability and therefore have a strong dispersion interaction with the interface [46,

47]. As a result, they can effectively accumulate at the air-water interface and screen the


68

surface charge of the adsorbed molecules. Thus, electrostatic repulsion between the

charged headgroup substantially reduced favoring closer molecular packing in the

monolayer. Consequently, chaotropic ions are more effective in lowering the 𝛾CMC

compared to the kosmotropic ions. In our present study contrary to the usual trend

kosmotropic C7H5O3− is found most effective in lowering the 𝛾CMC. Due to hydrotropic in

nature this ion interact with surfactant ion via weak vander waals interactions in the bulk

of the solution [40]. Therefore, they show strong tendency to migrate to the air-water

interface. Their relative tendency depends on the size of the hydrophobic part on the

respective molecules. The larger the hydrotropic part greater will be the tendency to

migrate at the interface. As C7H5O3− has the larger hydrophobic part, it prevails in the

competition of substantial reduction of surface charge of the adsorbed surfactant

molecules. Thus it facilitates closer packing of OTAB molecules at the air-water interface

with a consequent decrease 𝛾CMC value. It is to be mentioned here that 𝛾CMC of F− and

SO42− is slightly higher than that of pure OTAB. Both the ions are kosmotropes.

Therefore, they preferentially remain in the bulk due to their extreme tendency for

hydration. Their hydrated shell is much thicker than those of Cl− and Br− [50]. This

hydrated layer, considering acting as steric hindrance and diffuse layer of F− are more

significant than the screening of the electric charge of the adsorbed monolayer [3]. The

origin of this steric hindrance probably arises from the fact that the water molecules in

the hydration shells are oriented differently toward the positively charged headgroup of

OTAB and negatively charged F− and SO42−. As a result, the monolayer attains a loose

molecular packing resulting in higher 𝛾CMC compared to that of pure OTAB.

Furthermore, at a given temperature molecular motion of F− is higher than SO42−. So

loose packing in monolayer for F− is higher than SO42− with a consequent higher 𝛾CMC for

F− than SO42−.

𝛾CMC values for SDS were found to increase with increasing temperature while for OTAB

values were found to decrease with increasing temperature. The lower the value of 𝛾CMC,

the higher will be the surface excess concentration (max) of the adsorbed molecules.

With increasing temperature, the molecular motion and chain flexibility increase and

result in a poorer packing of the molecules in adsorbed monolayers of SDS. In the case of


69

OTAB the dehydration effect prevails over molecular motion of monomeric ions and

facilitates them closer packing with a consequent decrease in 𝛾CMC.

The CMC value for OTAB is found to be lower than that of SDS. Longer carbon chain

length of OTAB facilitates them to entangle with minimum number of monomer and thus

lowering the CMC. It is well known that temperature is one of the major factors affecting

the CMC. The effect of temperature on the CMC of amphiphiles in aqueous solution is

usually a consequence of two opposing phenomena [48, 49]. First, with the increase in

temperature, dehydration of the ionic head group increases, leading to an increased

hydrophobicity of the amphiphile molecule. This favors the aggregation process and

lowers the CMC. On the other hand, an increase in temperature results in the breakdown

of the structured water surrounding the hydrophobic chain along with thermal motion of

the molecules. This is unfavorable for aggregation and the CMC increases [49, 50]. These

two factors determine whether the CMC will decrease or increase over a particular

temperature range. The first factor dominates usually in the low temperature range where

above a certain temperature, the second factor starts to dominate. However, the literature

also contains examples of a continuous increase in CMC with temperature [51, 52].

Rosen [53] reported that the CMC decreased with temperature initially and then increased

with increasing temperature. With increasing temperature dehydration of the hydrophilic

group occurred which favors micellization.


70

Tab

le 3

.1:

CM

C v

alu

es o

f S

DS

at d

iffe

ren

t te

mp

erat

ure

s i

n p

ure

wat

er

and

in

th

e p

rese

nce

of

0.0

05

io

nic

str

engt

h

solu

tio

ns

of

so

me

elec

tro

lyte

s

308

K

𝛾C

MC

38.6

38.0

37.5

35.8

35.6

CM

C (m

M)

Surfa

ce

Tens

iom

etry

8.13

7.24

6.61

5.89

5.49

Con

duct

om

etry

8.30

7.40

6.90

5.90

5.60

303K

𝛾C

MC

38.3

37.9

37.2

35.5

35.5

CM

C (m

M)

Surfa

ce

Tens

iom

etry

7.94

7.08

6.46

5.37

5.13

Con

duct

om

etry

8.20

7.20

6.60

5.70

5.30

298K

𝛾C

MC

37.9

37.7

37.0

35.3

35.3

CM

C (m

M)

Surfa

ce

Tens

iom

etry

7.76

6.76

5.89

5.49

4.79

Con

duct

om

etry

8.04

7.00

6.20

5.60

5.00

293K

𝛾C

MC

37.7

37.5

36.7

-

35.0

CM

C (m

M)

Surfa

ce

Tens

iom

etry

7.94

6.92

6.02

-

4.89

Con

duct

om

etry

8.10

7.10

6.40

-

5.10

Syst

em

Pure

SD

S

LiC

l

NaC

l

KC

l

CsC

l


71

Tab

le 3

.2:

CM

C v

alu

es o

f O

TAB

at

dif

fere

nt

tem

per

atu

res

in

pu

re w

ate

r an

d i

n t

he

pre

sen

ce o

f 0

.00

5 i

on

ic s

tren

gth

so

luti

on

s o

f s

eve

ral e

lect

roly

tes

318K

𝛾C

MC

36.3

0

32.3

33.4

30.2

36.5

33.1

33.1

35.6

38.3

CM

C (m

M)

Surfa

ce

Tens

iom

etry

0.30

0.02

6

0.03

8

0.03

5

0.06

8

0.06

9

0.07

9

0.07

9

0.09

5

Con

duct

om

etry

0.30

0.03

3

0.03

9

0.04

3

0.07

9

0.07

9

0.08

7

0.09

3

0.09

7

313K

𝛾C

MC

37.1

5

33.3

33.9

30.8

37.4

33.5

33.8

36.8

38.9

CM

C (m

M)

Surfa

ce

Tens

iom

etry

0.25

0.01

8

0.02

8

0.03

1

0.05

8

0.05

6

0.05

9

0.07

2

0.10

Con

duct

om

etry

0.28

0.02

3

0.02

9

0.03

6

0.06

1

0.06

6

0.06

7

0.07

5

0.11

308K

𝛾C

MC

37.8

33.9

34.8

31.3

38.0

34.3

-

37.3

39.4

CM

C (m

M)

Surfa

ce

Tens

iom

etry

0.

24

(310

)

0.01

6

0.02

2

0.02

1

0.05

0

0.05

0

-

0.05

8

0.11

Con

duct

om

etry

0.25

(3

10)

0.01

9

0.02

3

0.02

3

0.05

1

0.06

2

-

0.06

1

0.13

303K

𝛾C

MC

36.4

34.2

35.1

31.9

38.7

- -

36.0

-

CM

C (m

M)

Surfa

ce

Tens

iom

etry

0.

28

(316

)

0.01

3

0.01

5

0.01

6

0.04

5

- -

0.08

1 (3

16)

-

Con

duct

om

etry

0.30

(3

16)

0.01

4

0.02

1

0.01

8

0.04

7

- -

0.08

8

(316

)

-

Syst

em

Pure

OTA

B

C6H

5SO

3Na

C7H

5O2N

a

C7H

5O3N

a

Na 2

SO4

NaN

O3

NaB

r

NaC

l

NaF


72

However temperature increase also causes disruption of the structured water surrounding

of the hydrophobic group, an effect that disfavors micellization. The relative magnitude

of these two opposing effects, therefore, determines whether the CMC increases or

decreases over a particular temperature range. In the present work, we have found that

CMC of SDS decreases upto room temperature and then increases with increasing

temperature for all pure surfactant and in the presence of added salts over the studied

temperature range [Table 3.1]. Now it is obvious that dehydration of hydrophilic group

dominates over disruption of the water structure around hydrophobic group upto room

temperature and disruption of the water structure around hydrophobic group prevail over

the dehydration of hydrophilic group beyond the room temperature. For OTAB, CMC

values were found to increase with temperature [Table 3.2] in the presence of all added

salts except NaF over the studied temperature range. So it is assumed that disruption of

the water structure around hydrophobic group prevails over the first one. Because of high

charge density F− remains extensively hydrated. With increasing temperature dehydration

of hydrophilic group dominates over the disruption of the structured water surrounding of

the hydrophobic group. That is why increasing temperature favors micellization of NaF

with a consequent decrease of CMC. Addition of electrolytes in surfactant solution has

significant influence on lowering the CMC. Table 3.1 shows the CMC of SDS in the

presence of the added electrolytes. The uncertainty in the CMC values is found to be

within 1-2%. From the Table 3.1 and 3.2 it is clear that the CMC decreases significantly

in the presence of the added electrolytes favoring assembling of the surfactant molecules

in the bulk of the aqueous solution. A significant number of papers have dealt with the

effect of electrolytes (containing a common ion to that of the surfactant) on the CMC of

ionic surfactants [54-56]. These studies have shown that the CMC decreases in the

presence of added electrolytes, which has been attributed to partial neutralization of

surface charge by the excess counter-ions. When counter-ions adsorb at micelle surface,

they screen the charge of surfactant headgroups. Thus electrostatic repulsion between the

surfactant molecules is substantially reduced. The screening of the micelle surface charge

reduces electrostatic repulsion between the charged headgroups and promotes axial

growth of micelles [33]. The effectiveness of lowering the CMC appears to be dependent

on the nature of the added counter-ion.


73

The chaotropic Cs+ is the most effective and kosmotropic Li+ is the least effective at

lowering the CMC. The counter-ions remain in the solution contribute to minimize the

net charge of surfactant headgroups and thus electrostatic repulsion between the head

groups which paves the way for micelle formation at lower SDS concentration in the

presence of added electrolytes. Onoratoa proposed that kosmotropic and chaotropic

interact differently with their counterparts [57]. Weakly hydrated ion interact most

strongly with oppositely charged head groups with a result of close packing of head

groups in the micelles. Weakly hydrated chaotropes preferentially adsorb at the

hydrophobic surface and directly disturb hydrophobic hydration [29]. On the other hand,

strongly hydrated kosmotrope do not show any tendency to lose its hydrated sphere. As a

result, kosmotropic Li+ cannot interact strongly with oppositely charged head groups and

cannot reduce the CMC as much as chaotropic K+ and Cs+ can. Thus chaotropic K+ and

Cs+ are much more effective in lowering the CMC compared to the other cations in this

work. Na+ being a weak kosmotrope is less effective to lower the CMC. The

effectiveness in lowering the CMC the ions follows the order: Cs+ > K+ > Na+ > Li+.

Table 3.2 shows the CMC of OTAB in the presence of the added electrolytes.

Hydrotropic C6H5SO3− is the most effective and kosmotropic F− is the least effective in

lowering the CMC. Chaotropic NO3− and Br− are weakly hydrated. So they can form

contact ion pair quickly with the surfactant headgroup with a consequent decrease in

headgroup charge. This phenomena facilitates closer packing of surfactant in the micelle

and thus decreases CMC. On the other hand, kosmotropic Cl− and F− are strongly

hydrated and do not show any tendency to lose their hydration sphere. Therefore, they do

not come into close contact with the micelle surface. Thus chaotropic NO3− and Br− are

more effective in lowering the CMC compared to Cl− and F−. When there is more than

one counter-ions present in the surfactant solution, a competition of counter-ions

adsorption on micelle surface occurs. In such a case, multivalent ion prevails over the

monovalent ion [33, 58] and this is also true for SO42− ion. Doubly charged SO4

2− interact

with OTAB head groups more efficiently and doubly effective compared to monovalent

counterparts in screening the charge of OTAB head group with reduction of electrostatic

repulsion between the charged headgroups. This paves the way for easier micelle

formation from the monomeric surfactant molecules. Mason and coworkers reported that


74

the SO42− behaves like a chaotrope and shifts from its usual position in the Hofmeister

series when it interacts with positively charged nitrogen [59]. In the present study SO42−

interacts with Octadecyltrimethylammonium ion and behave like a chaotrope shifting

from its usual position in the Hofmeister series and thus lowering the CMC.

Hydrotropes are most effective at lowering the CMC of the surfactant. They do not

aggregate in well-arranged structures such as micelles, but somewhat form dimers,

trimers, etc [60]. It suggests that the mixed micelles are formed due to attractive

interactions between the surfactant and hydrotropes. These hydrotropes are tied on the

ionic headgroup of the surfactant, reducing the headgroup repulsions and favoring

micellization. Among the three hydrotropes used in this study C6H5SO3− being more

chaotropre decrease the CMC more effectively than that of kosmotropic C7H5O2− and

C7H5O3−. The effectiveness in lowering the CMC the ions follows the order: C6H5SO3

− >

C7H5O2− > C7H5O3

− > SO42− > NO3

− > Br− > Cl− > F−

3.3 SURFACE EXCESS CONCENTRATION

The surface excess concentration () is an important physical property of adsorbed

molecules which is closely related to formation of an oriented surfactant monolayer. This

is defined as the concentration of surfactant molecules at the surface, relative to that in

the bulk. Monolayer formation by surfactant system is of theoretical interest and

industrial importance. One of the most important aspects of surfactant adsorption at the

air–water interface is its relationship to surface tension reduction. Monolayer formation

affects contact angle with a solid surface (affecting flotation), rate of wetting of a solid,

and foaming (with applications in enhanced oil recovery or fire extinguishers). So it is

important to understand monolayer composed of surfactant as well as surface excess

concentration () [61].

Figures 3.13 and 3.14 show the variation of the surface excess concentration of SDS and

OTAB at different temperatures in the presence of NaCl respectively.


75

Figure 3.13: Surface excess concentration of SDS (i) in pure and (ii) in 0.005M aqueous solution of NaCl

Figure 3.14: Surface excess concentration of OTAB (i) in pure and (ii) in 0.005M NaCl solution.

292 296 300 304 3081.8

2.0

2.2

2.4

2.6

2.8

(ii)

(i)

Su

rface E

xcess C

on

c. (1

0-6m

ol/m

2)

Temperature (K)

308 310 312 314 316 318

1.95

2.00

2.05

2.10

2.15

2.20

(ii)

(i)

Su

rface E

xcess C

on

c. (1

0-6m

ol/m

2)

Temperature (K)


76

The Г values of SDS and OTAB at a definite temperature was calculated from the slope

of the straight line of the surface tension vs. log10C plot before the CMC with the help of

the following equation [62].

𝛤1

2.303𝑛𝑅𝑇

∂γ

∂logC)𝑇,𝑃 (3.1)

where the pre-factor, n is the number of species formed in solution by the dissociation of

the surfactant (for a non-ionic candidate, n= 1; for totally dissociated ionic surfactant, n=

2), (∂γ/∂logC) is the maximum slope, R is the gas constant (8.314JK-1mol-1), T is the

absolute temperature in Kelvin, C is the surfactant concentration in the bulk. All the Г

values were calculated within 1-2% error. In all cases the surface excess concentration is

positive indicating that the surfactant has more concentration at the surface as compared

to that in the bulk. This is termed as positive adsorption and is exhibited by all the

surfactant molecules which accumulate mostly at the surface. It can be seen from the

Figure 3.13 that there is a decreasing trend in Г values with increasing temperature while

Figure 3.14 shows increasing trend Г values with an increase in temperature. It can be

explained firstly, the dehydration of the hydrophilic head-group and secondly, the

thermal motions of the adsorbed molecules at the air-water interface. The dehydration

effect results in shrinkage of the head-group size and provides a close molecular packing

in the adsorbed monolayer. On the other hand, with an increase in temperature the

adsorbed molecules at the air-water interface become disorganized due to an increase in

kinetic energy, thermal motion and chain flexibility [63, 64]. For SDS, in the presence of

added NaCl as the temperature increase van der Waals interactions between the alkyl

chains become more and more unfavorable. Besides, an increase in the temperature

brings about disturbance in the adsorbed molecules that dominates over the dehydration

effect and hinders closer molecular packing of the monolayer at the air-water interface.

Consequently, the Г values show a gradual decreasing trend with increasing temperature.

It is assumed that for OTAB with salt as the temperature increases dehydration of the

hydrophilic head dominates over molecular motion which helps closer packing of the

molecules in the interface. As a result, Г values show a gradual increasing trend with

increasing temperature. Besides, for solutions of ionic surfactants an electrostatic surface


77

potential acts as a barrier for the adsorption of additional molecules as they migrate from

the bulk of the solution to the air-water interface. When an electrolyte is introduced to the

surfactant solution electrostatic screening of surface potential occurs at the air-water

interface [65, 66]. As a result, the obstruction for further adsorption of surfactant

molecules is substantially reduced, giving higher surface excess concentration of

the adsorbed molecules in the presence of NaCl for both SDS and OTAB. This higher

surface excess concentration can also be attributed to the lower equilibrium surface

tension of SDS and OTAB in the presence of NaCl.

3.4 THERMODYNAMICS OF MICELLIZATION

The thermodynamics of micellization processes of ionic surfactants with different

additives received much attention in recent years as the thermodynamic parameters are

powerful means for elucidating the mechanism of the micellization and effects of

additives on the micellization process [67]. The Thermodynamics of micelle formation of

the cationic surfactant OTAB and anionic surfactant SDS in water and aqueous NaCl

solutions were investigated. Conductometric method has been used to study the effect of

the added NaCl on the critical micelle concentration, CMC and enthalpy of micellization,

∆Hm° between 293 and 308 K for SDS and between 308 and 318 K for OTAB. Gibbs free

energy, ∆Gm° and entropy, ∆Sm° were deduced by taking into account the counterion

binding. Estimates of the thermodynamic parameters of micellization the free energy

(∆Gm°), the entropy (∆Sm°), the enthalpy (∆Hm°) have been determined for anionic

surfactant SDS and cationic surfactant OTAB from the following expression [68].

∆Gm° = (1 + 𝛽) RT ln Xcmc (for pure surfactant) (3.2)

∆Gm° = RT [lnXcmc + (1−𝛼) ln (Xcmc + Xs)] (in the presence of salt) (3.3)

∆Sm° = − 𝜕(∆𝐺𝑚

° )

𝜕𝑇 P (3.4)

∆Hm° = 𝑇∆Sm° + ∆Gm° (3.5)


78

Where β is the degree of counter-ion binding, Xcmc is mole fraction of the surfactants

and Xs is the mole fraction of salts at the CMC. Details of calculation will be found in the

calculation part at the end of this paper. All the thermodynamic parameters have been

calculated within 2-3% error. The data of thermodynamics parameters during aggregates

formation are listed in Tables 3.3, 3.4, 3.5 and 3.6 from which it can be seen that:

Table 3.3: Thermodynamic parameters of adsorption and micellization* of the SDS surfactants

solution.

Temp/K

ΔHm°/

kJmol-1 ΔHad°/ kJmol-1

ΔSm°/ Jmol-1K-1

ΔSad°/ Jmol-1K-1

ΔGm°/ kJmol-1

ΔGad°/ kJmol-1

293 -5.18 0.60 100.44 171.01 -34.61 -49.50

298 -15.33 2.75 66.34 178.51 -35.10 -50.44

303 -25.46 5.11 32.24 186.01 -35.23 -51.26

308 -35.96 7.33 -1.86 193.51 -35.38 -52.27

Table 3.4: Thermodynamic parameters of adsorption and micellization* of the SDS – 0.005M NaCl surfactants solution.

Temp/K ΔHm°/


ΔSm°/ Jmol-1K-1

ΔSad°/ Jmol-1K-1

ΔGm°/ kJmol-1

ΔGad°/ kJmol-1

293 -27.78 71.48 23.02 407.7 -34.53 -47.98

298 -27.48 49.26 23.62 331.7 -34.52 -49.59

303 -27.49 26.06 24.22 255.7 -34.83 -51.41

308 -27.18 3.09 24.82 179.7 -34.82 -52.26

*The CMC values were taken in mole fractions for the calculation of the thermodynamic parameters.


79

Table 3.5: Thermodynamic parameters of adsorption and micellization* of the OTAB surfactants solution.

Temp/K

ΔHm°/


ΔSm°/ Jmol-1K-1

ΔSad°/ Jmol-1K-1

ΔGm°/ kJmol-1

ΔGad°/ kJmol-1

310 -69.70 -131.64 -59.33 -205.96 -51.31 -67.79

313 -68.05 -112.40 -54.03 -144.21 -51.14 -67.26

316 -66.09 -85.31 -47.19 -60.41 -50.99 -66.98

318 -65.26 -79.93 -45.20 -41.29 -50.89 -66.80

Table 3.6: Thermodynamic parameters of adsorption and micellization* of the OTAB – 0.005M

NaCl surfactants solution.

Temp/K

ΔHm°/


ΔSm°/ Jmol-1K-1

ΔSad°/ Jmol-1K-1

ΔGm°/ kJmol-1

ΔGad°/ kJmol-1

308 -74.89 -152.97 -74.78 -276.52 -51.86 -67.80

313 -63.34 -100.87 -37.58 -108.72 -51.58 -66.84

316 -55.78 -69.1 -8.54 -65.34 -51.53 -66.74

318 -51.61 -47.92 -0.38 59.08 -51.49 -66.71

*The CMC values were taken in mole fractions for the calculation of the thermodynamic

parameters.

(i) The values of free energy change (∆Gm°) during micelle formation for pure SDS and

OTAB as well as in the presence of NaCl are found to be negative. This means that

micelle formation is a spontaneous process for the surfactants. Based on Equation (3.2

and 3.3), ∆Gm° values are affected by both critical micelle concentration (CMC,

expressed as molar fraction) and miceller degree of ionization (α). The values of ∆Gm°

for OTAB are found to be more negative than that of SDS in pure water and in the

presence of NaCl. This indicates OTAB form micelle more spontaneously than SDS.

Longer alkyl chain lengths result in considerably more negative values of ∆Gm° [68]. It is


80

found that the changes in ∆Gm° with increasing the temperature are very small. In the

case of SDS ∆Gm° is found to become slightly more negative while in case of OTAB the

negative values are found to decrease with increasing temperature. This suggests

that spontaneity of micellization increases for SDS while decreases for OTAB with

temperature.

(ii) All ∆Hm° values for both the surfactants over the studied temperature range are

negative, indicating that micelle formation is an exothermic process. ∆Hm° values are

found to be more negative for OTAB than SDS. The negative contribution to the ∆Hm° is

indicative of the transfer of the hydrocarbon chains into the micelles and restoring the

hydrogen bonding structure of the water around the micelles [69]. In addition, the

negative ∆Hm° values can be taken as the evidence of London dispersion force, a

major attractive force for micellization which becomes more and more dominant with

increasing the hydrocarbon chain length [38]. With increasing temperature, the three-

dimensional structure of the hydration water is partially broken down and, consequently,

the role of hydrophobic and other dehydration becomes weaker because less energy is

required to break up the three-dimensional water structure. Thus ∆Hm° became more

exothermic [70]. It is evident from the Tables 3.3, 3.4, 3.5 and 3.6 that all ∆Hm° values

are negative for SDS and OTAB. With increasing the chain length of a surfactant

molecule, the enthalpy of micellization becomes more negative. This suggests that the

enthalpy term for OTAB is more effective in contributing to the free energy term

than the SDS.

(iii) Entropy makes a major contribution to ∆Gm°. Over the investigated temperature

range, the entropy change of micellization (∆Sm°)for SDS and OTAB presents different

trends. The ∆Sm° values for SDS are found to be positive while for OTAB the values are

negative. The values are found to decrease with increasing temperature except SDS in the

presence of NaCl. The negative ∆Sm° values means that there is a reduction of disorder at

the molecular level, probably because the effect of the liberation of surfactant hydration

water molecules on micellization becomes less important than the loss of freedom when

monomers join each other to form micelles [71]. The positive ∆Sm° values indicate that

the micellization process is associated with the destruction of the iceberg around the


81

hydrophobic alkyl chain. On the other hand, lower values of ∆Sm° for OTAB compared

to those of SDS are probably a result of the organization of a greater number of OTAB

molecules from randomly oriented monomers to well organized micelle structure.

3.5 THERMODYNAMICS OF SURFACE ADSORPTION

Before the conductance and surface tension measurement, TK were measured to ensure

the absolute dissolution for the surfactants in water and in the presence of salts at the

experimental temperature. Based on the surface and aggregation properties, the

thermodynamics of adsorption during the aggregates formation in the aqueous solution of

surfactant was studied. Tables 3.3, 3.4, 3.5 and 3.6 also show the thermodynamic of

adsorption of OTAB and SDS at the air-water interface. The free energy of

adsorption (∆Gad°), enthalpy of adsorption (∆Had°) and entropy of adsorption (∆Sad°)

values were calculated from the following expression [46, 53, 72]

(∆𝐺ad°) = (∆𝐺m°)−( 𝜋cmc / 𝛤max) (3.6)

where, 𝜋cmc and𝛤max are the equilibrium surface pressure and the surface concentration of

the adsorbed molecules, respectively, at and above the CMC. The ∆𝑆ad° and ∆𝐻ad° were

calculated from the relationships corresponding to Equation 3.4 and 3.5 like that-

∆Sad° = − 𝜕(∆𝐺𝑎𝑑

° )

𝜕𝑇 p (3.7)

∆Had° = 𝑇∆Sad° + ∆Gad° (3.8)

The free energy of adsorption is the energy required to transfer 1 mol of surfactant in

solution to the surface at unit surface pressure. The adsorption of the surfactant molecule

at the solution–air interface causes a decrease in free energy, indicating that the head of

the adsorbed surfactant molecule is orientated towards the interface, so that the chains

move away from the aqueous phase [73].

The ∆𝐺ad° values for SDS and OTAB are negative which indicates that the adsorption of

monomeric surfactant from bulk of the solution to the surface is a spontaneous process.

For SDS with added NaCl, the values are found to become more negative, suggesting that


82

adsorption becomes more spontaneous with increasing temperature. This result is in line

with the increases in the hydrophobicity of the molecules caused by the dehydration of

the headgroup with increasing temperature [63]. On the other hand ∆𝐺ad° values for

OTAB solution and in the presence of NaCl become a bit less negative with increasing

temperature suggesting molecular motion of monomer at high temperature dominates

over dehydration. At a given temperature, the ∆𝐺ad° values of OTAB are found to be

more negative than the corresponding ∆𝐺ad° values of SDS (Tables 3.3, 3.4, 3.5 and 3.6).

This may due to longer hydrocarbon tail present in OTAB than that of SDS. Moreover, at

a definite temperature the ∆𝐺ad° values are found to be more negative than the

corresponding ∆Gm° values, suggesting that adsorption of monomeric surfactant

molecules at the air-water interface is more spontaneous than micelle formation in the

bulk.

The ∆𝐻ad° values for SDS are found to be positive while for OTAB the values are

negative. This result implies stronger van der Waals interaction between alkyl chains

during micellization due to the presence of longer hydrophobic chain in OTAB.

The ∆𝐻ad° values become more positive for pure SDS and less positive in the presence of

NaCl and for OTAB become less negative with increasing temperature. Moreover at

lower temperature surfactant remains hydrated and require more energy to adsorb at the

air-water interface while at high temperature less energy is required to adsorb at the

interface [63, 74, 75]. This is why over the studied temperature range the ∆𝐻ad° values

become positive for SDS and negative for OTAB. The more negative value of ∆𝐻ad° than

∆𝐻m° for OTAB suggests easier adsorption than micellization of the surfactant monomer.

The ∆𝐻ad° values are positive and ∆𝐻m° is negative for SDS. This indicates that fewer

bonds between surfactant molecules and water molecules are broken in the process of

adsorption at the air/aqueous solution interface than in micellization [76].

The ∆Sad° values for SDS are positive while for OTAB the values are negative. This may

due to longer hydrocarbon chain present in OTAB than that of SDS. The positive

∆Sad° value suggests that the adsorption at the air/liquid interface is favored by entropy

effect [77]. The ∆Sad° values are higher than ∆Sm° for SDS. This may reflect greater

freedom of motion of the hydrocarbon chains at the planar air/aqueous solution interface


83

compared to that in the relatively cramped interior beneath the convex surface of the

micelle [76]. The lower ∆Sad° values than ∆Sm° for both SDS and OTAB indicate

domination of the relative molecular motion in the bulk than surface. The ∆Sad° values

are found to increase except SDS in the presence of NaCl with increasing temperature.

The ∆Sad° value is governed by the following competitive factors: A positive ∆Sad° values

can arise from the destruction of the ordered ice-berg structure around the hydrophobic

alkyl chain and the subsequent dangling of the alkyl chains of the adsorbed surfactant

molecules at the air-water interface. On the contrary, a negative ∆Sad° value can arise

from the spontaneous adsorption of the surfactant molecules in the form of organized

monolayer and the concomitant loss of one degree of rational freedom of the adsorbed

molecules at the air-water interface.

Figures 3.15 and 3.16 show the linear relationships between enthalpy changes

(∆Hm°, ∆Had°) and the entropy changes (∆Sm°, ∆Sad°) of SDS and OTAB, respectively.

This linear relationship is called the enthalpy–entropy compensation phenomenon [78,

79]. The micellization/adsorption of SDS and OTAB in the presence of NaCl also exhibit

such a compensation phenomenon. Lumry and Rajender [80, 81], reported that the

micellization/adsorption involves a two-part process: (1) the „desolvation‟ part, i.e., the

dehydration of the hydrocarbon tail of surfactant molecules, and (2) the „chemical‟ part,

i.e., aggregation of the hydrocarbon tails of surfactant molecules in the formation of

micelle. The study of enthalpy–entropy compensation phenomena can provide a measure

of the desolvation part of the micellization/adsorption process through the temperature of

compensation Tc, having the dimension of the Kelvin temperature, which is the slope of

the plot [82]. This parameter is a characteristic of solute–solute and solute–solvent

interactions, as suggested by Chen et al [83].

In general the compensation between the enthalpy and entropy changes can be described

as, ∆H°m/ad = ∆H*m/ad + Tc∆S°m/ad, where ∆H*

m/ad is the intercept on the enthalpy axis that

represent the solute–solute interactions and can be considered as an index of the chemical

part of the process of micellization/adsorption, and it stands for the enthalpy effect under

the condition ∆S°m/ad = 0. ∆S°m/ad stands for the entropy effect under the condition

∆H*m/ad = 0.


84

Figure 3.15: Enthalpy-Entropy compensation plot for (a) Micellization (b) surface Adsorption of SDS in aqueous solution

0 20 40 60 80 100-40

-35

-30

-25

-20

-15

-10

-5

(a)

H

m(k

Jm

ol-1

)

Sm(Jmol

-1K

-1)

170 175 180 185 190 1950

1

2

3

4

5

6

7

8

(b)

H

ad(k

Jm

ol-1

)

Sad

(Jmol-1K

-1)


85

Figure 3.16: Enthalpy-Entropy compensation plot for (a) Micellization (b) surface Adsorption of OTAB in aqueous solution

-60 -58 -56 -54 -52 -50 -48 -46 -44

-70

-69

-68

-67

-66

-65 (a)

H

m(k

Jm

ol-1

)

Sm(Jmol

-1K

-1)

-210 -180 -150 -120 -90 -60 -30-140

-130

-120

-110

-100

-90

-80(b)

H

ad(k

Jm

ol-1

)

Sad

(Jmol-1K

-1)


86

The enthalpy–entropy compensation plots for micellization/adsorption for SDS and

OTAB are parallel to one another. The enthalpy and entropy terms are found to

compensate each other for both micellization and adsorption at the air-water interface

and the linear relationship indicates same mechanism for all the processes. As shown

in Table 3.7, the compensation temperature Tc, is within the range of 300.5–300.7 K for

SDS in pure water and 298.3– 300.5 K for SDS in the presence of 0.005M NaCl solution.

For OTAB in pure water, Tc is within the range of 314.2–314.4 and in the presence of

0.005M NaCl Tc is within the range of 312.9–313.0. The Tc values obtained from the

slopes of Figures 3.15 & 3.16 are shown in Table 3.7. The values obtain for OTAB and

SDS for both adsorption and micelle formation is found to lie in the suggested literature

values [84-86]. When the entropy contributes less to the free energy, its counterpart, the

enthalpy term contributes more to keep the negative free energy change to a

nearly constant value. Such a behavior has been observed for aqueous solution of ionic

surfactant previously [84-86].

Table 3.7: Tc value for OTAB and SDS in water and 0.005M NaCl solution

System Process Compensation

Temperature (Tc)

SDS in pure water Adsorption 300.7

Micellization 300.5

SDS-0.005M NaCl Adsorption 300.5

Micellization 298.3

OTAB in pure water Adsorption 314.2

Micellization 314.4

OTAB-0.005M NaCl Adsorption 313.0

Micellization 312.9


87

3.6 SOLUBILIZATION STUDY OF SUDAN RED B (SRB)

It is well known that surfactants self-aggregate spontaneously to form micelles in

aqueous solution and this property is the basis of their use in many industrial processes.

Micelle-enhanced solubilization of nonpolar compounds is one of the more significant

applications of surfactants. Micelles of surfactants, containing an inner hydrophobic core

and an extended interfacial region called mantle, can incorporate other molecular species

into their structure, which is known as micellar solubilization [87, 88]. Below the

surfactant‟s critical micelle concentration (CMC), surfactants exist as monomers and

have only minimal effects on the aqueous solubility of organics. Micellar solubilization

occurs at the CMC and increases almost linearly with the surfactant concentration

[89]. When the surfactant concentration exceeds the CMC, the aqueous solubility of

organics is enhanced by the incorporation of hydrophobic molecules into surfactant

micelles [90, 91].

The solubilizations of SRB in both pure water and in the presence of added electrolytes

were studied. Figures 3.17 and 3.18 show the absorption spectra of solubilization of SRB

in pure OTAB solution and in the presence of Na2SO4 solution respectively. Several

surfactant concentrations ranging from 0.05 to 2 mM for pure OTAB solution and 0.01 to

0.8 mM for OTAB solution in 0.005 ionic strength Na2SO4 solution (some of which are

below the CMC and some are above the CMC) were used to carry out the solubilization

study. Figures 3.19 and 3.20 show the absorption spectra of solubilization of SRB in

pure SDS solution and in the presence of NaCl solution, respectively. In this case

surfactant concentrations ranging from 4 to 30 mM for pure SDS solution and 3 to 20

mM for SDS solution in 0.005M NaCl solution (some of which are below the CMC and

some are above the CMC) were used. Fixed amount but excess to that of the

solubilization equilibrium of the SRB dye with OTAB and SDS micelle was used for

solubilization study. It is important to note here that no significant absorbance was found

below the CMC. On the other hand above the CMC value the absorbance increases with

increasing the surfactant concentration (Figures 3.17- 3.20) which is in line with the

previous observation [92]. This means below the CMC there is no incorporation of


88

Figure 3.17: Effect of surfactant concentration on the absorption spectra of SRB: i 0.4, ii 0.6, iii 1.0, iv 1.5 and v 2.0 mM OTAB solutions in pure water.

Figure 3.18: Effect of surfactant concentration on the absorption spectra of SRB: i 0.06, ii 0.1, iii 0.2, iv 0.4 and v 0.8 mM OTAB solutions in 0.005 ionic strength Na2SO4

400 450 500 550 600 650 700 750

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

v

iv

iii

ii

i

508 (max

)

517 (max

)

Ab

so

rban

ce

Wavelength (nm)

400 450 500 550 600 650 700

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8v

iv

iii

ii

i

512 (max

)

520 (max

)

Ab

so

rban

ce

Wavelength (nm)


89

Figure 3.19: Effect of surfactant concentration on the absorption spectra of SRB: i 8, ii 9, iii 10, iv 15, v 20 and vi 30 mM SDS solutions in pure water.

Figure 3.20: Effect of surfactant concentration on the absorption spectra of SRB: i 6, ii 7, iii 8, iv 9, v 10 and vi 20 mM SDS solutions in 0.005M NaCl

400 500 600 700 800

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

vi

v

iv

iii

iii

520 (max

)

524 (max

)

Ab

so

rban

ce

Wavelength (nm)

400 500 600 700

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

vi

v

iv

iii

ii

i

517 (max

)

524 (max

)

Ab

so

rban

ce

Wavelength (nm)


90

Figure 3.21: Solubilization of SRB in OTAB solution in (a) pure water and (b) 0.005 ionic strength aqueous Na2SO4 solution. The break point in the curve shows the CMC below which no significant absorbance was observed. This indicates the SRB solubilized only when OTAB forms micelles

-0.2 0.0 0.2 0.4 0.5 1.0 1.5 2.0

0.00

0.01

0.02

0.03

0.04 (a)

Co

nc. o

f S

ud

an

Red

B (mM

)

OTAB Conc. (mM)

-0.02 0.00 0.02 0.040.05 0.10 0.15 0.20

0.000

0.001

0.002

0.003

0.004

0.005(b)

Co

nc. o

f S

ud

an

Red

B (mM

)

OTAB Conc. (mM)


91

Figure 3.22: Solubilization of SRB in SDS solution in (a) pure water and (b) 0.005M NaCl solution. The break point in the curve shows the CMC below which no significant absorbance was observed. This indicates the SRB solubilized only when SDS forms micelles

4 5 6 7 8 9 100.000

0.001

0.002

0.003

0.004

0.005

(a)

Co

nc. o

f S

ud

an

Red

B (mM

)

SDS Conc. (mM)

2 3 4 5 6 7 8 9 10 110.000

0.001

0.002

0.003

0.004

0.005

0.006

0.007

(b)

Co

nc. o

f S

ud

an

Red

B (mM

)

SDS Conc. (mM)


92

hydrophobic SRB molecules into micelle but above the CMC higher amount of SRB

molecules are taken up by the surfactant micelle. Such a solution of an apolar substance

in a micellar solution is thermodynamically stable [93]. There are several possible

locations for a solubilizate in a surfactant micelle: the very hydrophobic inner core, the

less hydrophobic environment just below the head group region, the head group

palisade layer, and the surface of the micelles. Non-polar molecules will be

solubilized in the micellar core and substances with intermediate polarity will be

distributed along the surfactant molecules in certain intermediate positions [94].

Solubilized molecule may pass completely inside the hydrophobic core or penetrate a

particular depth into the surface layer (solute can be adsorbed on the surface of the

micelle or, in the case of molecules containing polar substituents, be oriented with the

polar portion of the molecule situated in the surface layer and the non-polar portion

directed into the micelle) [95]. From the molecular structure of SRB, it can be seen that

the π-electron present in the aromatic ring of the dye make it suitable for electrostatic

attraction to the cationic headgroup of the OTAB in the micellar surface. The solubilizate

molecules are thereby incorporated into the micellar surface. In the case of SDS,

negatively charged headgroup facilitate the dye molecule to locate in the palisade layer of

the micelle which results in a red shift in the UV–visible spectrum [96].

A gradual red shift of the λmax was observed as the SRB molecules are solubilized in the

micelles. This shift indicates that dye interact with surfactant molecules. Just above the

CMC, OTAB in pure water and aqueous Na2SO4 solution the λmax for the solubilization

of SRB was found to be 508 and 512nm. The λmax was found to be 517 nm for 2mM

OTAB in pure water and 520 nm for 0.8 mM OTAB in aqueous 0.005 ionic strength

Na2SO4 solution. The λmax for the solubilization of SRB was found to be 520 and 517 in

pure SDS and in the presence of NaCl respectively at the CMC. For 30 mM SDS in pure

water and 20 mM SDS in NaCl solution, the λmax for the solubilization of SRB was found

to be 524 nm. Thus red shift can be attributed as SRB solubilization in the oil like

environment of the micellar core. Awan M. A. et al. observed such type of bathochromic

shift for solubilization of hydrophobic dyes in cationic and anionic surfactant micelles

[97].


93

To quantify the effectiveness of a surfactant in solubilizing a given solubilizate, the molar

solubilization ratio, MSR, is defined as the number of moles of organic compound

solubilized per mole of surfactant added to the solution [98]. When solute concentration

is plotted against surfactant concentration above the CMC, MSR can be determined from

the slope of the linearly fitted line. Figure 3.21 shows the effect of OTAB concentration

on SRB solubilization in pure water and in aqueous Na2SO4 solution, respectively and the

Figure 3.22 shows the effect of SDS concentration on SRB solubilization in pure water

and in aqueous NaCl solution respectively. The molar solubilization ratio of OTAB in

pure water and in 0.005 ionic strength of Na2SO4 solution are found to be 0.0194 and

0.0259, respectively (Table 3.8). On the other hand, the molar solubilization values of

SDS in pure water and in 0.005M NaCl solution are found to be 0.00113 and 0.00121,

respectively (Table 3.9). This indicates that the solubilizing power of OTAB and SDS

increases in aqueous salt solution. The solubilizing power of OTAB and SDS in the

presence of added salts is found to be 1.33 and 1.07 fold than that of respective pure

surfactant respectively. Counter-ion present in aqueous solution reduced electrostatic

repulsion between the charged head groups at the micelle surface and thus imparts an

increase in the micellar aggregation number resulting in an increase in the solubilization

capacity [99, 100]. This occurs due to the decrease in the CMC of the surfactant solution

in the presence of counter ion.

Table 3.8: Molar Solubilization Ratio (MSR) values of SRB in SDS

Surfactant NaCl Concentration (M)

Regression coefficient (R2)

MSR

SDS 0.00 0.968 0.00113

0.005 0.998 0.00121

Table 3.9: Molar Solubilization Ratio (MSR) values of SRB in OTAB

Surfactant Na2SO4 Concentration (M)

Regression coefficient (R2)

MSR

OTAB 0.00 0.996 0.0194

0.005 0.994 0.0259


94

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conclusions

Conclusions

99

CONCLUSIONS

The Krafft temperature (TK) and critical micelle concentration (CMC) of SDS and OTAB

were found to be functions of added counter-ions in the aqueous phase. The study

brought out that the TK can be increased or decreased depending on the nature of added

electrolytes and the CMC can be tuned to lower value with added electrolytes and

associated interactions. TK was also found to vary with the concentration of the added

salts. Some system increases while some decreases TK with concentration of the ions. In

the case of OTAB the TK decreases in the presence of C6H5SO3−, C7H5O2

−, C7H5O3−,

SO42−, NO3

−, Cl−, F− while Br−, SCN−, I− are found to increase TK with increase in

concentration. Li+ ion decreases while K+, Cs+, Na+ ions increase the TK of SDS with

increase in concentration. The hydrotropic C7H5O3−, C7H5O2

−, C6H5SO3−, less chaotropic

NO3− and Cl− as well as kosmotropic SO42− and F− increase the solubility of the OTAB

due to salting in effect with a consequent decrease in the TK. Li+ ion being kosmotrope

decrease the TK of the SDS. On the other hand the common ion effect of Br− in OTAB

solution and Na+ in SDS solution negatively affects the solubility of surfactant, resulting

in an increase in the TK. More chaotropic SCN− and I− form contact ion pair with the

octadecyltrimethylammonium ion and K+, and Cs+ form contact ion pair with the dodecyl

sulfate ion due to their matching water affinities, and thereby reduce the electrostatic

repulsion between the surfactant ions. This leads to a decrease in the solubility with a

consequent increase in the TK of the surfactant. The same explanation can be attributed

for the decreasing of CMC of surfactant solution in the presence of these ions. These ions

screen the surface charge of micelle and thus contribute for the closer packing of

surfactant molecules with a consequent decrease in CMC. It appears that C6H5SO3− and

Cs+ is the most effective in lowering the CMC with F− and Li+ being least effective in

lowering the CMC of OTAB and SDS respectively. The thermodynamic parameters of

the studied compounds were estimated. The ∆Gm° is negative over the studied

temperature range measured for all the system. This indicates a spontaneous process of

micelle formation of the ionic liquid in aqueous solution. ∆Hm° and ∆Sm° have an

opposite effect on ∆Gm°, so thus the value of ∆Gm° is dependent on relative changes of

enthalpy and entropy in the system. The negative ∆𝐺ad° values for SDS and OTAB

Conclusions

100

indicate the adsorption of monomeric surfactant from bulk of the solution to the surface is

spontaneous process. The more negative ∆𝐺ad° value than ∆Gm° suggesting that the

adsorption of surfactant at the air-water interface is more spontaneous than the

micellization in the bulk. The ∆𝐻ad° values for SDS are found to be positive while for

OTAB the values are negative. The positive ∆𝐻ad° value for SDS implies non spontaneity

while negative ∆𝐻ad° value for OTAB indicates spontaneity of the system. The ∆Sad°

values for SDS are positive while for OTAB the values are negative. The positive ∆Sad°

value suggests that the adsorption at the air/liquid interface is favored by entropy effect.

Higher surface excess concentration (Г) for SDS than OTAB indicates lower equilibrium

surface tension of the system. However except some anomaly added salts contribute to

lower the equilibrium surface tension of the system than that of respective pure state.

Solubilization study showed that the molar solubilization ratio (MSR) increases in the

presence of added salts. From this result, it is evident that the presence of salts helps to

increase the hydrophobic interaction of the surfactant by reducing the surface potential of

the micelles and thereby increases the oil-like environment of the micelle core. This helps

to solubilize more SRB molecules compared to that in the case in aqueous OTAB and

SDS solution. Since many of the industrial applications of surfactants are governed by the

TK and the formation of CMC, it can be emphasized that the depression of the Krafft

temperature and lowering of the CMC in the presence of C7H5O3−, C7H5O2

−, C6H5SO3−,

NO3−, F−, Cl−, SO4

2− and Li+ will pave the way for wider industrial applications of OTAB

and SDS, respectively.

Appendix

Appendix

101

DATA OF SDS

Krafft Temperature

Table 1: Krafft Temperature For Pure SDS Solution

Pure SDS

0.0075M 0.01M

Tempera ture (°C)

Conductance

(S/cm)

Tempera ture (°C)

Conductance

(S/cm)

3 381 3 387

3.5 382 3.5 388

4 383 4 390

4.5 385 4.5 392

5 387 5 394

5.5 389 5.5 397

6 392 6 400

6.5 395 6.5 404

7 399 7 409

7.5 403 7.5 414

8 407 8 421

8.5 411 8.5 428

9 416 9 437

9.5 422 9.5 443

10 428 10 452

10.5 434 10.5 458

11 441 11 468

11.5 448 11.5 474

12 456 12 484

12.5 466 12.5 494

13 478 13 513

13.5 488 13.5 530

14 498 14 556

14.5 504 14.5 572

15 507 15 587

15.5 507 15.5 592

16 506 16 593

16.5 506 16.5 593

17 507 17 593

17.5 506 17.5 592

18 506 18 592

18.5 505 18.5 592

Table 2: Krafft Temperature For SDS + LiCl Solution

SDS (0.0075)-LiCl

0.0025M 0.005M 0.0075M 0.01M

Temperature (°C)

Conductance

(S/cm)

Temperature (°C)

Conductance

(S/cm)

Temperature (°C)

Conductance

(S/cm)

Temperature (°C)

Conductance

(S/cm)

2 591 2 808 2 1036 2 1319

2.5 594 2.5 809 2.5 1037 2.5 1320

3 597 3 811 3 1038 3 1321

3.5 600 3.5 813 3.5 1039 3.5 1323

4 604 4 815 4 1041 4 1325

4.5 607 4.5 817 4.5 1044 4.5 1327

5 611 5 819 5 1047 5 1330

5.5 615 5.5 821 5.5 1050 5.5 1334

6 619 6 824 6 1054 6 1338

6.5 624 6.5 828 6.5 1059 6.5 1343

7 629 7 832 7 1066 7 1349

7.5 635 7.5 838 7.5 1074 7.5 1354

8 642 8 846 8 1082 8 1359

8.5 649 8.5 854 8.5 1092 8.5 1362

9 657 9 864 9 1102 9 1365

9.5 665 9.5 874 9.5 1111 9.5 1366

10 672 10 885 10 1121 10 1366

10.5 682 10.5 895 10.5 1129 10.5 1365

11 692 11 909 11 1130 11 1364

11.5 699 11.5 922 11.5 1131 11.5 1363

12 711 12 932 12 1131 12 1361

12.5 725 12.5 934 12.5 1130 12.5 1360

13 730 13 935 13 1130 13 1359

13.5 731 13.5 935 13.5 1129 - -

14 731 14 934 14 1129 - -

14.5 730 14.5 934 14.5 1128 - -

15 729 15 933 15 1128 - -

15.5 728 15.5 932 15.5 1128 - -

16 727 16 932 - - - -

16.5 726 16.5 931 - - - -

17 725 17 931 - - - -

Table 3: Krafft Temperature For SDS + NaCl Solution

SDS (0.0075)-NaCl

0.0025M 0.005M 0.0075M 0.01M

Temperature (°C)

Conductance

(S/cm)

Temperature (°C)

Conductance

(S/cm)

Temperature (°C)

Conductance

(S/cm)

Temperature (°C)

Conductance

(S/cm)

3 633 3 911 3 1226 3 1484

3.5 633 3.5 912 3.5 1227 3.5 1482

4 634 4 913 4 1228 4 1481

4.5 635 4.5 914 4.5 1229 4.5 1480

5 636 5 915 5 1230 5 1479

5.5 639 5.5 916 5.5 1230 5.5 1479

Appendix

102

6 641 6 917 6 1231 6 1478

6.5 644 6.5 918 6.5 1231 6.5 1478

7 647 7 919 7 1231 7 1478

7.5 650 7.5 922 7.5 1232 7.5 1479

8 655 8 926 8 1232 8 1480

8.5 661 8.5 929 8.5 1233 8.5 1481

9 668 9 932 9 1235 9 1482

9.5 675 9.5 936 9.5 1238 9.5 1484

10 683 10 940 10 1243 10 1486

10.5 691 10.5 945 10.5 1246 10.5 1488

11 700 11 951 11 1249 11 1491

11.5 708 11.5 957 11.5 1253 11.5 1494

12 716 12 968 12 1257 12 1497

12.5 727 12.5 982 12.5 1261 12.5 1501

13 740 13 996 13 1266 13 1507

13.5 759 13.5 1007 13.5 1273 13.5 1514

14 785 14 1019 14 1289 14 1520

14.5 800 14.5 1031 14.5 1314 14.5 1526

15 803 15 1041 15 1328 15 1537

15.5 803 15.5 1043 15.5 1336 15.5 1548

16 802 16 1043 16 1338 16 1563

16.5 801 16.5 1042 16.5 1338 16.5 1564

17 801 17 1041 17 1337 17 1564

17.5 800 17.5 1041 17.5 1337 17.5 1563

18 800 18 1040 18 1337 18 1562

18.5 800 18.5 1040 18.5 1336 18.5 1561

19 800 19 1039 19 1336 19 1560

- 19.5 1038 19.5 1335 19.5 1559

- 20 1038 20 1335 20 1558

Table 4: Krafft Temperature For SDS + KCl Solution

SDS (0.0075)-KCl

0.0025M 0.005M 0.0075M 0.01M

Temperature (°C)

Conductance

(S/cm)

Temperature (°C)

Conductance

(S/cm)

Temperature (°C)

Conductance

(S/cm)

Temperature (°C)

Conductance

(S/cm)

4 672 7 866 10 1149 11 1435

4.5 673 7.5 867 10.5 1151 11.5 1437

5 674 8 868 11 1153 12 1439

5.5 675 8.5 869 11.5 1156 12.5 1441

6 677 9 871 12 1160 13 1444

6.5 678 9.5 873 12.5 1164 13.5 1446

7 679 10 876 13 1168 14 1449

7.5 681 10.5 878 13.5 1170 14.5 1451

8 682 11 881 14 1173 15 1455

8.5 684 11.5 884 14.5 1175 15.5 1458

9 686 12 890 15 1178 16 1462

9.5 688 12.5 894 15.5 1182 16.5 1466

10 691 13 899 16 1188 17 1470

10.5 694 13.5 903 16.5 1192 17.5 1473

11 697 14 907 17 1196 18 1477

11.5 699 14.5 911 17.5 1199 18.5 1479

12 702 15 916 18 1202 19 1482

12.5 705 15.5 920 18.5 1206 19.5 1485

13 708 16 926 19 1210 20 1488

13.5 711 16.5 929 19.5 1215 20.5 1491

14 715 17 933 20 1220 21 1497

14.5 718 17.5 938 20.5 1225 21.5 1505

15 723 18 944 21 1232 22 1509

15.5 726 18.5 948 21.5 1238 22.5 1515

16 729 19 952 22 1243 23 1519

16.5 733 19.5 956 22.5 1248 23.5 1524

17 737 20 963 23 1255 24 1528

17.5 742 20.5 970 23.5 1260 24.5 1533

18 748 21 977 24 1266 25 1538

18.5 753 21.5 982 24.5 1273 25.5 1545

19 759 22 992 25 1284 26 1553

19.5 765 22.5 997 25.5 1290 26.5 1560

20 771 23 1002 26 1300 27 1569

20.5 776 23.5 1010 26.5 1312 27.5 1579

21 783 24 1024 27 1327 28 1592

21.5 790 24.5 1034 27.5 1345 28.5 1600

22 799 25 1044 28 1361 29 1613

22.5 804 25.5 1054 28.5 1372 29.5 1629

23 810 26 1066 29 1388 30 1653

23.5 813 26.5 1075 29.5 1398 30.5 1666

24 815 27 1087 30 1411 31 1682

24.5 816 27.5 1093 30.5 1418 31.5 1694

25 817 28 1099 31 1427 32 1703

25.5 818 28.5 1103 31.5 1429 32.5 1709

26 819 29 1107 32 1430 33 1718

26.5 819 29.5 1108 32.5 1432 33.5 1720

27 820 30 1109 33 1434 34 1721

27.5 820 30.5 1110 33.5 1436 34.5 1723

28 821 31 1112 34 1436 35 1724

28.5 821 31.5 1113 34.5 1437 35.5 1725

29 822 32 1114 35 1438 36 1727

- - 32.5 1115 - - 36.5 1728

- - - - - - 37 1729

Table 5: Krafft Temperature For SDS + CsCl Solution

SDS (0.0075)-CsCl

0.0025M 0.005M 0.0075M 0.01M

Temperature (°C)

Conductance

(S/cm)

Temperature (°C)

Conductance

(S/cm)

Temperature (°C)

Conductance

(S/cm)

Temperature (°C)

Conductance

(S/cm)

3 709 4 995 3 1201 5 1482

3.5 711 4.5 996 3.5 1200 5.5 1480

4 712 5 996 4 1200 6 1479

4.5 714 5.5 997 4.5 1200 6.5 1480

5 716 6 997 5 1200 7 1481

5.5 718 6.5 998 5.5 1201 7.5 1482

6 720 7 998 6 1201 8 1483

6.5 723 7.5 999 6.5 1201 8.5 1483

7 726 8 999 7 1203 9 1483

7.5 729 8.5 1000 7.5 1202 9.5 1484

Appendix

103

8 733 9 1000 8 1203 10 1484

8.5 738 9.5 1001 8.5 1204 10.5 1484

9 744 10 1003 9 1205 11 1484

9.5 752 10.5 1005 9.5 1206 11.5 1485

10 763 11 1007 10 1207 12 1486

10.5 770 11.5 1009 10.5 1208 12.5 1487

11 778 12 1012 11 1209 13 1488

11.5 783 12.5 1015 11.5 1211 13.5 1489

12 788 13 1018 12 1215 14 1492

12.5 792 13.5 1021 12.5 1220 14.5 1495

13 793 14 1026 13 1225 15 1499

13.5 793 14.5 1029 13.5 1230 15.5 1503

14 792 15 1036 14 1236 16 1509

14.5 792 15.5 1039 14.5 1240 16.5 1515

15 791 16 1043 15 1246 17 1521

15.5 791 16.5 1047 15.5 1252 17.5 1528

16 791 17 1052 16 1259 18 1538

16.5 790 17.5 1056 16.5 1270 18.5 1551

17 790 18 1064 17 1282 19 1563

- - 18.5 1070 17.5 1292 19.5 1573

- - 19 1078 18 1304 20 1585

- - 19.5 1082 18.5 1312 20.5 1595

- - 20 1087 19 1320 21 1606

- - 20.5 1089 19.5 1327 21.5 1618

- - 21 1091 20 1337 22 1631

- - 21.5 1091 20.5 1349 22.5 1641

- - 22 1092 21 1363 23 1654

- - 22.5 1092 21.5 1375 23.5 1665

- - 23 1092 22 1389 24 1677

- - 23.5 1091 22.5 1394 24.5 1685

- - 24 1091 23 1399 25 1693

- - 24.5 1091 23.5 1403 25.5 1697

- - 25 1091 24 1407 26 1697

- - - - 24.5 1407 26.5 1697

- - - - 25 1407 27 1697

- - - - 25.5 1407 27.5 1698

- - - - 26 1406 28 1698

- - - - 26.5 1406 28.5 1698

- - - - 27 1406 29 1698

Critical Micelle Concentration (CMC)

Conductometric Method

Table 6: Plot of conductance vs. concentration of aqueous SDS solution at different temperatures (293K, 298K, 303K, 308K)

Concentration (mM)

293K 298K 303K 308K

0.98 68.5 70.1 71.7 72.1

1.92 132.4 133.7 135.9 137.3

2.83 192.9 195 197.8 197

3.7 248 249 250 256

4.54 303 306 307 308

5.35 357 360 364 367

6.14 408 410 415 415

6.9 455 457 463 467

7.63 497 499 506 510

8.33 528 531 538 545

9.02 553 557 564 573

9.68 572 577 584 596

10.32 590 595 603 616

10.94 606 610 618 633

11.54 621 626 636 650

12.12 636 640 650 667

12.69 649 655 665 680

13.24 661 666 680 697

13.77 675 -- -- 710

Table 7: Plot of conductance vs. concentration of aqueous SDS-0.005M LiCl solution at different temperatures (293K, 298K, 303K, 308K)

Concentration (mM)

293K 298K 303K 308K

0.78 413 412 411 412

1.54 462 460 461 465

2.26 508 506 507 514

2.96 552 552 553 558

3.64 597 595 598 605

4.29 639 639 639 645

4.91 679 679 680 688

5.52 717 717 719 725

6.1 752 754 754 765

6.67 783 785 789 798

7.21 809 811 814 825

7.74 829 831 836 848

8.25 845 846 852 865

8.75 860 862 868 882

9.23 874 876 883 899

9.7 886 890 896 912

10.1 898 902 909 925

10.6 910 914 922 938

11 921 925 933 951

11.4 931 936 944 964

11.8 941 947 955 976

12.2 951 958 967 986

Appendix

104

Table 8: Plot of conductance vs. concentration of aqueous SDS-0.005M NaCl solution at different temperatures (293K, 298K, 303K, 308K)

Concentration (mM)

293K 298K 303K 308K

0.78 647 645 644 646

1.54 696 694 692 696

2.26 742 739 739 743

2.96 785 784 784 788

3.64 828 826 826 831

4.29 868 867 867 872

4.91 907 907 908 915

5.52 945 933 946 951

6.1 975 974 977 985

6.67 999 998 1003 1011

7.21 1018 1016 1022 1033

7.74 1032 1031 1039 1050

8.25 1044 1046 1054 1064

8.75 1057 1059 1068 1080

9.23 1070 1072 1080 1094

9.7 1082 1084 1092 1108

10.1 1092 1096 1103 1122

10.6 1103 1106 1115 1132

11 1113 1116 1126 1143

11.4 1123 1126 1137 1154

11.8 1132 1136 1148 1165

Table 9: Plot of conductance vs. concentration of aqueous SDS-0.005M KCl solution at different temperatures (298K, 303K, 308K)

Concentration (mM)

298K 303K 308K

0.78 758 754 755

1.54 805 804 804

2.26 852 850 851

2.96 896 896 896

3.64 938 938 940

4.29 979 979 982

4.91 1018 1015 1022

5.52 1047 1050 1057

6.1 1069 1071 1082

6.67 1082 1087 1102

7.21 1096 1102 1116

7.74 1107 1114 1130

8.25 1117 1124 1141

8.75 1126 1135 1152

9.23 1135 1146 1162

9.7 1145 1156 1173

10.1 1154 1166 1183

10.6 1164 1175 1192

11 1173 1184 1204

11.4 1181 1195 --

Table 10: Plot of conductance vs. concentration of aqueous SDS-0.005M CsCl solution at different temperatures (293K, 298K, 303K, 308K)

Concentration (mM)

293K 298K 303K 308K

0.78 757 763 765 759

1.54 804 811 814 810

2.26 849 856 862 857

2.96 893 899 905 903

3.64 934 941 947 946

4.29 970 979 987 988

4.91 1005 1015 1018 1024

5.52 1026 1033 1044 1052

6.1 1037 1047 1063 1071

6.67 1048 1059 1077 1088

7.21 1058 1072 1088 1101

7.74 1067 1083 1099 1113

8.25 1076 1093 1108 1126

8.75 1085 1102 1118 1139

9.23 1094 1111 1128 1149

9.7 1103 1120 1138 1158

10.1 1111 1129 1148 1169

10.6 1119 1138 1157 1179

11 -- -- 1166 1189

11.4 -- -- 1175 1198

Surfacetensiometric Method

Table 11: Surface tension vs. logarithm of

concentration of aqueous solutions of SDS at

different temperatures (293K, 298K, 303K, 308K)

SDS Concentration

(M)

Log (C) Surface Tension (mN/m)

293K 298K 303K 308K

0.000594 -3.226 65.9 66.1 64.1 63.2

0.001178 -2.929 59.1 59.3 57.5 57.2

0.002307 -2.637 51.5 51.9 50.9 50.6

0.003396 -2.469 46.9 47.3 47.3 46.5

0.004446 -2.352 44 44.5 44.5 44.1

0.005458 -2.263 41.2 41.4 41.7 42.1

0.006427 -2.192 39.9 39.7 39.9 40.4

Appendix

105

0.007362 -2.133 38.4 38.3 38.9 39.2

0.008279 -2.082 37.8 37.9 38.3 38.5

0.009162 -2.038 37.7 37.8 38.2 38.5

0.01 -2 37.7 37.7 38.2 38.4

0.0108 -1.966 37.6 37.8 38.2 38.4

0.0116 -1.935 37.6 37.7 38.1 38.5

0.01238 -1.907 37.5 37.6 38.1 38.4

0.01312 -1.882 37.5 37.6 38 38.3

0.01383 -1.859 37.5 37.6 38 38.4

0.01455 -1.837 37.5 37.6 38 38.4

0.01524 -1.817 37.5 37.6 38 38.4

Table 12: Surface tension vs. logarithm of concentration of aqueous solutions of SDS-NaCl (0.005M) at different temperatures (293K, 298K, 303K, 308K)

SDS Concentration

(M)


293K 298K 303K 308K

0.000594 -3.226 64.7 60.6 58.9 58.4

0.001178 -2.929 57.3 54.9 53.5 53.5

0.002307 -2.637 49.2 47.7 47 46.9

0.003396 -2.469 44.3 43.1 42.8 43.4

0.004446 -2.352 40.6 40.1 40.2 40.5

0.005458 -2.263 37.9 37.7 38 38.6

0.006427 -2.192 36.8 37 37.3 37.6

0.007362 -2.133 36.8 36.9 37.2 37.5

0.008279 -2.082 36.7 36.9 37.2 37.5

0.009162 -2.038 36.7 37 37.1 37.4

0.01 -2 36.6 36.9 37.1 37.4

0.0108 -1.966 36.7 36.9 37.2 37.3

0.0116 -1.935 36.7 36.8 37.1 37.3

0.01238 -1.907 36.6 36.8 37 37.3

0.01312 -1.882 36.6 36.8 37 37.3

Table 13: Surface tension vs. logarithm of concentration of aqueous solutions of SDS-LiCl (0.005M) at different temperatures (293K, 298K, 303K, 308K)

SDS Concentration

(M)


293K 298K 303K 308K

0.000594 -3.226 64.6 62.2 59.4 56.4

0.001178 -2.929 57.6 56.3 53.9 51.9

0.002307 -2.637 49.9 49 47.8 46.7

0.003396 -2.469 44.7 44.2 43.8 43.2

0.004446 -2.352 41.7 41.4 41.3 41.5

0.005458 -2.263 39.5 39.8 39.8 39.8

0.006427 -2.192 38.2 38.3 38.7 38.9

0.007362 -2.133 37.5 37.7 37.9 38.1

0.008279 -2.082 37.5 37.6 37.8 38

0.009162 -2.038 37.4 37.7 37.9 38

0.01 -2 37.4 37.6 37.9 37.9

0.0108 -1.966 37.3 37.6 37.8 37.9

0.0116 -1.935 37.4 37.5 37.8 38

0.01238 -1.907 37.4 37.5 37.7 38

0.01312 -1.882 37.3 37.6 37.7 37.9

0.01383 -1.859 37.3 37.5 37.7 37.9

0.01455 -1.837 37.3 37.5

Table 14: Surface tension vs. logarithm of concentration of aqueous solutions of SDS-KCl (0.005M) at different temperatures ( 298K, 303K, 308K) SDS Concentration

(M) Log (C) Surface Tension (mN/m)

298K 303K 308K

0.000594 -3.226 59.3 54.2 50.7

0.001178 -2.929 52.5 48.7 46.5

0.002307 -2.637 44.6 43.3 42.2

0.003396 -2.469 40.1 39.6 39.2

0.004446 -2.352 37.5 37.3 37.9

0.005458 -2.263 35.7 35.9 36.2

0.006427 -2.192 35.4 35.6 35.8

0.007362 -2.133 35.4 35.6 35.8

0.008279 -2.082 35.3 35.4 35.7

0.009162 -2.038 35.3 35.5 35.7

0.01 -2 35.3 35.5 35.8

0.0108 -1.966 35.4 35.4 35.7

0.0116 -1.935 35.4 35.4 35.6

0.01238 -1.907 35.3 54.2 35.6

0.01312 -1.882 35.2 48.7 35.6

Table 15: Surface tension vs. logarithm of concentration of aqueous solutions of SDS-CsCl (0.005M) at different temperatures (293K, 298K, 303K, 308K)

SDS Concentration

(M)


293K 298K 303K 308K

0.000594 -3.226 57.6 55.5 52.7 49.4

0.001178 -2.929 50.5 49.4 47.7 45.5

0.002307 -2.637 46.1 45.5 44.6 42.9

0.003396 -2.469 42.9 42.1 42.1 41.1

0.004446 -2.352 38.5 38.2 38.4 38.6

0.005458 -2.263 35.9 35.9 36.5 36.8

0.006427 -2.192 34.9 35.3 35.5 35.9

0.007362 -2.133 34.9 35.3 35.4 35.7

0.008279 -2.082 34.8 35.2 35.5 35.7

0.009162 -2.038 34.9 35.3 35.5 35.6

0.01 -2 34.9 35.2 35.4 35.6

0.0108 -1.966 34.8 35.2 35.4 35.7

0.0116 -1.935 34.8 35.1 35.3 35.6

0.01238 -1.907 34.7 35.1 35.3 35.5

0.01312 -1.882 34.7 35.1 35.3 35.5

Appendix

106

DATA OF OTAB

Krafft Temperature

Table 16: Krafft Temperature For Pure OTAB Solution

Pure OTAB

0.005M 0.0075M 0.01M

Tempera ture (°C)

Conductance

(S/cm)

Tempera ture (°C)

Conductance

(S/cm)

Tempera ture (°C)

Conductance

(S/cm)

10 12.33 10 12.44 10 13.62

10.5 12.44 10.5 12.73 10.5 13.82

11 12.63 11 12.86 11 13.96

11.5 12.77 11.5 12.99 11.5 14.09

12 12.87 12 13.11 12 14.13

12.5 12.98 12.5 13.29 12.5 14.41

13 13.11 13 13.55 13 14.79

13.5 13.26 13.5 13.69 13.5 14.94

14 13.46 14 13.81 14 15.19

14.5 13.66 14.5 14.94 14.5 15.39

15 13.86 15 14.12 15 15.54

15.5 13.92 15.5 14.34 15.5 15.73

16 13.98 16 14.63 16 15.81

16.5 14.48 16.5 14.84 16.5 16

17 15.24 17 15.03 17 16.27

17.5 15.42 17.5 15.4 17.5 16

18 15.62 18 15.7 18 17.01

18.5 15.97 18.5 16 18.5 17.33

19 16.21 19 16.35 19 17.82

19.5 16.56 19.5 16.41 19.5 18.01

20 16.88 20 17.1 20 18.47

20.5 17.1 20.5 17.36 20.5 18.85

21 17.25 21 17.65 21 19.02

21.5 17.75 21.5 18.12 21.5 19.29

22 18.27 22 18.45 22 19.65

22.5 18.65 22.5 18.82 22.5 19.7

23 18.97 23 19.16 23 19.9

23.5 19 23.5 19.39 23.5 20

24 19.08 24 19.79 24 20.7

24.5 19.44 24.5 20.1 24.5 21.2

25 19.7 25 20.5 25 21.8

25.5 20.3 25.5 20.87 25.5 22.2

26 20.8 26 21.2 26 22.6

26.5 21.3 26.5 21.8 26.5 23.1

27 21.6 27 22.2 27 23.5

27.5 22.3 27.5 22.7 27.5 23.9

28 22.8 28 23.1 28 24.4

28.5 23.4 28.5 23.7 28.5 25

29 24 29 24.3 29 25.7

29.5 24.6 29.5 24.8 29.5 26.3

30 25.1 30 25.5 30 26.9

30.5 25.7 30.5 26.1 30.5 27.7

31 26.5 31 26.6 31 28.5

31.5 27.7 31.5 27.4 31.5 30.1

32 29.1 32 28.1 32 31.5

32.5 30.7 32.5 29 32.5 33.7

33 32.5 33 30.2 33 35.8

33.5 34.5 33.5 32.3 33.5 39.1

34 36.7 34 34.7 34 42.6

34.5 39.3 34.5 41.5 34.5 48

35 43.5 35 47.4 35 53.7

35.5 52.5 35.5 55 35.5 62.7

36 71 36 77.5 36 79.5

36.5 106.3 36.5 116.5 36.5 174.4

37 128.4 37 174 37 217

37.5 129.8 37.5 178.7 37.5 225

38 130.9 38 179.9 38 228

38.5 131.9 38.5 181.3 38.5 230

39 132.9 39 182.3 39 232

39.5 133.7 39.5 183.5 39.5 233

40 134.8 40 184.2 40 234

40.5 135.5 40.5 185.3 40.5 235

41 136.7 41 186.5 41 236

41.5 137.6 41.5 187.4 41.5 237

42 138.4 42 188.4 42 238

Table 17.1: Krafft Temperature For Pure OTAB + NaF Solution

OTAB(0.005M)-NaF

0.0025M 0.005M 0.0075M 0.01M

Temperature (°C)

Conductance

(S/cm)

Temperature (°C)

Conductance

(S/cm)

Temperature (°C)

Conductance

(S/cm)

Temperature (°C)

Conductance

(S/cm)

10 260 10 488 10 733 10 1004

10.5 260 10.5 487 10.5 731 10.5 1003

11 260 11 487 11 729 11 1002

11.5 261 11.5 486 11.5 728 11.5 1001

12 260 12 486 12 727 12 1000

12.5 260 12.5 485 12.5 726 12.5 999

13 260 13 485 13 726 13 998

13.5 260 13.5 485 13.5 725 13.5 998

14 261 14 485 14 725 14 997

14.5 261 14.5 485 14.5 724 14.5 997

15 261 15 485 15 724 15 996

15.5 261 15.5 485 15.5 724 15.5 996

16 261 16 484 16 724 16 995

16.5 262 16.5 484 16.5 723 16.5 995

17 262 17 484 17 723 17 995

Appendix

107

17.5 262 17.5 484 17.5 723 17.5 995

18 262 18 484 18 723 18 995

18.5 262 18.5 484 18.5 723 18.5 996

19 262 19 485 19 723 19 997

19.5 262 19.5 485 19.5 723 19.5 998

20 262 20 485 20 723 20 999

20.5 263 20.5 485 20.5 724 20.5 1000

21 263 21 486 21 724 21 1001

21.5 263 21.5 486 21.5 725 21.5 1002

22 264 22 487 22 726 22 1003

22.5 264 22.5 488 22.5 727 22.5 1004

23 264 23 489 23 728 23 1005

23.5 265 23.5 490 23.5 729 23.5 1006

24 266 24 491 24 731 24 1008

24.5 267 24.5 492 24.5 733 24.5 1010

25 268 25 493 25 735 25 1012

25.5 269 25.5 495 25.5 737 25.5 1016

26 270 26 497 26 739 26 1020

26.5 271 26.5 499 26.5 742 26.5 1024

27 273 27 501 27 745 27 1029

27.5 275 27.5 503 27.5 749 27.5 1035

28 277 28 506 28 755 28 1041

28.5 279 28.5 509 28.5 760 28.5 1047

29 281 29 513 29 766 29 1054

29.5 284 29.5 516 29.5 771 29.5 1064

30 286 30 520 30 779 30 1072

30.5 289 30.5 529 30.5 787 30.5 1084

31 292 31 537 31 798 31 1094

31.5 294 31.5 547 31.5 814 31.5 1104

32 298 32 557 32 823 32 1123

32.5 306 32.5 572 32.5 837 32.5 1141

33 317 33 591 33 854 33 1152

33.5 331 33.5 607 33.5 865 33.5 1156

34 353 34 622 34 872 34 1158

34.5 364 34.5 625 34.5 874 34.5 1159

35 379 35 627 35 876 35 1161

35.5 383 35.5 628 35.5 878 35.5 1163

36 385 36 630 36 880 36 1164

36.5 386 36.5 632 36.5 881 36.5 1166

37 387 37 633 37 883 37 1168

37.5 389 37.5 635 37.5 884 37.5 1169

38 390 38 637 38 885 38 1171

38.5 391 38.5 639 38.5 887 - -

39 392 39 641 39 888 - -

39.5 394 39.5 642 - - - -

40 395 40 644 - - - -

Table 17.2: Krafft Temperature For Pure OTAB + NaCl Solution

OTAB(0.005M)-NaCl

0.0025M 0.005M 0.0075M 0.01M

Temperature (°C)

Conductance

(S/cm)

Temperature (°C)

Conductance

(S/cm)

Temperature (°C)

Conductance

(S/cm)

Temperature (°C)

Conductance

(S/cm)

10 406 10 702 10 836 10 1044

10.5 405 10.5 701 10.5 833 10.5 1040

11 405 11 700 11 830 11 1036

11.5 404 11.5 699 11.5 826 11.5 1032

12 404 12 698 12 824 12 1028

12.5 403 12.5 697 12.5 822 12.5 1024

13 402 13 696 13 820 13 1022

13.5 401 13.5 695 13.5 818 13.5 1018

14 401 14 694 14 816 14 1016

14.5 400 14.5 693 14.5 814 14.5 1014

15 400 15 692 15 812 15 1012

15.5 399 15.5 691 15.5 811 15.5 1011

16 399 16 690 16 810 16 1010

16.5 398 16.5 689 16.5 809 16.5 1009

17 398 17 689 17 808 17 1009

17.5 397 17.5 688 17.5 807 17.5 1008

18 397 18 688 18 807 18 1008

18.5 396 18.5 687 18.5 806 18.5 1007

19 396 19 687 19 805 19 1007

19.5 395 19.5 686 19.5 805 19.5 1006

20 395 20 686 20 804 20 1006

20.5 395 20.5 685 20.5 804 20.5 1005

21 395 21 685 21 804 21 1005

21.5 395 21.5 685 21.5 804 21.5 1005

22 396 22 685 22 805 22 1005

22.5 396 22.5 686 22.5 806 22.5 1006

23 396 23 686 23 808 23 1007

23.5 397 23.5 687 23.5 810 23.5 1010

24 397 24 688 24 814 24 1014

24.5 398 24.5 689 24.5 820 24.5 1020

25 398 25 691 25 826 25 1028

25.5 399 25.5 693 25.5 833 25.5 1036

26 400 26 696 26 840 26 1049

26.5 401 26.5 700 26.5 849 26.5 1060

27 402 27 705 27 858 27 1074

27.5 404 27.5 711 27.5 866 27.5 1091

28 406 28 719 28 875 28 1108

28.5 409 28.5 726 28.5 883 28.5 1120

29 413 29 735 29 894 29 1135

29.5 417 29.5 743 29.5 906 29.5 1149

30 422 30 751 30 915 30 1164

30.5 427 30.5 761 30.5 926 30.5 1174

31 433 31 772 31 936 31 1186

31.5 439 31.5 781 31.5 944 31.5 1200

32 447 32 791 32 949 32 1207

32.5 456 32.5 804 32.5 952 32.5 1211

33 465 33 814 33 954 33 1215

33.5 479 33.5 820 33.5 957 33.5 1219

34 492 34 823 34 959 34 1223

34.5 499 34.5 825 34.5 961 34.5 1226

35 502 35 827 35 964 35 1230

35.5 504 35.5 829 35.5 966 35.5 1233

36 506 36 831 36 968 36 1236

36.5 508 36.5 833 36.5 970 36.5 1238

37 509 37 835 37 972 37 1241

37.5 510 37.5 837 - - - -

Appendix

108

38 512 38 839 - - - -

38.5 513 38.5 841 - - - -

39 514 39 843 - - - -

Table 18: Krafft Temperature For Pure OTAB + NaBr Solution

OTAB(0.005M)-NaBr

0.0025M 0.005M 0.0075M 0.01M

Temperature (°C)

Conductance

(S/cm)

Temperature (°C)

Conductance

(S/cm)

Temperature (°C)

Conductance

(S/cm)

Temperature (°C)

Conductance

(S/cm)

10 226 10 477 10 676 10 908

10.5 225 10.5 476 10.5 675 10.5 906

11 225 11 476 11 673 11 904

11.5 224 11.5 474 11.5 671 11.5 902

12 224 12 472 12 670 12 900

12.5 223 12.5 471 12.5 669 12.5 898

13 223 13 470 13 668 13 897

13.5 223 13.5 469 13.5 667 13.5 895

14 223 14 469 14 666 14 894

14.5 222 14.5 468 14.5 665 14.5 892

15 222 15 468 15 664 15 890

15.5 222 15.5 467 15.5 663 15.5 888

16 222 16 467 16 662 16 887

16.5 222 16.5 467 16.5 660 16.5 885

17 221 17 467 17 659 17 884

17.5 221 17.5 466 17.5 658 17.5 883

18 221 18 465 18 658 18 882

18.5 220 18.5 464 18.5 657 18.5 881

19 220 19 464 19 656 19 880

19.5 220 19.5 464 19.5 655 19.5 879

20 220 20 464 20 654 20 878

20.5 219 20.5 463 20.5 654 20.5 877

21 219 21 463 21 653 21 877

21.5 219 21.5 463 21.5 653 21.5 876

22 219 22 463 22 652 22 876

22.5 219 22.5 462 22.5 652 22.5 875

23 219 23 462 23 652 23 875

23.5 219 23.5 462 23.5 652 23.5 874

24 220 24 462 24 652 24 874

24.5 220 24.5 463 24.5 652 24.5 874

25 220 25 463 25 651 25 874

25.5 220 25.5 463 25.5 651 25.5 873

26 221 26 463 26 651 26 873

26.5 221 26.5 463 26.5 651 26.5 873

27 221 27 463 27 651 27 873

27.5 221 27.5 464 27.5 651 27.5 873

28 221 28 464 28 651 28 873

28.5 221 28.5 464 28.5 651 28.5 873

29 222 29 464 29 651 29 873

29.5 222 29.5 464 29.5 651 29.5 874

30 222 30 464 30 652 30 874

30.5 222 30.5 465 30.5 652 30.5 874

31 223 31 465 31 652 31 874

31.5 223 31.5 466 31.5 652 31.5 875

32 224 32 466 32 653 32 875

32.5 224 32.5 467 32.5 653 32.5 875

33 225 33 467 33 653 33 875

33.5 225 33.5 468 33.5 653 33.5 876

34 226 34 468 34 654 34 876

34.5 227 34.5 469 34.5 655 34.5 876

35 230 35 469 35 657 35 876

35.5 235 35.5 472 35.5 662 35.5 877

36 249 36 476 36 670 36 878

36.5 277 36.5 487 36.5 682 36.5 892

37 310 37 521 37 691 37 904

37.5 314 37.5 563 37.5 724 37.5 951

38 316 38 567 38 736 38 964

38.5 317 38.5 571 38.5 742 38.5 970

39 319 39 573 39 744 39 972

39.5 320 39.5 574 39.5 746 39.5 973

40 321 40 575 40 747 40 974

40.5 323 40.5 576 40.5 748 40.5 975

41 324 41 577 41 749 41 976

41.5 325 41.5 578 41.5 750 41.5 977

42 326 42 579 42 751 42 978

- - 42.5 580 42.5 752 42.5 979

- - 43 581 43 753 43 980

- - 43.5 582 43.5 754 - -

- - - - 44 755 - -

Table 19: Krafft Temperature For Pure OTAB + Na2SO4 Solution

OTAB(0.005M)-Na2SO4

0.0025 Ionic Strength



0.01 Ionic Strength

Temperature (°C)

Conductance

(S/cm)

Temperature (°C)

Conductance

(S/cm)

Temperature (°C)

Conductance

(S/cm)

Temperature (°C)

Conductance

(S/cm)

10 226 10 432 10 627 10 812

10.5 226 10.5 432 10.5 626 10.5 812

11 225 11 431 11 625 11 811

11.5 225 11.5 431 11.5 624 11.5 811

12 225 12 431 12 624 12 810

12.5 225 12.5 431 12.5 623 12.5 810

13 225 13 430 13 623 13 809

13.5 225 13.5 430 13.5 622 13.5 809

14 226 14 430 14 622 14 808

14.5 226 14.5 430 14.5 622 14.5 808

15 226 15 430 15 622 15 808

15.5 227 15.5 430 15.5 622 15.5 808

16 227 16 430 16 622 16 808

16.5 227 16.5 430 16.5 622 16.5 808

17 227 17 430 17 623 17 809

17.5 228 17.5 430 17.5 623 17.5 809

18 228 18 431 18 624 18 810

18.5 228 18.5 431 18.5 624 18.5 810

19 228 19 431 19 625 19 811

Appendix

109

19.5 229 19.5 432 19.5 625 19.5 812

20 229 20 432 20 626 20 813

20.5 230 20.5 433 20.5 627 20.5 814

21 230 21 433 21 628 21 815

21.5 231 21.5 434 21.5 629 21.5 816

22 231 22 435 22 631 22 818

22.5 232 22.5 436 22.5 633 22.5 820

23 233 23 438 23 635 23 822

23.5 234 23.5 440 23.5 637 23.5 824

24 235 24 442 24 639 24 827

24.5 236 24.5 444 24.5 641 24.5 830

25 237 25 446 25 644 25 833

25.5 238 25.5 448 25.5 647 25.5 837

26 239 26 450 26 650 26 840

26.5 240 26.5 453 26.5 653 26.5 844

27 241 27 456 27 656 27 848

27.5 243 27.5 459 27.5 659 27.5 851

28 246 28 462 28 662 28 855

28.5 248 28.5 465 28.5 665 28.5 857

29 252 29 468 29 669 29 859

29.5 256 29.5 471 29.5 672 29.5 860

30 260 30 475 30 675 30 861

30.5 263 30.5 480 30.5 676 30.5 861

31 267 31 484 31 677 31 862

31.5 271 31.5 486 31.5 678 31.5 862

32 276 32 487 32 678 32 863

32.5 281 32.5 488 32.5 679 32.5 863

33 286 33 488 33 679 33 863

33.5 290 33.5 489 33.5 680 33.5 864

34 295 34 489 34 680 34 864

34.5 297 34.5 490 34.5 681 34.5 864

35 298 35 490 35 681 35 864

35.5 299 35.5 491 35.5 682 - -

36 300 36 491 36 682 - -

36.5 301 36.5 492 - - - -

37 302 37 492 - - - -

37.5 302 37.5 493 - - - -

38 303 38 493 - - - -

38.5 303 - - - - - -

39 304 - - - - - -

Table 20: Krafft Temperature For Pure OTAB + NaNO3 Solution

OTAB(0.005M)-NaNO3

0.0025M 0.005M

Temperature (°C)

Conductance

(S/cm)

Temperature (°C)

Conductance

(S/cm)

Temperature (°C)

Conductance

(S/cm)

Temperature (°C)

Conductance

(S/cm)

10 329 25.5 306 10 633 25.5 596

10.5 326 26 306 10.5 630 26 596

11 324 26.5 306 11 628 26.5 596

11.5 322 27 306 11.5 626 27 596

12 320 27.5 307 12 624 27.5 596

12.5 318 28 307 12.5 622 28 596

13 317 28.5 307 13 621 28.5 597

13.5 315 29 308 13.5 619 29 598

14 314 29.5 309 14 618 29.5 599

14.5 313 30 310 14.5 616 30 600

15 312 30.5 311 15 615 30.5 602

15.5 311 31 312 15.5 614 31 605

16 310 31.5 315 16 612 31.5 611

16.5 310 32 320 16.5 610 32 626

17 309 32.5 330 17 608 32.5 637

17.5 309 33 342 17.5 607 33 650

18 308 33.5 356 18 605 33.5 655

18.5 308 34 375 18.5 604 34 660

19 308 34.5 382 19 602 34.5 662

19.5 308 35 393 19.5 601 35 664

20 307 35.5 395 20 600 35.5 667

20.5 307 36 397 20.5 599 36 670

21 307 36.5 398 21 598 36.5 671

21.5 307 37 399 21.5 598 37 673

22 307 37.5 400 22 597 37.5 675

22.5 307 38 401 22.5 597 38 678

23 307 38.5 402 23 596 38.5 680

23.5 306 39 403 23.5 596 39 684

24 306 39.5 404 24 596 - -

24.5 306 40 404 24.5 596 - -

25 306 40.5 405 25 596 - -

Table 21: Krafft Temperature For Pure OTAB + C7H5O2Na Solution

OTAB(0.005M)-C7H5O2Na

0.0025M 0.005M 0.0075M 0.01M

Temperature (°C)

Conductance

(S/cm)

Temperature (°C)

Conductance

(S/cm)

Temperature (°C)

Conductance

(S/cm)

Temperature (°C)

Conductance

(S/cm)

10 285 10 570 10 795 10 980

10.5 284 10.5 569 10.5 793 10.5 978

11 284 11 568 11 792 11 976

11.5 283 11.5 566 11.5 790 11.5 973

12 283 12 565 12 789 12 971

12.5 282 12.5 563 12.5 788 12.5 969

13 282 13 562 13 786 13 968

13.5 281 13.5 561 13.5 785 13.5 966

14 281 14 560 14 784 14 965

14.5 280 14.5 559 14.5 782 14.5 964

15 279 15 559 15 781 15 963

15.5 279 15.5 558 15.5 780 15.5 962

16 278 16 558 16 779 16 961

16.5 278 16.5 557 16.5 777 16.5 960

17 279 17 557 17 776 17 959

17.5 277 17.5 556 17.5 776 17.5 958

18 277 18 556 18 775 18 957

18.5 276 18.5 556 18.5 775 18.5 956

19 276 19 556 19 774 19 955

19.5 276 19.5 555 19.5 774 19.5 955

20 276 20 555 20 774 20 954

Appendix

110

20.5 276 20.5 555 20.5 774 20.5 954

21 276 21 555 21 773 21 954

21.5 276 21.5 555 21.5 773 21.5 954

22 276 22 555 22 773 22 954

22.5 277 22.5 556 22.5 773 22.5 954

23 277 23 556 23 773 23 954

23.5 278 23.5 558 23.5 773 23.5 954

24 283 24 563 24 773 24 954

24.5 285 24.5 568 24.5 773 24.5 954

25 291 25 578 25 774 25 954

25.5 301 25.5 583 25.5 774 25.5 955

26 318 26 584 26 774 26 955

26.5 321 26.5 585 26.5 775 26.5 956

27 324 27 585 27 775 27 957

27.5 328 27.5 585 27.5 777 27.5 958

28 333 28 585 28 779 28 960

28.5 338 28.5 586 28.5 783 28.5 965

29 342 29 586 29 783 29 972

29.5 347 29.5 586 29.5 784 29.5 981

30 351 30 586 30 784 30 982

30.5 354 30.5 586 30.5 785 30.5 983

31 356 31 587 31 785 31 984

31.5 358 31.5 587 31.5 786 31.5 984

32 360 32 587 32 786 32 984

32.5 362 - - 32.5 787 32.5 985

33 365 - - 33 787 33 985

33.5 367 - - - - 33.5 986

34 369 - - - - 34 986

- - - - - - 34.5 987

- - - - - - 35 987

Table 22: Krafft Temperature For Pure OTAB + C7H5O3Na Solution

OTAB(0.005M)-C7H5O3Na

0.0025M 0.005M 0.0075M 0.01M

Temperature (°C)

Conductance

(S/cm)

Temperature (°C)

Conductance

(S/cm)

Temperature (°C)

Conductance

(S/cm)

Temperature (°C)

Conductance

(S/cm)

10 290 10 631 10 829 10 1037

10.5 290 10.5 629 10.5 827 10.5 1035

11 290 11 627 11 826 11 1033

11.5 290 11.5 625 11.5 823 11.5 1031

12 291 12 623 12 821 12 1029

12.5 291 12.5 621 12.5 820 12.5 1027

13 291 13 619 13 818 13 1025

13.5 291 13.5 617 13.5 817 13.5 1023

14 292 14 616 14 816 14 1021

14.5 292 14.5 615 14.5 814 14.5 1019

15 292 15 614 15 813 15 1018

15.5 293 15.5 613 15.5 812 15.5 1016

16 293 16 612 16 811 16 1015

16.5 294 16.5 612 16.5 810 16.5 1014

17 294 17 611 17 809 17 1013

17.5 295 17.5 611 17.5 808 17.5 1012

18 296 18 610 18 807 18 1011

18.5 297 18.5 610 18.5 806 18.5 1010

19 298 19 610 19 806 19 1010

19.5 299 19.5 609 19.5 805 19.5 1009

20 301 20 609 20 805 20 1009

20.5 305 20.5 609 20.5 804 20.5 1009

21 308 21 609 21 804 21 1008

21.5 315 21.5 609 21.5 804 21.5 1008

22 323 22 609 22 804 22 1008

22.5 333 22.5 610 22.5 804 22.5 1008

23 349 23 610 23 804 23 1007

23.5 352 23.5 611 23.5 804 23.5 1007

24 354 24 613 24 804 24 1007

24.5 356 24.5 615 24.5 805 24.5 1007

25 357 25 619 25 805 25 1008

25.5 359 25.5 623 25.5 806 25.5 1008

26 360 26 623 26 806 26 1009

26.5 362 26.5 623 26.5 813 26.5 1010

27 363 27 622 27 823 27 1012

27.5 365 27.5 622 27.5 823 27.5 1017

28 367 28 622 28 823 28 1023

28.5 367 28.5 621 28.5 822 28.5 1023

29 368 29 621 29 822 29 1023

29.5 368 29.5 621 29.5 822 29.5 1022

30 369 - - 30 821 30 1022

30.5 369 - - 30.5 821 30.5 1022

31 369 - - 31 821 31 1021

31.5 370 - - - - 31.5 1021

32 370 - - - - 32 1021

32.5 370 - - - - 32.5 1020

33 370 - - - - 33 1020

- - - - - - 33.5 1020

- - - - - - 34 1020

Table 23: Krafft Temperature For Pure OTAB + C6H5SO3Na Solution

OTAB(0.005M)-C6H5SO3Na

0.0025M 0.005M 0.0075M 0.01M

Temperature (°C)

Conductance

(S/cm)

Temperature (°C)

Conductance

(S/cm)

Temperature (°C)

Conductance

(S/cm)

Temperature (°C)

Conductance

(S/cm)

10 275 10 552 10 787 10 977

10.5 275 10.5 551 10.5 785 10.5 976

11 274 11 550 11 783 11 975

11.5 273 11.5 549 11.5 781 11.5 973

12 273 12 549 12 780 12 971

12.5 273 12.5 548 12.5 779 12.5 969

13 273 13 547 13 778 13 968

13.5 272 13.5 546 13.5 776 13.5 966

14 273 14 545 14 774 14 964

14.5 273 14.5 544 14.5 773 14.5 963

15 273 15 544 15 772 15 962

15.5 273 15.5 543 15.5 771 15.5 960

16 273 16 543 16 770 16 959

Appendix

111

16.5 274 16.5 542 16.5 769 16.5 958

17 273 17 542 17 769 17 957

17.5 274 17.5 541 17.5 768 17.5 957

18 273 18 541 18 768 18 956

18.5 274 18.5 540 18.5 767 18.5 956

19 274 19 540 19 767 19 955

19.5 275 19.5 540 19.5 766 19.5 955

20 275 20 540 20 765 20 954

20.5 276 20.5 541 20.5 765 20.5 954

21 276 21 541 21 764 21 953

21.5 277 21.5 542 21.5 764 21.5 953

22 277 22 543 22 763 22 952

22.5 278 22.5 544 22.5 763 22.5 952

23 279 23 545 23 763 23 951

23.5 280 23.5 547 23.5 763 23.5 951

24 283 24 550 24 763 24 951

24.5 292 24.5 554 24.5 763 24.5 951

25 304 25 558 25 763 25 951

25.5 314 25.5 562 25.5 763 25.5 952

26 327 26 566 26 764 26 952

26.5 330 26.5 571 26.5 764 26.5 952

27 333 27 575 27 765 27 952

27.5 336 27.5 579 27.5 766 27.5 952

28 339 28 581 28 768 28 953

28.5 343 28.5 582 28.5 769 28.5 954

29 346 29 583 29 771 29 955

29.5 348 29.5 583 29.5 774 29.5 956

30 349 30 584 30 780 30 957

30.5 350 30.5 584 30.5 787 30.5 960

31 351 31 585 31 788 31 965

31.5 352 31.5 585 31.5 789 31.5 970

32 353 32 585 32 789 32 978

32.5 354 32.5 586 32.5 790 32.5 983

33 355 33 586 33 790 33 988

33.5 356 33.5 586 33.5 791 33.5 989

34 357 - - 34 791 34 989

- - - - 34.5 791 34.5 990

- - - - - - 35 990

- - - - - - 35.5 990

- - - - - - 36 991

- - - - - - 36.5 991

- - - - - - 37 991

Critical Micelle Concentration (CMC)

Conductometric Method

Table 24: Plot of conductance vs. concentration of aqueous OTAB solution at different temperatures (310K, 313K, 318K)

Concentration (mM)

310K 313K 316K 318K

0.0392 5.22 4.8 8.3 5.43

0.0769 8.66 8.36 11.42 9.04

0.1132 11.98 11.67 15 12.45

0.1481 15.13 14.93 18.1 15.62

0.18181 18.27 18.1 21.1 18.72

0.2142 20.9 20.3 24.11 21.23

0.2456 23.8 23.1 26.58 23.7

0.2759 24.9 25.7 29.38 26.5

0.3051 26.1 27.2 31.38 28.5

0.3333 26.8 28.4 32.68 29.8

0.3607 27.8 29.2 33.88 31

0.3871 28.7 30.3 35.08 32.2

0.4127 29.5 31.1 35.88 33

0.4375 30.4 32 36.78 33.9

0.4615 31.4 32.9 37.58 34.7

0.4848 32.2 33.5 38.38 35.5

0.5075 -- 34.2 39.18 36.3

0.5294 -- 34.9 39.88 37

0.5507 -- 40.68 37.8

0.5742 -- 41.38 38.5

0.5915 -- 42.18 39.3

Table 25: Plot of conductance vs. concentration of aqueous OTAB-0.005M NaF solution at different temperatures (308K, 313K, 318K)

Concentration (mM)

308K 313K 318K

0.0098 505 509 513

0.0192 506 510 516

0.0283 507 512 518

0.037 509 513 520

0.0454 510 515 522

0.0536 511 516 524

0.0614 512 518 525

0.069 513 519 527

0.0763 514 521 529

0.0833 515 522 530

0.0902 516 523 531

0.0968 517 525 532

0.103 518 526 533

0.109 519 527 534

0.115 520 527 535

0.121 521 528 536

0.127 522 528 536

0.132 522 529 537

0.138 523 529 537

0.143 523 530 538

Appendix

112

0.148 524 530 538

0.153 524 531 539

0.158 525 531 539

0.162 525 532 540

0.167 526 532 540

0.171 526 533 541

0.175 527 533 541

0.179 527 534

0.184 528 -- --

0.188 528 -- --

Table 26: Plot of conductance vs. concentration of aqueous OTAB-0.005M NaCl solution at different temperatures (308K, 313K, 316K, 318K)

Concentration (mM)

308K 313K 316K 318K

0.0098 595 598 599 601

0.0192 596 599 600 603

0.0283 597 601 601 605

0.037 599 602 602 606

0.0454 600 603 603 608

0.0536 601 605 604 610

0.0614 602 606 605 612

0.069 602 607 606 613

0.0763 603 608 607 615

0.0833 603 608 608 616

0.0902 603 609 608 617

0.0968 604 609 609 618

0.103 604 609 609 618

0.109 604 610 610 619

0.115 604 610 610 619

0.121 605 610 611 620

0.127 605 611 611 620

0.132 605 611 611 621

0.138 605 611 612 621

0.143 606 611 612 621

0.148 606 612 612 622

0.153 606 612 612 622

0.158 606 612 613 622

0.162 606 612 613 622

Table 27: Plot of conductance vs. concentration of aqueous OTAB-0.005M NaBr solution at different temperatures (313K, 318K)

Concentration (mM)

313K 318K

0.0098 449 451

0.0192 450 452

0.0283 451 453

0.037 452 454

0.0454 453 455

0.0536 454 456

0.0614 455 457

0.069 455 458

0.0763 456 459

0.0833 456 460

0.0902 456 460

0.0968 457 461

0.103 457 461

0.109 457 462

0.115 457 462

0.121 458 462

0.127 458 463

0.132 458 463

0.138 458 463

0.143 458 463

0.148 -- 464

0.153 -- 464

0.158 -- 464

0.162 -- 464

Table 28: Plot of conductance vs. concentration of aqueous OTAB-0.005 Ionic Strength Na2SO4 solution at different temperatures (308K, 313K, 318K)

Concentration (mM)

308K 313K 318K

0.0098 404 408 410

0.0192 405 409 411

0.0283 406 410 412

0.037 407 411 413

0.0454 408 412 414

0.0536 409 413 415

0.0614 409 414 416

0.069 410 414 417

0.0763 410 415 418

0.0833 410 415 418

0.0902 411 416 419

0.0968 411 416 419

0.103 411 416 419

0.109 411 417 420

0.115 412 417 420

0.121 412 417 420

0.127 412 417 421

0.132 412 418 421

0.138 418 421

0.143 -- 418 422

0.148 -- 418 422

0.153 -- 419 422

0.158 -- 419 422

0.162 -- 419 --

0.167 -- 419 --

Table 29: Plot of conductance vs. concentration of aqueous OTAB-0.005M NaNO3 solution at different temperatures (308K, 313K, 318K)

Concentration (mM)

308K 313K 318K

0.0098 565 566 571

0.0192 566 567 572

Appendix

113

0.0283 567 568 573

0.037 568 569 574

0.0454 569 570 575

0.0536 570 571 576

0.0614 571 572 577

0.069 571 573 578

0.0763 572 573 579

0.0833 572 574 580

0.0902 572 574 580

0.0968 573 575 581

0.103 573 575 581

0.109 573 576 582

0.115 574 576 582

0.121 574 577 583

0.127 574 577 583

0.132 574 577 584

0.138 575 578 584

0.143 575 578 --

0.148 575 578 --

0.153 575 579 --

0.158 579 --

0.162 -- 579 --

Table 30: Plot of conductance vs. concentration of aqueous OTAB-0.005M C7H5O2Na solution at different temperatures (303K, 308K, 313K, 318K)

Concentration (mM)

303K 308K 313K 318K

0.00588 376 379 383 386

0.01154 377 380 384 387

0.01698 378 381 385 388

0.02222 378 382 386 389

0.02727 379 382 387 390

0.03214 379 383 387 391

0.03684 379 383 388 392

0.04138 380 384 388 392

0.04576 380 384 389 393

0.05 380 384 389 393

0.05409 380 384 389 394

0.05806 381 385 390 394

0.0619 381 385 390 394

0.06563 381 385 390 395

0.06923 381 385 390 395

0.07272 382 386 391 395

0.07612 382 386 391 396

0.07941 382 386 391 396

0.08261 382 386 391 396

0.08571 382 392 396

0.08873 -- -- 392 397

0.09167 -- -- 392 397

0.09452 -- -- 392 397

0.09729 397

Table 31: Plot of conductance vs. concentration of aqueous OTAB-0.005M C7H5O3Na solution at different temperatures (303K, 308K, 313K, 318K)

Concentration (mM)

303K 308K 313K 318K

0.00588 387 390 394 398

0.01154 388 391 395 399

0.01698 389 392 396 400

0.02222 389 393 396 401

0.02727 390 393 397 402

0.03214 390 394 398 403

0.03684 391 394 398 404

0.04138 391 395 399 405

0.04576 391 395 399 406

0.05 392 395 399 406

0.05409 392 396 400 407

0.05806 392 396 400 407

0.0619 393 396 400 408

0.06563 393 397 401 408

0.06923 393 397 401 409

0.07272 393 397 401 409

0.07612 394 397 401 410

0.07941 394 398 402 410

0.08261 394 398 402 411

0.08571 394 398 402 411

0.08873 -- 398 403 412

0.09167 -- -- 403 412

0.09452 -- -- 403 413

0.09729 403 413

Table 32: Plot of conductance vs. concentration of aqueous OTAB-0.005M C6H5SO3Na solution at different temperatures (303K, 308K, 313K, 318K)

Concentration (mM)

303K 308K 313K 318K

0.00588 384 388 392 400

0.01154 385 389 393 401

0.01698 386 390 394 402

0.02222 386 391 395 403

0.02727 387 391 395 404

0.03214 387 392 396 405

0.03684 387 392 396 406

0.04138 388 392 397 406

0.04576 388 393 397 407

0.05 388 393 398 407

0.05409 388 393 398 408

0.05806 389 394 398 408

0.0619 389 394 399 409

0.06563 389 394 399 409

0.06923 389 395 399 410

0.07272 389 395 400 410

0.07612 -- 395 400 410

0.07941 -- 395 400 411

0.08261 -- 400 411

0.08571 411

Appendix

114

Surfacetensiometric Method

Table 33: Surface tension vs. logarithm of concentration of aqueous solutions of OTAB at different temperatures (310K, 313K, 318K)

OTAB Concentrat

ion (M)

Log(C) Surface Tension (mN/m)

310K

313K 316K 318K

0.0000389 -4.41 56.1 56.9 56.6 56.3

0.0000776 -4.11 49.5 49.9 50.2 50.2

0.0001122 -3.95 45.7 45.6 46.1 46.5

0.0001479 -3.83 42.8 42.5 43.5 43.2

0.0001819 -3.74 40.9 40.5 41.1 41.3

0.0002138 -3.67 39.2 39.1 40 39.7

0.0002455 -3.61 38.3 38 38.3 38.4

0.0002754 -3.56 37.9 37.2 37.2 37.3

0.0003019 -3.52 37.9 37.2 36.7 36.6

0.0003311 -3.48 37.9 37.1 36.7 36.6

0.0003631 -3.44 37.9 37.1 36.7 36.5

0.0003890 -3.41 37.9 37.1 36.6 36.5

0.0004169 -3.38 37.8 37 36.5 36.6

0.0004365 -3.36 37.8 37 36.6 36.6

0.0004571 -3.34 37.9 37 36.6 36.5

0.0004786 -3.32 37.8 36.9 -- 36.5

0.0005012 -3.30 37.8 36.9 36.5 36.4

0.0005248 -3.28 37.8 36.9 36.5 36.4

Table 34: Surface tension vs. logarithm of concentration of aqueous solutions of OTAB-Na2SO4 (0.005M) at different temperatures (303K, 308K, 313K, 318K)

OTAB Concentration

(M)

Log (C)

Surface Tension (mN/m)

303K 308K 313K 318K

0.00001 -5 46.9 47.3 47.5 47.5

0.00001905 -4.72 43.5 43.6 43.7 44.5

0.00002818 -4.55 40.9 41.5 41.6 42.4

0.00003715 -4.43 39.6 39.7 39.9 40.3

0.00004571 -4.34 38.7 38.6 38.5 39

0.00005370 -4.27 38.7 38.1 37.8 38

0.00006166 -4.21 38.6 38.1 37.5 36.9

0.00006918 -4.16 38.6 38 37.4 36.6

0.00007586 -4.12 38.7 38 37.5 36.5

0.00008318 -4.08 38.6 38 37.4 36.5

0.00009120 -4.04 38.6 37.9 37.4 36.5

0.00009772 -4.01 38.5 37.9 37.5 36.5

0.0001023 -3.99 38.5 38 37.4 36.5

0.0001096 -3.96 38.5 37.9 37.4 36.5

0.0001148 -3.94 38.5 37.9 37.4 36.5

Table 35: Surface tension vs. logarithm of concentration of aqueous solutions of OTAB-NaF (0.005M) at different temperatures (308K, 313K, 318K)

OTAB Concentration

(M)


308K 313K 318K

0.00001 -5 54.3 53.6 53.8

0.00001905 -4.72 50.7 49.7 50

0.00002818 -4.55 48.3 47.2 47.5

0.00003715 -4.43 46.2 45.3 45.3

0.00004571 -4.34 44.8 44.2 44.2

0.00005370 -4.27 43.9 43.1 42.8

0.00006166 -4.21 42.8 42.2 42

0.00006918 -4.16 42.2 41.2 41

0.00007586 -4.12 41.6 40.8 40

0.00008318 -4.08 41 39.9 39.4

0.00009120 -4.04 40.5 39.3 38.8

0.00009772 -4.01 40 38.9 38.4

0.0001023 -3.99 39.7 38.9 38.4

0.0001096 -3.96 39.5 38.9 38.4

0.0001148 -3.94 39.4 38.8 38.3

0.0001202 -3.92 39.4 38.8 38.3

0.0001259 -3.9 39.3 38.9 38.3

0.0001318 -3.88 39.3 38.8 38.3

0.0001380 -3.86 39.3 38.8 38.3

Table 36: Surface tension vs. logarithm of concentration of aqueous solutions of OTAB-C6H5SO3Na (0.005M) at different temperatures (303K, 308K, 313K, 318K)

OTAB Concentration

(M)

Log (C)


303K 308K 313K 318K

0.000001995 -5.7 47.9 48.7 45 50.5

0.000003981 -5.4 43.2 44.2 39.6 46.2

0.000007762 -5.11 38.1 39.1 36 41.1

0.00001175 -4.93 34.4 35.8 34.4 37.8

0.00001549 -4.81 34.2 34.1 33.5 35.8

0.00001905 -4.72 34.2 34 33.3 34.4

0.00002239 -4.65 34.1 34 33.3 33

0.00002630 -4.58 34.1 34 33.2 32.2

0.00002951 -4.53 34 33.9 33.2 32.2

0.00003311 -4.48 34 33.9 33.1 32.1

Appendix

115

0.00003631 -4.44 33.8 33.1 32.2

0.00003981 -4.4 -- 33.8 -- 32.1

Table 37: Surface tension vs. logarithm of concentration of aqueous solutions of OTAB-NaCl (0.005M) at different temperatures (308K, 313K, 318K)

OTAB Concentration

(M)


308K 313K 316K 318K

0.00001 -5 56.7 57.5 51.9 57.7

0.00001905 -4.72 50 51.3 47 51.9

0.00002818 -4.55 46 46.8 43.9 47.4

0.00003715 -4.43 42.8 43.7 41.8 44.2

0.00004571 -4.34 40.4 41.2 40.3 41.8

0.00005370 -4.27 38.8 39.6 39 39.9

0.00006166 -4.21 37.6 37.9 38.1 38.2

0.00006918 -4.16 37.4 36.9 37.3 36.9

0.00007586 -4.12 37.4 36.4 36.6 36.1

0.00008318 -4.08 37.4 36.4 36.2 35.8

0.00009120 -4.04 37.3 36.3 36.2 35.6

0.00009772 -4.01 37.3 36.3 36.2 35.6

0.0001023 -3.99 37.3 36.3 36.1 35.5

0.0001096 -3.96 36.2 36.1 35.5

0.0001148 -3.94 36.2 36.1 35.5

0.0001202 -3.92 36.2 36 35.5

0.0001259 -3.9 36.2 36 35.5

Table 38: Surface tension vs. logarithm of concentration of aqueous solutions of OTAB-C7H5O2Na (0.005M) at different temperatures (303K, 308K, 313K, 318K)

OTAB Concentration

(M)

Log (C)


303K 308K 313K 318K

0.000001995 -5.7 45.7 47.9 49.7 51.4

0.000003981 -5.4 42.3 44.3 45.7 47.3

0.000007762 -5.11 38.8 40.4 41.7 43.5

0.00001175 -4.93 36.3 38.2 38.6 41.1

0.00001549 -4.81 35.1 36.3 37.1 38.9

0.00001905 -4.72 35.1 35.4 36 37.9

0.00002239 -4.65 35.1 34.8 35 37

0.00002630 -4.58 35 34.7 34.2 36

0.00002951 -4.53 35 34.7 34 35.1

0.00003311 -4.48 35 34.6 34 34.3

0.00003631 -4.44 34.9 34.6 33.9 33.7

0.00003981 -4.4 34.9 34.5 33.9 33.5

0.00004266 -4.37 34.8 34.5 33.8 33.5

0.00004571 -4.34 34.8 34.5 33.7 33.4

0.00004898 -4.31 34.8 34.5 33.7 33.4

0.00005248 -4.28 -- -- 33.8 33.4

0.00005495 -4.26 -- -- 33.7 33.3

Table 39: Surface tension vs. logarithm of concentration of aqueous solutions of OTAB-NaNO3 (0.005M) at different temperatures (308K, 313K, 318K)

OTAB Concentration

(M)


308K 313K 318K

0.00001 -5 51.7 50.9 50.1

0.00001905 -4.72 45.3 44.3 45

0.00002818 -4.55 41 40.4 41.2

0.00003715 -4.43 37.6 37.3 38.9

0.00004571 -4.34 35.2 35.4 36.9

0.00005370 -4.27 34.5 33.9 35.5

0.00006166 -4.21 34.2 33.5 34.6

0.00006918 -4.16 34.2 33.4 33.7

0.00007586 -4.12 34.1 33.4 33.1

0.00008318 -4.08 34.2 33.5 33.1

0.00009120 -4.04 34.1 33.4 33

0.00009772 -4.01 34.1 33.4 33

0.0001023 -3.99 34.1 33.4 33

Table 40: Surface tension vs. logarithm of concentration of aqueous solutions of OTAB-NaBr (0.005M) at different temperatures (313K, 318K)

OTAB Concentration

(M)


313K 318K

0.00001 -5 45.1 44.9

0.00001905 -4.72 41.4 41.5

0.00002818 -4.55 38.4 39.4

0.00003715 -4.43 36.5 37.6

0.00004571 -4.34 35.3 36.6

0.00005370 -4.27 34.3 35.5

0.00006166 -4.21 33.8 34.8

0.00006918 -4.16 33.8 34.1

0.00007586 -4.12 33.8 33.5

0.00008318 -4.08 33.7 33.2

0.00009120 -4.04 33.8 33.2

0.00009772 -4.01 33.7 33.1

0.0001023 -3.99 33.7 33.2

0.0001096 -3.96 33.6 33.1

0.0001148 -3.94 33.6 33.1

0.0001202 -3.92 33.6 33

0.0001259 -3.9 33.6 33

0.0001318 -3.88 33.6 33

Appendix

116

Table 41: Surface tension vs. logarithm of concentration of aqueous solutions of OTAB-C7H5O3Na (0.005M) at different temperatures (303K, 308K, 313K, 318K)

OTAB Concentration

(M)

Log (C)


303K 308K

313K 318K

0.000001479 -5.83 48.5 47.5 48.3 48.1

0.000002951 -5.53 44.5 44 44.3 44.3

0.000005888 -5.23 38.7 40.1 40.3 40.7

0.000008709 -5.06 36.4 37.5 37.8 38.1

0.00001148 -4.94 34.3 35.8 36.2 36.7

0.00001413 -4.85 32.8 34.4 35.5 35.4

0.00001698 -4.77 32.1 33.2 34.1 34.1

0.00001949 -4.71 32 32.4 33.4 33.5

0.00002239 -4.65 31.9 31.6 32.3 32.9

0.00002455 -4.61 31.9 31.3 32.1 32.2

0.00002754 -4.56 31.9 31.3 31.4 31.5

0.00002951 -4.53 31.8 31.3 31 31.1

0.00003236 -4.49 31.8 31.3 30.8 30.8

0.00003467 -4.46 31.8 31.2 30.8 30.6

0.00003715 -4.43 31.7 31.2 30.8 30.4

0.00003890 -4.41 31.7 31.1 30.7 30.3

0.00004074 -4.39 31.7 31.1 30.7 30.3

0.00004365 -4.36 -- 31 30.7 30.2

0.00004571 -4.34 -- 31 30.7 30.2

0.00004786 -4.32 -- -- 30.7 30.2

Table 42: Temperature dependence on counter-ion binding parameter and Surface Excess Concentration of SDS. T/K Counter-ion binding () Surface Excess

Concentration()/ 10-6

Pure SDS

SDS-0.005M NaCl

Pure SDS

SDS-0.005M NaCl

293 0.607 0.599 2.36 2.70

298 0.602 0.569 2.22 2.34

303 0.585 0.564 2.05 2.06

308 0.568 0.544 1.88 1.90

Table 43: Temperature dependence on counter-ion binding parameter and Surface Excess Concentration of OTAB. T/K Counter-ion binding () Surface Excess

Concentration()/ 10-6

Pure OTAB

OTAB-0.005M NaCl

Pure OTAB

OTAB-0.005M

NaCl

308 0.702 2.09

310 0.617 1.96

313 0.611 0.678 1.99 2.16

318 0.587 0.664 2.02 2.19

Table 44: Absorption vs. Wavelength data of SRB for SDS surfactant Concentration

(mM) Pure SDS SDS-0.005M NaCl

𝝀max Absorbance

𝝀max Absorbance

3 -- -- 517 0.019

4 517 0.018 517 0.025

5 518 0.031 518 0.029

6 518 0.041 517 0.051

7 519 0.046 521 0.093

8 520 0.074 521 0.145

9 519 0.116 523 0.189

10 523 0.18 523 0.241

15 524 0.504 -- --

20 523 0.85 524 0.606

30 524 1.495

Table 45: Absorption vs. Wavelength data of SRB for OTAB surfactant Concentration

(mM) Pure OTAB OTAB-0.005 ionic

strength Na2SO4

𝝀max Absorbance

𝝀max Absorbance

0.01 -- -- 511 0.006

0.02 -- -- 511 0.018

0.03 -- -- 511 0.025

0.04 -- -- 512 0.036

0.06 -- -- 512 0.094

0.1 505 0.053 517 0.129

0.15 -- -- -- --

0.2 506 0.083 515 0.245

0.3 507 0.13 -- --

0.4 508 0.23 515 0.399

0.6 512 0.47 -- --

0.8 -- -- 520 0.761

1 515 0.763 -- --

1.5 515 1.079 -- --

2.0 517 1.474 -- --

Table 46: Data of calibration curve of SRB in surfactant media

Amount of Dye (g) Absorbance

2E-5 0.047

4E-5 0.092

8E-5 0.177

1.2E-4 0.257

1.6E-4 0.341

2.4E-4 0.512

4E-4 0.82

6E-4 1.23

1E-3 2.059

Appendix

117

Table 47: Data of dye concentration in different SDS concentration

SDS Concentration (mM) Dye Concentration (mM)

4 4.589E-4

5 7.901E-4

6 0.00105

7 0.00117

8 0.00189

9 0.00296

10 0.00458

Table 48: Data of dye concentration in different SDS-0.005M NaCl concentration

SDS Concentration (mM) Dye Concentration (mM)

3 4.844E-4

4 6.377E-4

5 7.391E-4

6 0.0013

7 0.00237

8 0.0037

9 0.00482

10 0.00615

Table 49: Table: Data of dye concentration in different OTAB concentration OTAB Concentration (mM) Dye Concentration (mM)

0.1 0.0013

0.2 0.0021

0.3 0.0033

0.4 0.0058

0.6 0.012

1 0.019

1.5 0.028

2 0.038

Table 50: Data of dye concentration in different OTAB-0.005 ionic strength of Na2SO4 OTAB Concentration (mM) Dye Concentration (mM)

0.01 1.7E-4

0.02 4.1E-4

0.03 6.3E-4

0.04 7.9E-4

0.06 0.0014

0.1 0.0027

0.15 0.0037

0.2 0.005

Table 51: Krafft temperature of SDS in presence of electrolytes at different concentration

Concentration (Ionic strength)

Krafft temperature

LiCl NaCl KCl CsCl

0.0025 12.9 14.56 23.3 12.38

0.005 12 14.92 28 20.07

0.0075 10.6 15.43 30.6 22.8

0.01 9.27 16.1 32.2 24.96

Table 52: Krafft temperature of OTAB in presence of electrolytes at different concentration

Concentration (Ionic strength)

Krafft temperature

NaF NaCl NaBr Na2SO4 NaNO3 NaSCN NaI C6H5SO3Na C7H5O2Na C7H5O3Na

0.0025 34.87 34.39 37.3 34.14 34.76 41.25 57.5 28.9 29.84 28.09

0.005 33.92 33.1 37.69 31.18 33.36 47.5 63.5 28.2 25.6 25.3

0.0075 33.64 31.68 38.19 29.94 32.78 49.15 65.5 30.5 28.3 26.9

0.01 32.96 31.31 38.28 28.89 32.02 51.25 66.75 32.8 29.5 27.8

Appendix

118

CALCULATION

Micellization

Counter ion binding calculation: (from data of conductometric method used for CMC

measurement)

In aqueous solution and in salt solution the way of calculation of counter ion binding (𝛽) is

same.

𝛽 = (1 − 𝛼)

𝛼 = 𝑆𝑙𝑜𝑝𝑒 𝑜𝑓 𝑝𝑜𝑠𝑡𝑚𝑖𝑐𝑒𝑙𝑙𝑎𝑟 𝑧𝑜𝑛𝑒

𝑆𝑙𝑜𝑝𝑒 𝑜𝑓 𝑝𝑟𝑒𝑚𝑖𝑐𝑒𝑙𝑙𝑎𝑟 𝑧𝑜𝑛𝑒

Example: At 293K in SDS solution

The value of post-micellar region = 25.39650 (by taking the Fit Linear)

The value of pre-micellar region = 64.70421 (by taking the Fit Linear)

Now, 𝛼 = 25.39650

64.70421= 0.392502

So, 𝛽 = 0.607

Note: At all temperatures and medium 𝛼 value was calculated in the same way.

Thermodynamic parameters calculation for aqueous SDS solution

Mole fraction calculation

At 293K, the CMC of aqueous SDS solution = 8.09 mM. During this CMC value the total volume of the solution is (50+9.86) = 59.86 ml.

1000 ml 1M SDS solution contains = 288.37g SDS

So, 59.86 ml 0.00809 M SDS solution contains = 288.37×59.86×0.00809

1000 = 0.1396g SDS

Mole of SDS = 0.1396

288.37 = 4.841×10-4

Appendix

119

Mole of water = 59.86

18 = 3.326

Total mole = 3.326 + 4.841×10-4 = 3.326

Mole fraction of SDS at CMC = (4.841×10-4)/3.326 = 1.456×10-4

At 293K, ln(CMC) = −8.835

Note: All the calculation for mole fraction has been calculated in the same way for SDS and OTAB.

Free energy of micellization (∆𝑮m°)

At 293K, (∆𝐺m°)293 = (1 + β) RT ln(CMC) = (1+0.607)×8.314 ×293×(-8.835) = -34.606 kJmol-1

Note: All the calculation for free energy of micellization(∆𝐺m°) has been calculated in the same way for SDS and OTAB solution.

Entropy Calculation (∆Sm°):

We know (∆Sm°) = − 𝝏(∆𝑮𝒎

° )

𝝏𝑻

Now, plotting the T vs. (∆𝐺m°) graph for pure SDS solution in excel sheet we get an equation like

(∆𝐺m°) = 3.41T2−2098.7T

𝜕(∆𝐺𝑚° )

𝜕𝑇××−

When T = 293K then 𝜕(∆𝐺𝑚° )

𝜕𝑇

So for T = 293K

(∆Sm°) = 100.44 JK-1mol-1

Note: In the same way all calculations for different temperatures have been done for SDS and OTAB solution.

Enthalpy Calculation

We know, ∆Gm° = ∆Hm° − 𝑇∆Sm°

At 293K, 𝑇∆Sm° = 293×100.44

1000 = 29.428 kJ/mol

So ∆Hm° =−5.177 kJ/mol

Appendix

120

Note: Such a way we can calculate the value of ∆Hm° for SDS and OTAB at different temperature.

Thermodynamic parameters calculation for aqueous SDS-0.005M NaCl solution at

different temperatures

Mole fraction calculation

At 293K, the CMC of aqueous SDS solution = 6.31mM. During this CMC value the total volume of the solution is (50+9.65) = 59.65 ml.

1000 ml 1M SDS solution contains = 288.37g SDS

So, 59.65 ml 0.00631 M SDS solution contains = 288.37×59.65×0.00631

1000 = 0.1085g SDS

1000 ml 1M NaCl solution contains 58.44g NaCl

So, 59.65 ml 0.005 M NaCl solution contains 0.0174g NaCl

Mole of SDS = 0.1085

288.37 = 3.763×10-4

Mole of NaCl = 0.0174

58.44 = 2.977×10-4

Mole of water = 59.65

18 = 3.314

Total mole = 3.763 ×10-4+ 2.977×10-4 + 3.314= 3.314

Mole fraction of SDS at CMC (Xcmc) = 3.763×10-4/3.314 = 1.136×10-4

Mole fraction of NaCl (Xs) = 2.977 ×10-4/3.314 = 8.983×10-5

Free energy calculation in presence of 0.005M NaCl of SDS solution

∆Gm° = RT [lnXcmc + (1−α) ln (Xcmc + Xs)]

Here α = degree of dissociation

In the presence of 0.005M NaCl, the degree of dissociation of SDS solution is found to be 0.401

Now by putting the values from previous calculation we get

∆Gm° = 8.314×293 [(-9.083) + (1−0.401)×(-8.500)] = -34.529 kJmol-1K-1

Note: We can calculate the value of ∆Gm° for SDS and OTAB at different temperatures in the same way.

Appendix

121

Entropy calculation in the presence of 0.005M NaCl of SDS solution

Follow the same way of entropy calculation as pure SDS solution.

Enthalpy calculation in the presence of 0.005M NaCl of SDS solution

Follow the same way of enthalpy calculation as pure SDS solution.

Adsorption

Thermodynamic parameters calculation for aqueous SDS solution

Surface Excess Concentration: (from surface tension data)

Γ =1

2𝑅𝑇

𝜕𝛾

𝜕𝑙𝑛𝐶 )TP = − 1

2.303×2𝑅𝑇

𝜕𝛾

𝜕𝑙𝑜𝑔𝐶 )TP

In aqueous solution and in salt solution the way of calculation of Surface Excess Concentration Γ is same.

Example: At 293K in SDS solution

Here the value of 𝜕𝛾

𝜕𝑙𝑜𝑔𝐶)from the slope of the surface tension data, calculated by

Fit Linear)

Γ = −1×(−26.440)

2.303×2×8.314×293×1000= 2.356×10mol/m2

Note: At all temperatures and medium Γ value was calculated in the same way.

Equilibrium surface pressure

𝜋cmc = 𝛾o−𝛾cmc

At 293K for pure SDS solution

𝛾o = 72.8, 𝛾cmc = 37.7

So, 𝜋cmc = 35.1 mN/m

Note: At all temperatures and medium 𝜋cmc value was calculated in the same way.

Free energy of adsorption (∆𝑮ad°)

(∆𝐺ad°) = (∆𝐺m°)−( 𝜋cmc / Γmax)

At 293K for pure SDS solution

Appendix

122

(∆𝐺ad°) = (−34.606−14.898) = −49.504 kJ/mol

Note: At all temperatures and medium (∆𝐺ad°) value was calculated in the same way.

Entropy (∆𝑺ad°) calculation

Follow the same way of entropy calculation for pure SDS and in the presence of 0.005M NaCl solution at different temperature as micellization.

Enthalpy (∆𝑯ad°) calculation

Follow the same way of enthalpy calculation for pure SDS and in the presence of 0.005M NaCl solution at different temperature as micellization.

Solubilization

Calculation of MSR at 303K for SDS

To get a calibration curve, firstly, a fixed amount of dye was dissolved in a fixed amount of surfactant but several times higher concentration of CMC. The amount of dye to be like that it remains below to that of the solubilization equilibrium with surfactant micelle. Then carrying out the spectrophotogram we get the following data-

Table: 1

Amount of Dye (g) Absorbance 2E-5 0.047

4E-5 0.092

8E-5 0.177

1.2E-4 0.257

1.6E-4 0.341

2.4E-4 0.512

4E-4 0.82

6E-4 1.23

1E-3 2.059

Molar mass of SRB is 380.44g

380.44g 1000ml = 1M

Therefore, 0.001g 50ml = 5.26×10-5M

By solubilizing dye (fixed amount) at different concentration (above and below CMC value) of SDS we get the following data from spectrophotogram-

Appendix

123

Table: 2

Surfactant Concentration (M) Absorbance 0.004 0.018

0.005 0.031

0.006 0.041

0.007 0.046

0.008 0.074

0.009 0.116

0.01 0.18

Using absorbance value from Table-2 we can calculate corresponding amount of dye from Table-1 by plotting graph.

Table: 3

Surfactant Concentration (mM) Dye Concentration (mM) 4 4.589E-4

5 7.901E-4

6 0.00105

7 0.00117

8 0.00189

9 0.00296

10 0.00458

Published Articles

124

List of publications related to the present work

Journal Paper

1. Islam, M. N.; Sarker, K. C.; Sharker, K. K.; Influence of some Hofmeister anions on the Krafft

temperature and micelle formation of cetylpyridinium bromide in aqueous solution.

J Surfact Deterg. 18: 9-16, 2015

2. Islam, M. N.; Sharker, K. K.; Sarker, K. C.; Salt-Induced Modulation of the Krafft Temperature

and Critical Micelle Concentration of Benzyldimethylhexadecylammonium Chloride.

J Surfact Deterg. 18:651–659, 2015

3. Sharker, K. K.; Islam, M. N.; Das, S.; Influence of Some Counterions on the Krafft Temperature

and Related Physico-Chemical Properties of Aqueous Octadecyltrimethylammonium Bromide

Solution. J Surfact Deterg. (Submitted)

4. Sharker, K. K.; Islam, M. N.; Effect of some salts on the Krafft temperature and micellization of Sodium dodecyl sulfate and their thermodynamic studies (To be Submitted)

Conference Paper

1. Sharker, K. K.; Islam, M. N.; Counter-ion Effects on Krafft Temperature and Related Behavior of

Octadecyltrimethylammonium Bromide in Aqueous Solution (16th Asian Chemical Congress, 16-19

March, 2016, Dhaka, Bangladesh)

COUNTER ION EFFECTS ON THE KRAFFT TEMPERATURE AND … · counter-ion effects on the krafft temperature and micelle formation of ionic surfactants in aqueous solution. a dissertation

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