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Electrokinetic Theory Study of moving charged particles and their interactions with electric fields. 4 general types of phenomenon Electrophoresis applying.

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Presentation on theme: "Electrokinetic Theory Study of moving charged particles and their interactions with electric fields. 4 general types of phenomenon Electrophoresis applying."— Presentation transcript:

1 Electrokinetic Theory Study of moving charged particles and their interactions with electric fields. 4 general types of phenomenon Electrophoresis applying a field moves a charged particle Sedimentation potential opposite effect: moving particles create the field

2 Electro-osmosis movement of a liquid (containing aqueous ions) with respect to a charged surface in a field. Streaming Potential opposite effect: the moving liquid now creates the field

3 What can we measure? Mobility (u): how fast the particle moves (v) in an applied field, E. u=v/E Need to relate this to properties of the colloidal particles: charge, surface potential, and size. For a point particle q=ze For a colloid though, a large particle does not act like a point particle, so we must consider the surface potential.

4 Zeta Potential Potential at the surface of shear between charged surface and the liquid layer We assume that  and the Stern Potential   are approximately the same static moving

5 No longer are we concerned with a simple surface either: we assume that the charged particles are spheres  R s small Huckel limit  R s large Helmholtz- Smoluchowski limit

6 Huckel Limit (  R S small) (see Hiemenz and Rajagopalan) Want a relationship between q and  A general expression for the electric field is given by Coulomb’s law:  Since... Huckel Equation valid for  R S < 0.1 s ss ss R4 q )R1(R4 q )/1R(4 q R4 q       

7 Helmholtz-Smoluchowski Equation (  R S large)

8 Helmholtz- Smoluchowski Equation valid for  R S > 100

9 Henry’s Equation

10 Assumptions spherical particles low field conditions (D-H approximation) non-conducting particles double layer undistorted by flow (relaxation effect) + E + -

11

12 Surface Charge and Zeta Potential Recall from our discussion of the Huckel limit that... Again… spherical particles low field limit

13 Experimental Techniques Particle Electrophoresis

14 Electro-osmotic effects can also cause difficulties in interpretation.


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