Abstract
A molecular-level equilibrium partition coefficient model has been formulated to describe single and multicomponent ion uptake by a Nafion 117 cation exchange membrane. The model presumes a cylindrical pore structure for the membrane and considers ion solvation free energy changes which occur during solute partitioning and the orientation of solvent dipoles inside a membrane pore due to the strong electric field generated by the membrane's fixed-charge groups. Membrane structure parameters (radius of a membrane pore and the pore wall density of fixed charge sites) which are needed to obtain a model solution are determined from experimental membrane porosities and X-ray diffraction data in the literature. Without the use of adjustable parameters, the model predicts to within 6% equilibrium cation concentrations in Nafion for a variety of aqueous single salt and binary and ternary alkali metal chloride mixtures at a total external salt concentration of between 0.1 and 1.0 M. A close inspection of the computed electrostatic and hydration forces acting on membrane-phase cations in a mixed salt system indicates that the univalent cation with the highest surface charge density (smallest hard sphere ion radius for like-charged ions) is excluded from the pore wall region in a manner qualitatively similar to that observed for coions.
Published Version
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