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 solvents composed of methanol, methanol/water, and acetonitrile/water. The 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, and ion solvation free energy effects were taken into account. In the present study, 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). The model accurately predicted competitive K+/Na+ uptake from methanol and methanol/water mixed solvents and Cs+/K+ uptake selectivities for acetonitrile/water solvents. In the latter case, model calculations suggested that some acetonitrile absorbed into the amorphous PTFE phase of Nafion, when the external acetonitrile concentration was greater than 33 vol%. The addition of water to methanol caused a drop in the potassium/sodium selectivity (to its pure water value), whereas the addition of acetonitrile to water caused the Cs+/K+ selectivity to decrease.