Orientation dependent concentration potentials of asymmetrical cellulose acetate membranes

Abstract

Orientation dependent (asymmetrical) membrane potentials were observed on both reverse osmosis and ultrafiltration cellulose acetate and polymer sulfone membranes in contact with electrolyte solutions. It was shown that this phenomenon can be observed only on asymmetrical membranes with a gradient of fixed charge molality in the active layer (skin), provided that the activity of the more concentrated solution is comparable with or higher than the fixed charge molality in the active layer. This holds also for partly hydrolyzed reverse osmosis membranes. The origin of the orientation dependence of the membrane potential can be explained by means of an analysis of Donnan potentials in given conditions. Diffusion potentials cannot play the decisive role. The asymmetry of osmotic fluxes observed only on reverse osmosis (annealed) membranes does not contribute to the orientation dependence of the membrane potential, because the asymmetrical dilution effect is compensated by the arising streaming potential.