SURFACTANT ADSORPTION Surfactant = substance composed of surface active molecules (spontaneously adsorb and decrease surface tension)
Why are the surfactants so important? Ensure foam stability: Affect all foam properties! Can be used for control of foam behavior!
1. Low molecular mass ( 200 to 1000) Ionic Types of surfactant Nonionic 2. Polymeric (including proteins) Fibrilar Globular 3. Solid particles
Aims of presentation: 1. To describe the phenomena of surfactant adsorption and aggregation in solutions. 2. Role of surfactants in foams (introduction). CONTENTS A. Surface tension and surfactant adsorption. B. Kinetics of surfactant adsorption. C. Surfactants in foams illustrative examples.
A. Surface tension and surfactant adsorption Surface tension - energetical and force approaches. 1. Energetical approach Work = 2σA σ - interfacial energy per unit area [J/m 2 ]
Molecular interpretation of energetical approach Intermolecular interaction energy Surface energy as an excess energy ( 6 ) ( 5 ) G = N µ u + N µ u B 0 0 S 0 0 ( 0 6 0) { S 0 = NTOT µ u + N u 14243 µ B surface energy=σa σ Nu s 0 / A 200 mj/m 2
2. Force approach Surface tension of liquids as a tangential force σ F σ F = 2σL σ = (F / 2L) [N/m] F = mg + 2πRσcosθ
Where does this tangential force come from?
Surface tension of spread surfactant monolayers (Langmuir-trough) F = ( σ σ) 0 L Γ mol/m 2 surface concentration Effect of surfactants W/A W/S/A Surface pressure Π S ( ) σ σ 0 Related to Marangoni effect
Analogy between surface and bulk pressures Dilute monolayers Ideal adsorption layer Π S ( ) Γ = ΓkT B Ideal gas P( C) = CkBT Concentrated monolayers ( 2 S )( ) ( 2 )( ) Π β A A A = k T - 2D van der Waals P a v v v = k T B B - 3D van der Waals
Typical results for soluble surfactants and data interpretation Micelle formation CMC Gibbs adsorption isotherm dσ = Γ dµ Γk TdlnC B µ =µ 0 + kt B lnc (Gibbs-Duhem for the surface phase)
Model adsorption isotherms Γ C =Γ Langmuir ( ) KC 1+ KC dσ = k TΓdlnC B Surface tension isotherm C K σ =σ Γ =σ Γ + ( C) k T dc k TK ln( 1 KC) 0 B 0 1+ KC 0 B Assumptions: Localized adsorption. Non-interacting molecules.
Model adsorption isotherms Type of isotherm Surfactant adsorption isotherm Surface equation of state Henry KC Γ = Π =σ σ = kt Γ Γ S 0 B Langmuir Volmer Frumkin van der Waals Γ Γ KC = Π ln Γ Γ S = kt B Γ Γ Γ Γ Γ KC = exp Γ Γ Γ Γ Γ 2βΓ KC = exp Γ Γ kt B Γ Γ 2βΓ KC = exp Γ Γ Γ Γ kt B Γ Π S = kt B Γ Γ Γ Γ Π S = kt B Γ ln βγ Γ Γ Γ Π S = Γ βγ Γ kt B Γ 2 2
Gibbs elasticity of adsorption monolayers Insoluble monolayer E G =+ dσ dln A Soluble monolayer E G dσ = = dlnγ dπ S dlnγ Examples Henry Π S = kt B Γ EG = kt B Γ Volmer Γ Γ Π = ktγ E = ktγ Γ Γ Γ Γ S B G B 2 ( ) 2
1. Wilhelmy plate Methods for measuring the surface tension
2. Pendant drop method z P( z) P C = P 0 P( z) = ρgz 1 1 σ/ ρ g = f, R1 R2 P C 1 1 =σ + R R 1 2 Experimental determination of Γ(C): ellipsometry, neutron reflection, radioactivity
B. Kinetics of surfactant adsorption Interrelation between macroscopic and molecular levels of description
Stages of dynamic adsoprion Two consecutive stages Stage 1 - adsorption from the "subsurface layer" onto surface. Stage 2 - diffusion from the bulk to the subsurface layer Possible additional stages Molecule rearrangement Formation of intermolecular bonds
Diffusion control of adsorption: large deviation from equilibrium Γ = Γ(C S ) t = t 1 t = t 2 Diffusion time ( t) δ 2 ~2Dt δ(t 1 ) δ(t 2 ) Γ ~ C B δ max 1 Γ τdiff ~ D C B 2
Diffusion control of adsorption: exact solution and asymptotics Diffusion equation Surface mass balance Adsorption isotherm 2 C C dγ C = D = D 2 t x x= Γ =Γ( C ) dt x S 0 Solution (Ward & Tordai) ( t) t D C Γ () =Γ ( ) + S t 0 2Cb t dτ π 0 t τ t 0 Dt Γ() t 2C B t π t dσ τd 1 σ () t = Γ () t = Γ( 0) dγ πt t
Barrier control of adsorption Surface mass balance d Γ = Vads C Γ B V Γ des dt (, ) ( ) ads ads B 1 Example: Langmuir adsorption Rate of adsorption ( ) Rate of desorption V = k C Γ Γ Vdes = kdesγ () ( ) d Γ = kadsc B ( kadsc Γ + B kdes ) Γ dt () ( ) σ t Γ t t Γ = = exp τ b = σ 0 Γ 0 τb kdesγ + kadsc B
Adsorption from micellar solutions Characteristic times τ d diffusion of monomers τ dm - diffusion of micelles τ mic - supply of monomers from the micelles For ionic surfactants msec For nonionic surfactants - sec
Surfactant spreading and Marangoni effect Stress balance τ { surf = τ { visc surface stress viscous stress Viscous stress dv dz = r τ visc =µ z 0 Surface stress Spreading velocity dσ σ σ τ surf = 0 dr r Vr ( σ σ) 12 3 = 4ρ µ 0 14 t 12 12 Marangoni effect
Tears of wine and Marangoni effect Carlo Marangoni James Thomson, "On certain curious motions observable on the surfaces of wine and other alcoholic liquours, Philosophical Magazine, 10, 330 (1855). Carlo Marangoni, "On the expansion of a drop of liquid floating in the surface of another liquid, PhD Thesis, Pavia, Italy, 1865.
C. Role of surfactants in foams 1. Determine the equilibrium surface tension the static foam structure. Capillary pressure Hydrostatic pressure P = 2 σ/ R P = ρgh C B H Height of the wet foam H W 2σ = ρ gr B Water drainage: (ASJ, SCA next week) Proteins: σ ~ 50 mn/m Fluoro-surfactants: σ ~ 15 mn/m
2. Static stabilization of foam films Ionic surfactants Nonionic surfactants Electrostatic stabilization by ionic surfactants Steric stabilization by nonionic surfactants Static stabilization: ASJ and ND in Thursday
3. Dynamic stabilization of the foam films by Marangoni effect Damped local perturbations in the film thickness due to Marangoni effect Slower film drainage and higher stability
4. Effect of surfactants on the dynamic phenomena in foams Foaming foam volume and bubble size. Rate of water drainage from foams. Rate of foam film thinning. Viscous dissipation in foams. Foam-wall friction. Ostwald ripening. Important role of dynamic surface tension and interfacial rheology!
5. Role of micelles in foams (a) Reservoir of surfactant dynamic surface tension. (b) Oscillatory forces in foam films (stratification). (c) Solution rheology (shampoos, dish-washing gels) higher solution viscosity
Thursday Following related presentations: Role of surfactants in foam stabilization (including antifoams) Friday Interfacial rheology
SINCERE THANKS Dr. S. Tcholakova Help in preparation of the presentation. Miss M. Paraskova Preparation of some of the figures. Other colleagues The Laboratory of Chemical Physics & Engineering Faculty of Chemistry, University of Sofia, Sofia, Bulgaria
Derivation of Gibbs adsorption isotherm Surface excess quantities 0 α ( ) ( ) S β f = f z f dz f z f + dz 0 α V = V + V β α β S i = i + i + i N N N N α F = F + F + F α β β U = U + U + U S S 0 α β i i( ) i i( ) i 0 Γ = n z n dz n z n + dz 0 α T( ) β T( ) 0 σ= P z P dz P z P dz
For the total system: df = SdT pdv +σ da +Σµ dn F σ= A TV,, N F = PV +σ A+Σµ N i i i i Conditions for equilibrium: T = const µ i = const For the subsystems α β S α α α β β β = + + F = pv +Σµ ini ; F = pv +Σµ ini S S S S S S S F = U TS S F =σ A+Σµ N i S i α α α α β β β β i i i i df = s dt pdv + Σµ dn ; df = s dt pdv + Σµ dn S S S i i df = S dt +σ da +Σµ dn S dσ = S dt ΣΓ dµ i i