Impact of Relative Concentration of Sulfonate and Carboxylate Groups on Transport Properties in Ion-Exchange Membranes

Ion exchange membranes play a critical role in electrochemical energy storage and water purification technologies by mediating the selective transport of ionic solutes. Conventional cation exchange membranes, such as Nafion, typically feature a single type of fixed positive charge along the polymer backbone, which limits their tunability and ion selectivity. Introducing a second functional group with distinct acid-base or solvation characteristics provides an opportunity to tailor ion-membrane interactions and optimize transport properties. In this study, we investigate how varying the concentration of methacrylic acid comonomer in poly(3- sulfopropyl methacrylate) membranes influences ion transport and electrostatic environments. We systematically quantify counter-ion diffusion coefficients and ionic conductivities as functions of water volume fraction, revealing how local electrostatic interactions evolve with the relative abundance of sulfonate and carboxylate groups and with membrane hydration. Complementary molecular dynamics simulations provide molecular-level insight into the specific ion-ion and ion-membrane coordination environments responsible for the observed transport trends.