Poster: Effect of Zwitterion Architecture on Cation Selectivities in Zwitterion Grafted Nanopores

Ion separation is a critical challenge in applications such as water purification and energy recovery, where efficient cation selectivity is essential. Cross-linked zwitterionic amphiphilic copolymer (ZAC-X) membranes have garnered attention for their exceptional ion permselectivity, which is attributed to zwitterion-lined nanodomains. The sulfonate functional groups in ZAC-X are expected to enhance cation permselectivity by promoting specific interactions with various cations. In this study, we employed molecular dynamics simulations to investigate the transport properties of salt solutions in zwitterion-functionalized nanopores, focusing on how the dipole orientation of zwitterionic ligands influences cation diffusivity and permeability. Two ligand orientations were examined: Motif A (surface-cation–anion) and Motif B (surface-anion–cation). Our findings show that in Motif A, interactions between sulfonate groups and cations increase as the hydrated radius decreases from Li⁺ to K⁺, whereas these interactions weaken for larger cations such as Rb⁺ and Cs⁺. The overall diffusivity in Motif A reflects the combined effects of cation interactions with sulfonate groups and water molecules. In contrast, Motif B exhibits stronger ion pairing for all cations due to the overlap with anion distributions, leading to reduced diffusivity. Furthermore, we correlate these simulation results with experimental data by presenting cation flux measurements from non-equilibrium simulations.