The impact of ion-ion correlated motion on salt transport in solvated ion exchange membranes

Ion exchange membranes are widely used in water purification and energy generation applications to selectively and efficiently regulate salt transport, yet the influence of non-ideal ion-ion correlated motion on salt transport in these systems is often neglected and remains poorly understood. In this work, we study the impact of ion-ion correlations on salt transport in hydrated polystyrene sulfonate membranes and aqueous salt solutions via atomistic molecular dynamics simulations. Our results suggest that in both systems, cation-anion motion is positively correlated and increases significantly with doped salt concentration due to increased ion pairing. Similarly, the motion of distinct anions is found to be positively correlated and increase with salt concentration in the membrane system, but is negligible in the aqueous salt solution systems. Further, the anion diffusivities are lower than the cation diffusivities in the membrane, opposite to the trend in aqueous solution. We attribute these observations to the imbalance between the concentration of free (mobile) cations and anions in the membrane system.