Impact of dynamical correlations on salt transport in solvated ion exchange membranes

Ion exchange membranes are widely used in water purification and energy storage applications to selectively and efficiently regulate salt transport, yet the influence of dynamical ion-ion correlations and ion pairing/condensation on salt transport in charged polymer membranes remain poorly understood. In this study, we use the framework of Onsager transport coefficients within atomistic molecular dynamics simulations to study the impact of ion-ion correlated motion on salt transport in experimentally relevant hydrated polymer membranes. At sufficiently high salt concentrations, cation-anion dynamical correlations exert a significant influence on both salt diffusivities and conductivities. Anion-anion distinct correlations, arising from the imbalance between the concentration of free (mobile) cations and anions, and the retarding effect of the fixed charge groups on cations, proves to be an additional important feature for charged membranes. Our results demonstrate that dynamical correlations should become an important consideration in experimental measurements of salt diffusivities and conductivities for non-dilute salt solutions in polymer membranes.