Understanding the high Cl^-/ F^- permselectivity of random zwitterionic amphiphilic copolymer (r-ZAC) membranes

Random zwitterionic amphiphilic copolymer (r-ZAC) membranes have shown tremendous promise for facilitating ionic separation processes by virtue of their tunable morphology. Recent experiments reveal exceptionally high NaCl/NaF permselectivity for r-ZAC membranes with sub-nanometric zwitterionic pores. This is a result of the simultaneously higher solubility and diffusivity of NaCl (as compared to NaF) within these membranes, thereby circumventing the solubility-diffusivity tradeoff. We conducted molecular dynamics simulations to probe the transport of NaCl and NaF in r-ZAC membranes with varying pore morphologies (achieved by varying the water volume fraction). Our results indicate that the lower self-diffusivity of F^- ions (relative to Cl^- ions) within r-ZAC membranes stems from a higher dielectric drag acting on the F^- ions. The reduction in dielectric constant of water in conjunction with the sluggish re-orientational dynamics of water dipoles exacerbates the impact of dielectric drag in dictating ionic diffusivity within the membrane. Reduction of the water volume fraction leads to nonmonotonic variation in the Cl^-/ F^- diffusivity selectivity due to a complex interplay between the Stokes drag, dielectric drag, membrane-ion interactions and morphology-induced nanoconfinement.