Ion permeability of a membrane with soft polar interfaces .1. The hydrophobic layer as the rate-determining step

We have considered the ion transport across a membrane with extended soft permeable interfaces (polar zones), placed in an aqueous solution, under short-circuit conditions. We have taken into account the existence of fixed charges and dipoles in these membrane interphases. The membrane has been modeled as composed of three layers: an inner hydrophobic layer and two polar zones. Nernst-Planck's equation has been used for describing the ion transport, and Goldman's approximation has been assumed to be valid for the hydrophobic layer. In this paper the limiting case of the internal hydrophobic layer as the rate-controlling step for ion transport has been studied. The influence of the electrolyte concentration, the surface dipole density, and the thickness of the polar zone on the total ion flux and permselectivity has been analyzed. It has been shown that the permselectivity of such a system depends significantly on all these parameters. Particularly, the permselectivity of the same membrane changes with bulk electrolyte concentration. A general expression for cation selectivity has been obtained. The connection of the developed theory with experimental results has been discussed.