Abstract
We consider a simple model to evaluate mean activity coefficients in charged polymeric membranes with an inhomogeneous distribution of the fixed charge concentration on the macroscopic spatial scale, and apply the results obtained to the coion exclusion characteristic of the Donnan equilibrium. Model calculations with two fixed charge distributions of experimental interest (the asymmetric and the skin-core distributions) show that the inhomo-geneity effects can be important when the external salt solution concentration is lower than the membrane-fixed charge concentration, especially if spatial regions of low charge concentration exist within the membrane. The results obtained appear to be in qualitative agreement with previous experiments.
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