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
