(699f) Solvent Effects During Multicomponent Ion Uptake Into a Nafion® Cation-Exchange Membrane

A multicomponent ion partition coefficient model has been used to predict competitive alkali metal ion uptake into a Nafion® cation-exchange membrane from dilute external salt solutions, where the solvent was either methanol, water, methanol/water mixtures, or acetonitrile/water mixtures. The space-charge analysis was similar to that used previously for monovalent and divalent uptake and transport in Nafion, where the membrane was modeled as an array of parallel cylindrical pores of constant radius with a continuous distribution of fixed charges along the pore wall. Ion-ion electrostatic interactions, electric-field induced solvent dipole alignment in a membrane pore, and ion solvation free energy effects were considered. For mixed solvents, the model was modified to account for a different solvent composition in the membrane, as compared to that in the external solution (solvent uptake data were collected experimentally). Mass fraction mixing rules were used for solvent parameters in the model.

Without the use of adjustable parameters, the model accurately predicted selective K+/Na+ uptake from methanol and methanol/water mixed solvents and Cs+/K+ uptake for acetonitrile/water solvents. In the latter case, model calculations suggested that some acetonitrile absorbed into the amorphous hydrophobic phase of Nafion, when the external acetonitrile/water solvent contained more than 33 vol% acetonitrile. Experimental and computational results showed a marked difference in the K+/Na+ selectivity dependence on solvent composition for the methanol/water system, as compared to that for Cs+/K+ in acetonitrile/water solvents. K+/Na+ equilibrium sorption selectivities in water and methanol/water solvents were essentially identical, with a substantial increase in the selectivity coefficient when the solvent was neat methanol. The Cs+/K+ uptake behavior was different; the selectivity was constant for all acetonitrile/water mixtures, but increased when the solvent was pure water.