(264d) Overcoming the Permeability-Selectivity Tradeoff Via the Design of Heterogeneous Pore Wall Chemistries

Developing next-generation membranes for water treatment demands system-specific materials with precisely tuned functionality. In particular, the purification and reuse of highly contaminated water calls for solute-specific designs where a key challenge is the separation of small neutral solutes. One such solute is boric acid, which is strictly regulated in water reuse for agricultural irrigation. Precise tuning of membrane pore chemistry requires a fundamental understanding of the mechanism by which functional moieties at the pore wall modulate the transport of water and solutes. Simulations enable the nanoscopic investigation of the mechanism by which a chemically heterogeneous pore modulates hydration water and thus its water-mediated interactions with solute molecules. Here, we use molecular dynamics simulations to investigate water and boric acid structure, dynamics, and transport in a cylindrical nanopore patterned with nonpolar methyl and polar hydroxyl groups. We find that changes in the arrangement of the methyl and hydroxyl groups, in addition to the density of each group, affect the transport of both water and boric acid in the pore. These changes not only affect the selectivity of water over boric acid, but also the permeability of water through the pore, leading to improvements in the permeability-selectivity behavior, a pernicious trade-off in membrane technology. We thus apply an optimization procedure based on a genetic algorithm to design the chemical patterning of the pore wall to simultaneously optimize water selectivity and permeability. The optimization identifies non-intuitive patterns that significantly hinder the transport of boric acid through the pore, simply by altering the functional group patterning. This inverse design procedure and the accompanying molecular-level understanding of the effect of chemical heterogeneity on solute selectivity in a pore will inform the design of more efficient water purification membranes, as well as the modulation of water-mediated surface-solute interactions in other systems.