Specific Ion Selectivity in Sulfonated Polystyrene Membranes Near the Percolation Threshold

Engineering selective transport between ions of the same valence is critical for separations in water, energy, and the environment, but remains technically challenging for ion-exchange membranes (IEMs). In this study, we hypothesize that cation exchange membranes with sufficiently water-poor matrices can leverage differences in the ion dehydration free energy, −Δ G _hyd , to enhance selectivity between monovalent alkali metal cations. Thin films of sulfonated polystyrene random copolymers with controlled sulfonation levels were fabricated to tune membrane hydration. Mixed-cation electrodialysis experiments showed increased selectivity for K⁺ over Li⁺ as membrane hydration decreased, while mobility ratios remained unchanged—indicating that preferential sorption of K⁺, with lower −Δ G _hyd , governs the preferential transport. Selectivities up to 3.1 were achieved, surpassing commercial IEM benchmarks. However, excessive dehydration below a percolation threshold led to loss of ion transport due to disruption of ion cluster connectivity. These findings provide key structure-property insights into partial ion dehydration and hydration-dependent percolation, informing new design strategies for IEMs with enhanced specific ion selectivity.