Electrokinetic Phenomena

a group of effects that are observed in disperse systems and in capillaries and that entail either the movement of one phase relative to another under the influence of an applied electric field or the production of a potential difference in the direction of the relative movement of phases that is due to mechanical forces. Electroosmosis and electrophoresis are the electrokinetic phenomena of the first type; the production of a sedimentation potential, or the Dorn effect, and the production of a streaming potential are the electrokinetic phenomena of the second type.

Electrokinetic phenomena result from the existence, at a phase boundary, of excess charges that are grouped as two oppositely charged layers called an electrical double layer. An electric field applied in the direction parallel to a phase boundary causes the displacement of an ionic layer relative to another, resulting in the relative displacement of the phases, that is, in electroosmosis or electrophoresis. Effects that are the converse of electroosmosis or electrophoresis are similarly observed in the flow of a liquid or in the sedimentation of particles from a disperse phase. Such effects entail the relative movement of ionic layers or the spatial separation of charges in the direction in which the phases move. In other words, they entail the production of a streaming potential or the production of a sedimentation potential, respectively.

Any electrokinetic phenomenon may be used to determine the zeta potential, or electrokinetic potential, which is denoted ζ. In such a determination, the surface conductivity caused by the freely movable charges of an electrical double layer is assumed to be greater than the volume conductivity of a system.

The theory of electrokinetic phenomena, which was developed by M. Smoluchowski in 1903, establishes a linear relationship between the quantitative characteristics of such a phenomenon and those of the applied electric field. However, the theory does not take into account the departure of a double layer from equilibrium or the production of an induced dipole moment in dispersed particles. To take these effects into account, electrokinetic phenomena must be studied in conjunction with other electrical surface effects.