Chapter 6 - Electrophoresis and sedimentation potential

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This chapter intends to test linear phenomenological relations, to test the Onsager's reciprocal relation, and to examine the validity of conflicting theories of electrophoresis. Electrophoresis and sedimentation potential offer a test of predictions of thermodynamics of irreversible processes, provided these are supplemented by classical analysis of the data. The thermodynamics of irreversible processes may be helpful since the theory does not depend on any model. The kinetic theory is consistent with the thermodynamic theory. Experimental proof of Onsager relation would be obtained if the experiments on sedimentation potential and electrophoretic velocity are performed for the particles of the same size and shape. The experimental setup involves sedimentation column and optical assembly. Both sedimentation potential and electrophoretic velocity were measured with these setups. All the postulates of irreversible thermodynamics are found to be in agreement with experimental data over a wide range in the case of electro-kinetic phenomena (electro-osmosis, streaming potential, electrophoresis, and sedimentation potential). Similarly, thermo-osmosis provides support to the thermodynamic theory. However, a test of the Onsager reciprocity relation is difficult in this case. Thus, steady-state thermodynamics in the liner range for membrane phenomena (systems consisting two subsystems separated by a barrier) has a wide range of applicability. The only limitation is the range of validity of Gibbs equation. The limitation is observed in biological membranes, which are thinner and have much smaller pores. Nevertheless, even for such membranes, for getting an insight into the nature of physical processes in such systems, steady-state thermodynamics has proved to be a valuable guide.