MEMBRANE CHARGE DISTRIBUTION OF CELLULOSE ACETATE MEMBRANE AROUND THE INTERFACE BETWEEN THE MEMBRANE AND BULK SOLUTION

Abstract

To obtain an information on the membrane charge around the interface, we measured the membrane potential and ζ-potential of cellulose acetate in aqueous NaCl solution at 34℃. The ζ-potential was negative but nearly equal to zero. The Donnan potential obtained from the analysis of the membrane potential was also negative, but the absolute magnitude was much larger than ζ-potential. As the results of analysis, it was found that the cellulose acetate membrane itself had no fixed charge and the effective membrane charge was due to the Cl^- ions adsorbed by the membrane. The saturated adsorption amount expressed in surface density and the adsorption coefficient within the membrane were the same as those at the surface. It was, therefore, concluded that even if the surface density of adsorbed ions within the membrane is as small as at the membrane surface, the membrane charge strongly affects the Donnan potential. This is because the Donnan potential was affected not by the charge density expressed in surface density but by the charge density in volume density. Within the membrane, the charge density in volume density becomes very large since the pore radius is extremely small, even if the adsorbed amount in surface density was small.