Electrokinetics of colloids

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Slide 1: 

Electro-kinetics of Colloids

Slide 2: 

DEFINITION OF A COLLOID Ostwald: “die Welt der vernachlässigten Dimensionen” Chem E 455: Surface and Colloid Science Laboratory: Autumn and Spring Quarters

Slide 3: 

LYOPHOBIC COLLOIDS

Slide 4: 

INSTABILITY OF LYOPHOBIC COLLOIDS

Slide 5: 

LONG-RANGE VAN DER WAALS ATTRACTION s0 a

Slide 6: 

ELECTROSTATIC REPULSION RETARDS AGGREGATION van der Waals attraction

Slide 7: 

ELECTRICAL CHARGE AT AQUEOUS INTERFACES

Slide 8: 

ELECTRICAL DOUBLE LAYER

Slide 9: 

THE GUOY-CHAPMAN DOUBLE LAYER

Slide 10: 

Poisson equation (describes variation of potential in a spatial charge distribution) Boltzmann equation (describes variation of ion concentration in an electric field)

Slide 11: 

For a single, symmetric (z-z) electrolyte… for low potentials |z?0| = 25 mV, sinh y ˜ y, and “Debye-Hückel approximation”

Slide 12: 

ION CONCENTRATION PROFILES “Double layer thickness”

Slide 13: 

EFFECT OF ELECTROLYTE CONCENTRATION ON DOUBLE LAYER THICKNESS

Slide 14: 

REFINEMENTS OF DOUBLE LAYER MODEL

Slide 16: 

STERN MODEL (1924) ? ?? ?0 ?(x) Surface potential Stern potential Stern layer Stern layer thickness

Slide 17: 

STRONG SPECIFIC ADSORPTION IN THE STERN LAYER

Slide 18: 

ELECTROSTATIC REPULSION DEPENDS ON…. The Stern potential, ?? The thickness of the diffuse double layer, ?-1

Slide 19: 

Forces between particles (surfaces) s When (?/kT) ? 10, ? “stability” Derjaguin Landau Verwey Overbeek

Slide 20: 

Controlling DLVO interactions

Slide 21: 

Controlling electrolyte valence and concentration For z =2, divide all C’s by 64; for z =3, divide all C’s by 729

Slide 22: 

a = 0.1mm; T=298K; A212=10-19J; k-1=33Å; e=78.5 Controlling effective Stern potential, yd

Slide 23: 

HOW CAN WE MEASURE THE STERN POTENTIAL?

Slide 24: 

ELECTROKINETIC EFFECTS Electro-osmosis V (electro-osmotic velocity)

Slide 25: 

ELECTROPHORESIS Vp = electrophoretic velocity Relevant to colloidal dispersions

Slide 26: 

I. Externally imposed EMF produces motion 1. Liquid moves; solid is stationary 2. Solid moves; liquid is stationary II. Externally imposed motion produces EMF 1. Liquid forced to move past solid 2. Solid forced to move through liquid THE ELECTROKINETIC EFFECTS

Slide 27: 

At steady state: ?E = streaming potential i = streaming current

Slide 28: 

I. Externally imposed EMF produces motion 1. Liquid moves; solid is stationary 2. Solid moves; liquid is stationary II. Externally imposed motion produces EMF 1. Liquid forced to move past solid 2. Solid forced to move through liquid THE ELECTROKINETIC EFFECTS

Slide 29: 

“Dorn Effect”

Slide 30: 

Electro-osmosis

Slide 31: 

Integrating once:

Slide 32: 

Measurement of electro-osmotic velocity Typically, V ˜ a few mm/min

Slide 33: 

INTERPRETATION OF THE ZETA POTENTIAL

Slide 34: 

ELECTROPHORESIS Vp = electrophoretic velocity

Slide 35: 

MEASURING ? BY MICRO-ELECROPHORESIS Electrophoretic mobility

Slide 36: 

The measurement of electrophoretic mobility Rank Brothers Mark II Microelectrophoresis System FLOW CELL +

Slide 37: 

and vx = 0 at y = 0.2113w and 0.7887w

Slide 38: 

RELATING MEASURED uE TO ZETA POTENTIAL When double layer is thin relative to particle radius: ?a > 300 a ?-1 When double layer is thick relative to particle radius: ?a < 0.1

Slide 39: 

Spherical particles; 1-1 electrolyte Hückel Smoluchowski

Slide 40: 

Polyvalent electrolytes C(?a) ?a

Slide 41: 

Some ion properties* * Moelwyn-Hughes, E.A., Physical Chemistry, 2nd Ed., p. 859, Pergamon Pr., Oxford, 1961.

Slide 42: 

Approximate solution for symmetric (z-z) electrolytes* *O’Brien, R.W., and Hunter, R.J., Can. J. Chem., 59, 1878 (1981). Good to order (?a)-1

Slide 43: 

Aqueous KNO3 solutions, 25°C

Slide 44: 

Pen Kem Model 501 Lazer Zee Meter™

Slide 45: 

ZETA POTENTIAL AS A QC VARIABLE FOR WASTEWATER TREATMENT

Slide 46: 

MORE MODERN METHODS FOR MEASURING ELECTROPHORESIS Based on light scattering techniques

Slide 47: 

Quasi-elastic light scattering (QELS) Doppler shift in frequency (line broadening) of scattered light due to particle motion A = instrument constant D = particle diffusivity Q = magnitude of the scattering vector Measure ?w ? D ? particle size But ?w ˜ 103 Hz and w0 ˜ 1016 Hz

Slide 48: 

Heterodyning

Slide 49: 

Electrophoretic quasi-elastic light scattering Shifted autocorrelation function

Slide 50: 

Photon Correlation Spectroscopy (PCS) Intensity pair autocorrelation function homodyning Electrophoretic PCS

Slide 51: 

Laser Doppler Electrophoresis (LDE) + _

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