Mechanisms underlying Organic Solute Selectivity of Zwitterionic Ligand Functionalized Polymer Membranes

Chemical selective separation of small molecules is a key challenge in applications such as lignin monomer separation in biorefineries. We recently found that membranes with zwitterionic amphiphilic copolymer (ZAC) selective layers, featuring zwitterion-lined nanodomains, exhibit distinctive permeation trends for glucose, sucrose, and riboflavin depending on the zwitterionic monomer type (i.e., styrene vs. methacrylate). To better understand the mechanisms governing transport and selectivity in these materials, we conducted molecular dynamics simulations of solute solutions in zwitterion-functionalized nanopores. Specifically, we examined how differences in the functional groups near the pore surface influence solute diffusivity and partitioning by comparing two ligand configurations: surface-ester–zwitterion (SBMA) and surface-phenylene–zwitterion (SBMS). Our results revealed that SBMA exhibits higher solute diffusivity than SBMS, consistent with experimental selectivity trend for SBMA, while SBMS shows stronger solute partitioning, aligning with the experimental trend for SBMS. Radial distribution and spatial density analyses suggested that the rigid phenylene group in SBMS weakens the electrostatic interactions between the cationic and anionic functional groups of neighboring SBMS, thereby enhancing solute–zwitterion interactions. Such interactions shift solute distributions toward the pore wall, resulting in slower diffusion but improved partitioning.