Cation-polymer interactions and local heterogeneity govern the relative order of alkali cation diffusion coefficients in PEGDA hydrogels

Selective separation of monovalent cations from complex mixtures is an industrially relevant procedure necessary for the recovery of many commodity materials, such as lithium from salt brines. Unfortunately, most conventional membranes lack selectivity between monovalent ions, rendering their use in such applications infeasible. One approach to endowing membranes with ion-ion selectivity is to incorporate ion-polymer interactions into materials to bias the selective partitioning and or diffusivity of one species over another. However, little is known about the impact of such interactions on the mechanisms of ion transport. In this work, we probe the influence of cation-polymer interactions on cation, anion, and salt diffusivity in a model membrane material, poly(ethylene glycol) diacrylate (PEGDA). We do this by modeling concentrated poly(ethylene oxide) solutions via molecular dynamics simulations and compare the results to published experimental observations of LiCl, NaCl, and KCl diffusion in PEGDA. Experimentally, the order of salt and cation diffusion coefficients for LiCl, NaCl, and KCl are seen to deviate from the aqueous solution order. Molecular simulations attribute this deviation to cation-EO coordination in the membrane. Both the fraction of bound cations and the average lifetime of binding is observed to increase with decreasing hydration free energy of cations (moving down the alkali series), leading to different diffusivity trends in the membrane compared to solution. The experimentally observed diffusion order for cations and salt is recovered once membrane heterogeneity is explicitly included in our simulations. Our results reveal that cation-polymer interactions, as well as spatial heterogeneity within the membrane, play a critical role in dictating the order of alkali cation and salt diffusion coefficients in membranes.