Electrophoresis and Other Electrokinetic Phenomena

There is a constant attraction to the South. . . yet the hampering effect of the southward attraction is quite sufficient to serve as a compass in most parts of our earth. 

The word electrokinetic implies the combined effects of motion and electrical phenomena. Specifically, our interest in this chapter centers on those processes in which a relative velocity exists between two parts of the electrical double layer. This may arise from the migration of a particle relative to the continuous phase that surrounds it. Alternatively, it could be the solution phase that moves relative to stationary walls. 

There are four phenomena that are normally grouped under the term electrokinetic phenomena. 

Electrophoresis. This refers to movement of a particle (and any material attached to the surface of the particle) relative to a stationary liquid under the influence of an applied electric field. 

Electroosmosis Here, the liquid (an electrolyte solution) moves past a charged surface (e.g., the surface of a capillary tube or through a porous plug) under the influence of an electric field. Thus, electroosmosis is the complement of electrophoresis. The pressure needed to balance the electroosmotic flow is known as electroosmotic pressure. 

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John L. Anderson (Co-Chair) is a University Professor of Chemical Engineering and is affihated with the Center for Complex Fluids Engineering at Carnegie Mellon University. He is also the dean of the College of Engineering. He received his B.S. from the University of Delaware and his Ph.D. from the University of Illinois. His research interests are membranes, colloidal science, electrophoresis and other electrokinetic phenomena, polymers at interfaces, and biomedical engineering. He is a former co-chair of the BCST and is a member of the National Academy of Engineering. 

The studies of electrophoresis and other electrokinetic phenomena as well as the investigation of ion exchange (Chapter III), have shown a strong influence of electrolyte composition on the structure of electrical double layer and intensity of electrokinetic phenomena. One may subdivide electrolytes capable of causing such an influence into the following groups [13] ... 

Hiemenz P, Rajagopalan R (1997) Electrophoresis and other electrokinetic phenomena. In Principles of colloid and surface chemistry. Vol. 3, Marcel Dekker, Inc., New York, pp. 534-574. 

Applications of nonlinear electrokinetic motion are still largely unexplored. Aperiodic electrophoresis and other ICEP phenomena could be used to separate polarizable particles... 

For well-dispersed colloid systems, particle electrophoresis has been the classic method of characterization with respect to electrostatic interactions. However, outside the colloidal realm, i.e., in the rest of the known world, the measurement of other electrokinetic phenomena must be used to characterize surfaces in this respect. The term electrokinetic refers to a number of effects induced by externally applied forces at a charged interface. These effects include electrophoresis, streaming potential, and electro-osmosis. 

The underlying physical mechanisms for the electrokinetic motion of particles are described in other entries on Electroosmotic How (DC), Electrophoresis, Dielectrophoresis, Nonlinear Electrokinetic Phenomena, and Electrokinetic Motion of Polarizable Particles, along with various mathematical models. The effects of relaxing the assumptions above in these models, however, are often unexpected and have not yet been firUy explored, either theoretically or experimentally. Here, we simply give a few examples of how heterogeneous particles can move in electric fields. 

In addition to the foregoing, it is customary to include under electrochemistry (I) processes for which the net reaction is physical transfer, e g., concentration cells (2) electrokinetic phenomena, e.g.. electrophoresis. eleclroosmnsis, and streaming potential (3) properties ot electrolytic solutions, if they are determined by electrochemical or other means, e g.. activity coefficients and hydrogen ion concentration (4) processes in which electrical energy is first converted to heal, which in turn causes a chemical reaction that would not occur spontaneously at ordinary temperature. The... 

Section 4.6 may be considered the prototype of modem electrokinetics, because all relevant features were covered the coupling of hydrodynamic and electric fluxes and double layer polarization. However, the elaboration remained restricted to electrophoresis, which is the most familiar electrokinetlc phenomenon. Other types of electrokinetics, summarized in table 4.1. basically require the same theory, although there may be considerable differences in the elaboration (what is stationary what is moving boundiuy condition , etc.). With sec. 4.6 we consider the fundamentals sufficiently explained and illustrated and we shall therefore not repeat and apply this theory to other electrokinetlc phenomena. Instead, two important extensions will now be briefly reviewed inclusion of double layer overlap, as occurs in plugs, in the present section and measurement in alternating fields in the following. 

To estimate x, the decrease in equilibrium adsorption and the actual adsorption rate according to the electrostatic phenomena, have to be considered. The application of Boltzmann s law assumes equilibrium condition of the DL and neglects any transport within the diffuse layer. Thus, the classic Boltzmann law cannot be used to describe the distribution of adsorbing ions within the double layer in non-equilibrium systems. The presence of any ionic flux is connected with a non-equilibrium state of the DL and the approach given by Overbeek (1943) in his theory of electrophoresis has to be considered. In that theory, the non-equilibrium of the DL causes non-linear dependencies of electrophoresis on the electrokinetic potential, in contrast to the theory of Smoluchowski where this effect is not allocated for. The importance of the non-equilibrium state of the DL for many other surface phenomena was emphasised by Dukhin Deijaguin (1974), Dukhin Shilov (1974), and Dukhin (1993). 

Electroosmosis is one of several electrokinetic effects that deal with phenomena associated with the relative motion of a charged solid and a solution. A related effect is the streaming potential that arises between two electrodes placed as in Figure 9.8.1 when a solution streams down the tube (essentially the inverse of the electroosmotic effect). Another is electrophoresis, where charged particles in a solution move in an electric field. These effects have been studied for a long time (37, 38). Electrophoresis is widely used for separations of proteins and DNA (gel electrophoresis) and many other substances (capillary electrophoresis). 

Electrochemical Remediation Heavy metals and other contaminants can be removed from the soil and groundwater with the help of electrokinetic phenomena (electroosmosis, electrophoresis, electrolysis). In electrochemical remediation processes, a continuous electrical field is generated with electrodes that are inserted into the contaminated soil (Shapiro etal. 1989 Ottosen etal. 1995 Hansen etal. 1997). Laboratory and pilot tests have been conducted, for example, with acetic acid as cleaning solution (Renaud 1990). With elec-... 

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