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DOUBLE ELECTRICAL LAYER AND ZETA POTENTIAL Outline 1) Charge Development at Solid Interfaces 2) Electrical Double Layers 3) Zeta Potential Origin of Surface Charge Immersion of some materials in an electrolyte solution. Two mechanisms can operate. (1) Dissociation of surface sites. H O M OOO H H O M OOO H OH O M OOO + H2O (2) Adsorption of ions from solution. OH + + + M OOO Origin of Surface Charge (3) Some ionic crystals have a slight imbalance in number of lattice cations or anions on surface, eg. AgI, BaSO4, CaF2,NaCl, KCl (4) Surface charge due to crystal lattice defects, eg. lattice substitution in kaolin OH O O O Si Si Si Al HO O O O OH HO Other Surfaces with Charge The bacteria membrane is made of a self organizing lipid bi-layer. Most of the lipids head-groups are negative charged. Oil droplets develop negative charge. Bubbles develop negative charge since cationic species are more mobile, than anionic species and accumulate to the interface. Electrical Double Layer Distance from Surface Surface Potential 0 Water molecules x + + + Counter-Ions OHP + + Co-Ions Helmholtz (100+ years ago) proposed that surface charge is balanced by a layer of oppositely charged ions Gouy-Chapman Model 1910-1913 Distance from Surface 0 + + + + + Diffusion plane Assumed Poisson-Boltzmann distribution of ions from surface ions are point charges ions do not interact with each other Assumed that diffuse layer begins at some distance from the surface + x Stern (1924) / Grahame (1947) Model Distance from Surface 0 Diffusion layer x + + + + + + Stern Plane Shear Plane + Gouy Plane Bulk Solution Gouy/Chapman diffuse double layer and layer of adsorbed charge Salt Addition Distance from Surface 0 Diffusion layer Original Curve x + + + + + + + + + Stern Plane Shear Plane Gouy Plane Bulk Solution Salt will not effect the value of 0, but will effect the value of Salt Addition =0 0 Distance from Surface + + + + + + + + Shear Plane Stern Plane Gouy Plane Diffusion layer Original Curve x + + + + + Bulk Solution Further salt addition can collapse the EDL. Debye Length: EDL Thickness Debye-H ckel parameter ( ) describes the decay length e = kT r0 2 Ci Z i =1 n 2 i 1/ 2 Zi e Ci n r 0 k T = electrolyte valence = elementary charge (C) = ion concentration (#/m3) = number of ions = dielectric constant of medium = permittivity of a vacuum (F/m) = Boltzmann constant (J/K) = temperature (K) -1 (Debye length) has units of length Zeta Potential Effect of pH + + + + + + + + + + + + + + H+ OH- Shear Plane Low pH: Not enough OH- so there is a excess positive charge. Shear Plane + + + + + + + Shear Plane There is a point that there is enough OHand H+ to completely balance out the surface charge High pH: Not enough H+ so there is a excess negative charge. Zeta Potential Effect of pH (mV) + + + + + + + Isoelectric Point (IEP): pH at which = 0 pH + + + + + + + + + + + + + + + - Zeta Potential Effect of pH Question: What will happen to a mixed suspension of Alumina and Si3N4 particles in water at pH 4, 7 and Zeta 9? Potential -- Effect of Ionic Strength -50 40 30 20 10 0 -10 -20 -30 -40 -50 1 Increasing I.S. Zeta Potential (mV) Alumina 2 3 4 5 67 pH 8 9 10 11 Specific Adsorption Free energy decrease upon adsorption greater than predicted by electrostatics Have the ability to shift the isoelectric point Can reverse zeta potential Types of Specific Adsorption Ions Multivalent ions: Ca+2, Mg+2, La+3, hexametaphosphate, sodium silicate Self-assembling organic molecules: surfactants Polyelectrolytes + + + + + + +2 + + + + + + + + Specific Adsorption Ca2+ PO43- Multivalent cations shift IEP to right (calcite supernatant) Multivalent anions shift IEP to left (apatite supernatant) Amankonah and Somasundaran, Colloids and Surfaces, 15, 335 (1985). pH Electrokinetic Phenomena Electrophoresis Movement of particle in a stationary fluid by an applied electric field. Electroosmosis Movement of liquid past a surface by an applied electric field Streaming Potential Creation of an electric field as a liquid moves past a stationary charged surface Sedimentation Potential Creation of an electric field when a charged particle moves relative to stationary fluid Zeta Potential Measurements Electrophoresis determined by the rate of diffusion (electrophroretic mobility) of a charged particle in a applied DC electric field. PCS determined by diffusion of particles as measured by photon correlation spectroscopy (PCS) in applied field Acoustophoresis determined by the potential created by a particle vibrating in its double layer due to an acoustic wave Streaming Potential determined by measuring the potential created as a fluid moves past macroscopic surfaces or a porous plug Zeta Potential Measurements Electrophoresis Smoluchowski Folmula (1921) assumed a >> 1 = Debye length a = particle radius electrical double layer thickness much smaller than particle r 0 v= r 0 = v = velocity r = media dielectric constant 0 = permittivity of free space = zeta potential = medium viscosity E = electric field E = electrophoretic mobility Zeta Potential Measurements Electrophoresis Henry Formula (1931) expanded for arbitrary a assumed E field does not alter surface charge density ( o) - low 0 v = velocity r = media dielectric constant 2 r 0 f1 ( a ) v= 0 = permittivity of free space 3 = zeta potential 2 r 0 = medium viscosity = f1 ( a ) 3 E = electric field E = electrophoretic mobility Hunter, Foundations of Colloid Science, p. 560 Zeta Potential Measurements Streaming Potential = Debye parameter exp( Ze / 2 kT ) << 1 a = particle radius a = zeta potential r 0 E = p K E = Potential over capillary (V) r = media dielectric constant 0 = permittivity of free space (F/m) = medium viscosity (Pa s) KE= solution conductivity (S/m) p = pressure drop across capillary (Pa) Equipment for Measuring Zeta Potential Electrophoresis - Zeta Mark 21 particles ~ .10 microns- 300 m Equipment for Measuring Zeta Potential Electroacoustic - Colloidal Dynamics Acoustosizer powders/slurries < 10nm - up to 50 weight% Equipment for Measuring Zeta Potential Photocorrelation Spectroscopy - Brookhaven Zeta Plus dilute msuspensions <5microns light scattering technique Equipment for Measuring Zeta Potential Streaming Potential Paar Physica EKA granular particles, surfaces

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