Electroosmosis

ELECTROOSMOSIS, electric transfer of liquid through a porous membrane. E., similar to cataphoresis (see), belongs to electrokinetic phenomena that arise when a heterogeneous system is placed in an electric field. The electrokinetic potential (see), which usually occurs at the boundary surface of such heterogeneous systems, depends on the excess of ions of the same sign in the thin layer of liquid wetting the solid surface, and on a similar excess of oppositely charged ions in the freely mobile liquid layer. Due to the presence of such an electrical double layer (see), the solid body and liquid are attracted to opposite electrical poles. If the solid body is fixed immovably in the form of a solid porous membrane, then movement of liquid is observed in the electric field, which has been named E. The quantitative theory of E. was first given by Helmholtz and then thoroughly developed by Lamb, Smoluchowski, and Perrin. The simplest dependencies are obtained when studying E. through a thin cylindrical capillary. However, in practice, instead of a capillary, porous membranes (from unburnt clay, pressed powders, etc.) are usually used for E., the pores of which act as a collection of the smallest capillaries. If the total cross-sectional area of all pores of the membrane equals q, then the speed of E. (i.e., the amount of liquid transferred per unit time) v = ζηH/Dη where ζ is the electrokinetic potential, H is the voltage of the external field (i.e., the potential difference across both sides of the porous wall divided by its thickness), D is the dielectric constant and η is the viscosity of the liquid. Thus, the volume of electroosmotically transferred liquid is directly proportional to the total cross-sectional area of the pores of the membrane, the ζ-potential of its walls, and the voltage of the external electric field. In most cases, water is charged positively relative to the solid bodies present in it and therefore is transferred to the cathode in an electric field. By adding hydrogen ions (acids) or multivalent cations (salts of aluminum, lanthanum, thorium, etc.), it is possible to give a positive potential to the solid body and reverse the direction of E. of water.

D. Rubinshtein. 12