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Published byMatilda Sanders Modified over 9 years ago
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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
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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
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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.
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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
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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
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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
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Helmholtz-Smoluchowski Equation ( R S large)
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Helmholtz- Smoluchowski Equation valid for R S > 100
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Henry’s Equation
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Assumptions spherical particles low field conditions (D-H approximation) non-conducting particles double layer undistorted by flow (relaxation effect) + E + -
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Surface Charge and Zeta Potential Recall from our discussion of the Huckel limit that... Again… spherical particles low field limit
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Experimental Techniques Particle Electrophoresis
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Electro-osmotic effects can also cause difficulties in interpretation.
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