Confinement-Induced Dielectric Behavior of Water in Polymeric Nanofiltration Membranes: A Molecular Dynamics Study

Water confined within nanofiltration (NF) membrane pores experiences a dielectric environment that differs substantially from that of bulk water, and this pore-scale dielectric constant is a key parameter in NF transport models such as the Donnan and steric partitioning model with dielectric exclusion (DSPM-DE). However, real polyamide NF membranes possess highly irregular and tortuous pore structures, and the dielectric properties of water under such realistic confinement remain poorly quantified. In this study, molecular dynamics simulations were performed using five polyamide models based on piperazine-TMC and diethylenetriamine-TMC to clarify how nanoscale pore morphology influences the dielectric response of confined water. In all models, confinement significantly reduced the dielectric constant relative to bulk water due to suppressed dipole moment fluctuations and hindered rotational mobility. Analysis of directional dielectric components and orientational distributions revealed that pore geometry and connectivity, rather than polymer hydrophilicity, predominantly govern the dielectric behavior. These findings provide physically grounded dielectric parameters for improving NF transport models and offer molecular-level insight into polarization phenomena within realistic polymeric membrane pores.