Oral: Effect of Zwitterion Architecture on Anion Selectivities in Zwitterion Grafted Nanopores

The separation of ions with similar charges is a critical challenge in applications such as water treatment and resource recovery. Membranes with cross-linked zwitterionic amphiphilic copolymer (ZAC-X) selective layers offer distinct permselectivity between monovalent anions like Cl^– and F^–. To elucidate the mechanisms behind this permselectivity, we conducted molecular dynamics simulations of salt solutions in zwitterion-functionalized nanopores. This study examines how the dipole orientation of zwitterionic ligands impacts ion diffusivity, partitioning, and permeability, comparing two distinct zwitterion configurations: Motif A (surface-cation–anion) and Motif B (surface-anion–cation). Our simulations show that Motif A exhibits reduced ion pairing due to spatial separation in the cation-anion profiles, while Motif B demonstrates stronger ion pairing for smaller anions, driven by their overlap with cation distributions. This results in lower ion diffusivities in Motif B for both cations and anions, influenced by increased ion pairing and steric hindrance. Additionally, we present potential of mean force calculations to explain the effects of anion partitioning in zwitterion-functionalized nanopores. Our results reveal that anion partitioning increases with anion size in both motifs and predominantly governs anion permeability, with Motif B exhibiting significantly higher permselectivity for larger anions compared to Motif A.