Selective ionic transport in zwitterion-functionalized nanopores*

We investigate ionic transport in zwitterion-functionalized nanopores to illuminate the physics underlying selective salt transport in random zwitterionic amphiphilic copolymer (r-ZAC) membranes. By conducting molecular dynamics simulations, we probe the transport of various sodium halides in zwitterion-functionalized nanopores as a function of the (i) pore radius, (ii) zwitterionic chemistry, (iii) grafting density of the zwitterions, and (iv) orientation of the zwitterionic dipoles. The variation in density of water molecules within the nanopore with respect to the abovementioned parameters is accounted for in our work by allowing the pores to attain chemical equilibrium with two external reservoirs containing bulk water. Our results reveal the intriguing possibility for a complete reversal of the anionic diffusivity trends within the zwitterion-functionalized nanopores as compared to salt-in-water solutions at equivalent concentrations. By highlighting the key selectivity motifs underlying salt transport in zwitterion-functionalized nanopores, this study will help unravel the principles for designing novel r-ZAC membranes with exceptional ionic separation characteristics.