Electrolyte permeability and electromotive action of mosaic membranes composed of porous and liquid parts of high ionic selectivity for ions of opposite signs

Summary

A theory is presented of the electromotive and ion permeability properties of membranes which consist of a mosaic of highly ion selectiveporous membrane areas of ion exchanger nature and of areas of highly ion selectiveliquid ion exchanger membranes, the two types of areas being exclusively permeable to ions of opposite sign. It is demonstrated that, with properly chosen membranes, the preferential permeability of such porous-liquid mosaic membranes for ions of one sign of charge will be the opposite of that apparently indicated by their electromotive action.

The theory is based on the fact that the movement of ions across the porous membranes occurs in the dissociated state and in most instances is quantitatively linked to the resistance according to the Nernst-Einstein equation. The penetration of ions across liquid ion exchanger membranes, however, takes place essentially in a nondissociated state, and, as determined by self-exchange studies with radioactive tracers and stirred membranes, occurs at rates far in excess of those across porous membranes of the same resistance.

For the theoretical treatment the simplest case, two-membrane macro-model concentration cells, is discussed in detail. Qualitatively, it is evident that the ratio of the permeability of anions and cations across such porous-liquid mosaic membranes ordinarily will be strongly in favor of the ions which penetrate across its liquid parts; contrariwise, the electromotive actions of the mosaic membranes ordinarily are dominated by its porous parts.

Electric currents flow through all mosaic membranes; the strenghth of the current in a model cell may be calculated from the concentration potentials arising separately across the two membranes, and the resistances of the membranes and of the two solutions. From the strength of the current, the sign and the magnitude of the concentration potential arising in the model cell may be computed; in many instances it should closely approach the concentration potential across the porous membrane.

For the test of this theory with two-membrane macro-mosaic models, the electrolyte of choice for experimental reasons was RbSCN, tagged with⁸⁶Rb⁺ and S¹⁴CN⁻. The porous membranes were polystyrene sulfonic acid collodion matrix membranes; the liquid membranes consisted of 0.02m trioctyl propyl ammonium thiocyanate in 1-decanol. The ratios of the permeabilities across the model mosaic membranes determined by conventional rate of self-exchange measurements showed, as expected, that the permeability of the SCN⁻ ions is larger, up to 3600 times larger, than that of the Rb⁺ ions. The potentials arising in these models agreed within the limits of experimental error with those predicted by theory, closely approaching that arising at the cation selective porous membranes.