Origins of cation-cation selectivity in crown ether-functionalized polymer membranes

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 overcoming selectivity limitations is to incorporate ligands into membranes which specifically interact with target cations in an aqueous environment. In this work, we assess how incorporating crown ethers, which form host-guest complexes with monovalent cations, into poly(norbornene) networks impacts the selective partitioning and diffusion of alkali cations. For the case of a 12-Crown-4-functionalized membrane, atomistic molecular dynamics simulations reveal a strong coupling between cation hydration and their sorption and diffusion in the membrane. More specifically, we observe that alkali cation complexation tends to increase with decreasing cation hydration free energy. This complexation is observed to generally enhance salt partitioning, but decrease salt diffusivity. To rationalize these observations, we develop an approximate model which is able to qualitatively explain the trends observed in simulations.