Interplay of polymer hydrophilicity and charge density on water and ion solubility in hydrated polymer membranes

Ion exchange membranes (IEMs) have diverse applications related to water purification (electrodialysis, desalination) and energy storage (flow batteries, fuel cells). However, fundamental understanding of how chemical structure governs the rates of water and ion transport in IEMs remains poorly understood. In this work, we synthesize a series of tunable methacrylate copolymers with varying charge density and hydrophilicity. We measure water vapor swelling and salt solubility and analyze our results using several thermodynamic models. While conventional theoretical models fail to capture quantitative sorption trends, we achieve partial data collapse by incorporating interactions analogous to the Pitzer model. Furthermore, we show that dielectric and Donnan exclusion effects can be decoupled through explicit control of charge density. These results advance our ability to predict water and ion partitioning in charged polymer membranes and inform the rational design of next-generation materials for energy and separation applications.