OSMOTIC PROPERTIES OF POLYELECTROLYTE MEMBRANES - POSITIVE AND NEGATIVE OSMOSIS

A method is proposed and tested experimentally to predict the concentr ation dependence of the osmotic properties of a given cation exchange membrane with an inhomogeneous structure which separates two aqueous s olutions of a single electrolyte. These properties are characterized b y the reflection coefficient sigma(s). The prediction of the concentra tion dependence of sigma(s) is based on a modified version of the ''mo del of the membrane with narrow pores.'' In its original form this mod el assumes a homogeneous structure of the membrane matrix. The influen ce of structural inhomogeneities is taken into account by using an eff ective fixed ion concentration X(eff) instead of the analytically dete rmined fixed ion concentration X(a) (X(eff) < X(a)) in the calculation of sigma(s). The value of X(eff) is obtained from independent experim ents. From measurements of tracer flow across the membranes used in th is study it is known that the value of the ratio of the effective diff usion coefficients (($) over tilde D--/($) over tilde D-+) in the memb rane phase has approximately the same value as the ratio of the diffus ion coefficients (D--/D-+)infinity in free solution at infinite diluti on. Using values of X(eff) and (D--/D-+)infinity, the theoretically pr edicted concentration dependence of a, for aqueous solutions of three 2, -1 valent electrolytes is in satisfactory agreement with the experi mentally determined concentration dependence. Positive and negative os mosis is predicted and found experimentally. (C) 1995 Academic Press, Inc.