Interfacial behaviour of amphiphilic molecules: Adsorption of surfactants and polymers …stepitn.eu/.../uploads/2009/12/PM_12_FSU_Jena_Lindman.pdf · 2009-12-04 · * Adsorption

Interfacial behaviour of amphiphilic molecules: Adsorption of surfactants

and polymers on solid surfaces

Surfactant adsorption at solid surfaces. Polymer adsorption. Adsorption in mixed polymer-surfactant systems

STEP, Jena 2009

* Adsorption is quite different for associating (surfactants, amphiphilic polymers) and nonassociating (homopolymers) solutes

•  Adsorption depends on solute-surface interaction but also on solvation, and surface-solvent and solute-solute interactions.

•  Adsorption from aqueous solution is strongly dependent on surface polarity: hydrophobic vs hydrophilic surfaces

•  Surfactant adsorption on hydrophobic surfaces is noncooperative

•  Surfactant adsorption on hydrophilic surfaces is cooperative: organization into surface aggregates

•  Surfactant adsorption –desorption kinetics controlled by diffusion and micelle life-time

•  Polymer adsorption depends on solvency and molecular weight. It can be controlled by temperature and cosolutes

•  Block copolymer adsorption has features of surfactant adsorption

•  Adsorption from mixed solutions can be complex: Competitive adsorption, synergism

Amount adsorbed

Free surfactant

not full coverage

Monolayer adsorption Bilayer adsorption

Self-Assembly

Adsorption isotherms for different CnEm surfactants

Mean optical thickness versus concentration

0 to 2 surfactant molecule length directly!

No monolayers!

Nonionic adsorption •  Adsorption of nonionic surfactants on silica is a strongly co-

operative process resulting in the formation of surface aggregates (micelles) at a well defined concentration.

•  The structure of these aggregates parallels that of bulk surfactant-water systems closely. C12E8 small aggregates, C12E5 large.

•  No formation of monolayers.

•  No formation of bilayers. Approach to bilayers for short EO chains where lamellar phase forms in bulk.

Kinetics of adsorption and desorption of CnEm surfactants

Kinetic model

Adsorption from surfactant mixtures

Rinsing may lead to increased adsorption and thicker adsorbed layer.

Surfactant-surfactant and surfactant-oil mixtures

Binary CnEm surfactant and cosurfactant systems

Simultaneous adsorption and desorption of two non-ionic surfactants at silica surface

Redeposition�of the hydro-�phobic �surfactant

Evolution of adsorption and desorption of C12E5/alcane systems with time

Summary, nonionics

•  Adsorption isotherms: h-phobic vs. h-philic •  Adsorption increases with T •  Surface-induced self-assembly, admicelles •  Kinetics of adsorption and desorption •  Mixed systems: surfactant-surfactant,

surfactant-oil

Typical adsorption of an ionic surfactant at a polar surface

Adsorption of cationic surfactants at the silica surface

Adsorption of cationic surfactants at silica surface:�alkyl chain length effect

C12

Effect of added salt on ionic surfactant adsorption

Adsorption of SDS at three different salt concentrations

0 mM

Ionic surfactant packing denser with salt:�The molecular cross sectional surface area decreases

with salt concentration

Surfactant self-assembly relevant for adsorption in two ways:

1)  Bulk self-assembly is a competing process

2)  Surfactants at interfaces are often in a self-assembled state

Interaction of polymers with surfaces

Adsorption of polymers

Polymers at interfaces Adsorption – depletion

Interplay solvent-polymer-surface

Adsorption isotherm- high affinity vs low affinity

Adsorbed amount, thickness of adsorbed layer, structure of adsorbed layer, segment distribution, train-tail-loop picture

Kinetics of adsorption and desorption

Reversibility of adsorption

Competitive adsorption, high vs low molecular weight. Effect of polydispersity

Typical Adsorption isotherm

Equilibrium Polymer Concentration (ppm)

Am

ount

ads

orbe

d, Γ

(m

g/m

2 or m

g/g)

Low affinity

High affinity

Example:�Adsorption of EHEC

Solvent Polymer

Surface

Aqueous systems: Adsorption since water interacts unfavorably with polymer (clouding polymer) or surface (hydrophobic surface)

Polar/nonpolar surfaces

T dependence: Solvency

Polymer polarity. Solvency

Less polar

More polar

The poorer the solvent�the better the adsorption

Polymer Adsorption

Additives Surface Properties

Polymer Concentration

Temperature

Polymer Structure

Solution Properties

Solvency

The influence of the solvent

Increase in adsorption

Decrease in CP

Increase in CP

Decrease in adsorption

Adsorption of EHEC on SiO2: �Solvency effects due to cosolutes

Adsorption of EHEC on SiO2: Cosolute effects

Increase in adsorption

Decrease in CP

Decrease in adsorption

Increase in CP

The dependence on molecular weight

Kinetics

Reversible adsorption ?

Rinsing does not lead to desorption

Adsorption of polyethylene imine at an anionic surface

The apparent irreversibility can be used in order to control the charge of a surface

Polymer concentration profiles outside an interface

Adsorption/depletion Tails/loops/trains

Conformation of an adsorbed Polymer at an interface

Tails Loops

Trains

δ

Polymer adsorption kinetics

Flexible polymer

On adsorption, a flexible polymer loses its configurational freedom in 3 dimensions (∆S < 0):

Many loops and tails (steric stabilisation)

Stiff polymer

Little configurational freedom to lose: Flat on the surface

Adsorption of block copolymers: definitions

The adsorption of PVA on a polystyrene surface

The effect of adsorbent porosity on the rate of adsorption and on the molecular weight dependence

Adsorption of polyelectrolytes at solid surfaces

pH determines the adsorption mode

Case I: Polymer and surface have opposite charge

Add salt

Entropic gain of counterions

Adsorption�decreases

Case II: Polymer and surface have the same charge

+

+

+ - - -

+ + + - - - - - - - - - - - - - - - -

+

+ +

-

-

-

+ + + -

- -

+ + +

- -

+

+ + -

-

+ +

+

+

+ -

Add salt Adsorption increases

+ -

+

+

+ - - -

+ + +

- - - - - - - - + + +

- - - - - - - -

van der Waals�interaction + -

+ -

Adsorption of a cationic polymer on a negatively charged montmorillonite�

Case I

Adsorption of polystyrene sulphonate at a negatively charged surface�

Case II

Interfacial behavior of polymer-surfactant mixtures

Interfacial Polymer-Surfactant Interactions

Some parameters of importance

•  Solution Polymer-Surfactant Interactions

•  Affinity of the Polymer/Surfactant to the Surface

•  (Adsorbed Layer Structure)

•  …

A Slightly hydrophobic cellulose derivative

EHEC – Ethyl (HydroxyEthyl)Cellulose

The degree of ethyl and hydroxyethyl substitution determines the hydrophobicity of polymers in the EHEC family

General Swelling Isotherm �for ”Weakly Hydrophobic”

Nonionic Gel with Ionic Surfactant

5 10 15 20 25 30 35

0.1 1 10 100

V/V 0

C f,SDS 0 cac

EHEC/SDS on Hydrophobized Silica

•  Substrate:Silanol groups reacted with dimethyloctylchlorosilane

•  EHEC preadsorbed from 0.01 wt% solution. (Intermediate adsorption)

•  SDS adsorbs on hydrophobized silica; Competitive adsorption!

HMEHEC/SDS on the Hydrophobized Silica

•  Substrate: Silanol groups reacted with dimethyl-octyl-chloro-silane •  HMEHEC preadsorbed from 0.01 wt% solution. (Strong adsorption) •  SDS adsorbs on hydrophobized silica; Competitive adsorption!

•  Expansion observed at low SDS concentrations (around cac)

•  HMEHEC desorbs at higher SDS concentrations (above cac,cmc)

Summary

•  Solvency •  Polymer amphiphilicity •  Competition, synergism •  Molecular weight •  Thermodynamics vs. Kinetics •  Particle stabilisation