Interfacial behaviour of amphiphilic molecules: Adsorption of surfactants and polymers...
Transcript of Interfacial behaviour of amphiphilic molecules: Adsorption of surfactants and polymers...
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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
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* 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
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Amount adsorbed
Free surfactant
not full coverage
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Monolayer adsorption Bilayer adsorption
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Self-Assembly
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Adsorption isotherms for different CnEm surfactants
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Mean optical thickness versus concentration
0 to 2 surfactant molecule length directly!
No monolayers!
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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.
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Kinetics of adsorption and desorption of CnEm surfactants
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Kinetic model
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Adsorption from surfactant mixtures
Rinsing may lead to increased adsorption and thicker adsorbed layer.
Surfactant-surfactant and surfactant-oil mixtures
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Binary CnEm surfactant and cosurfactant systems
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Simultaneous adsorption and desorption of two non-ionic surfactants at silica surface
Redeposition�of the hydro-�phobic �surfactant
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Evolution of adsorption and desorption of C12E5/alcane systems with time
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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
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Typical adsorption of an ionic surfactant at a polar surface
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Adsorption of cationic surfactants at the silica surface
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Adsorption of cationic surfactants at silica surface:�alkyl chain length effect
C12
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Effect of added salt on ionic surfactant adsorption
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Adsorption of SDS at three different salt concentrations
0 mM
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Ionic surfactant packing denser with salt:�The molecular cross sectional surface area decreases
with salt concentration
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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
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Interaction of polymers with surfaces
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Adsorption of polymers
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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
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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
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Example:�Adsorption of EHEC
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Solvent Polymer
Surface
Aqueous systems: Adsorption since water interacts unfavorably with polymer (clouding polymer) or surface (hydrophobic surface)
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Polar/nonpolar surfaces
T dependence: Solvency
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Polymer polarity. Solvency
Less polar
More polar
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The poorer the solvent�the better the adsorption
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Polymer Adsorption
Additives Surface Properties
Polymer Concentration
Temperature
Polymer Structure
Solution Properties
Solvency
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The influence of the solvent
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Increase in adsorption
Decrease in CP
Increase in CP
Decrease in adsorption
Adsorption of EHEC on SiO2: �Solvency effects due to cosolutes
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Adsorption of EHEC on SiO2: Cosolute effects
Increase in adsorption
Decrease in CP
Decrease in adsorption
Increase in CP
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The dependence on molecular weight
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Kinetics
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Reversible adsorption ?
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Rinsing does not lead to desorption
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Adsorption of polyethylene imine at an anionic surface
The apparent irreversibility can be used in order to control the charge of a surface
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Polymer concentration profiles outside an interface
Adsorption/depletion Tails/loops/trains
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Conformation of an adsorbed Polymer at an interface
Tails Loops
Trains
δ
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Polymer adsorption kinetics
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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
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Adsorption of block copolymers: definitions
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The adsorption of PVA on a polystyrene surface
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The effect of adsorbent porosity on the rate of adsorption and on the molecular weight dependence
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Adsorption of polyelectrolytes at solid surfaces
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pH determines the adsorption mode
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Case I: Polymer and surface have opposite charge
Add salt
Entropic gain of counterions
Adsorption�decreases
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Case II: Polymer and surface have the same charge
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+ +
+
+
+ -
Add salt Adsorption increases
+ -
+
+
+ - - -
+ + +
- - - - - - - - + + +
- - - - - - - -
van der Waals�interaction + -
+ -
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Adsorption of a cationic polymer on a negatively charged montmorillonite�
Case I
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Adsorption of polystyrene sulphonate at a negatively charged surface�
Case II
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Interfacial behavior of polymer-surfactant mixtures
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Interfacial Polymer-Surfactant Interactions
Some parameters of importance
• Solution Polymer-Surfactant Interactions
• Affinity of the Polymer/Surfactant to the Surface
• (Adsorbed Layer Structure)
• …
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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
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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
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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!
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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)
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Summary
• Solvency • Polymer amphiphilicity • Competition, synergism • Molecular weight • Thermodynamics vs. Kinetics • Particle stabilisation