Membrane potential and permeation of salts across bipolar membranes

Abstract

This paper describes a theoretical consideration of membrane potential and permeabilities of uni-univalent salts in multi-lamellar ion exchange membranes. Model calculations are performed for bipolar membranes consisting of juxtaposed cation and anion exchange membranes. Donnan potential is predicted to be non-zero when the concentrations of external solution on the left- and right-hand sides of the bipolar membrane are equal, while this potential is estimated to be zero from TMS theory for single ion exchange membranes. Membrane potentials in both cases are, however, found to be zero. This is ascribed to the fact that the Donnan potential and the inter-Donnan potential, which is generated at the interfaces of anion and cation exchange layers in bipolar membranes, compensate each other. Concentration profiles and membrane potential profile in the bipolar membrane are calculated. The cation and anion concentration profiles are found to be approximately linear functions of distance from the surface of the membrane. The membrane potential profile, on the other hand, is convex. Permeability differences due to the direction of flux in bipolar membranes are also discussed in this study.