Investigating the impact of ionic correlations on selective salt transport in ligand-functionalized polymer membranes*

Functionalizing polymer membranes with ligands can be used as a method for imparting host-guest interactions into the membrane, thereby enabling selective transport of a particular ionic species over others. Prior experimental/computational studies on membrane transport have assumed an ideal salt transport scenario by neglecting any correlations between the constituent ions. However, ionic correlations can have a profound impact on salt diffusivity within the membranes, especially at high salt concentrations. We conduct coarse-grained molecular dynamics (CGMD) simulations of salt transport in ligand-functionalized polymer membranes for both single and mixed salt systems (binary salt mixture sharing a common anion). We specifically consider ligands that can bind to cations, and conduct a parametric study by systematically varying the ligand-cation interaction strength across our simulations. Our results indicate that ionic correlations speed up salt transport at high cation-ligand interaction strengths by promoting correlated hopping of the cations between the ligands. Furthermore, mixed salt systems exhibit much more selective salt transport than their single salt counterparts until a critical value of the cation-ligand interaction strength.