Breaking water-structure constraints by Hofmeister effects in interfacial polymerization for polyamide membranes

Hydration critically governs the physicochemical behavior of solutes in both the aqueous bulk and interfacial environments. As typical chaotropic Hofmeister ions, weakly hydrated anions (WHAs) can reshape hydration structures and accumulate at water–hydrophobe interfaces. Notably, WHAs paradoxically suppress the solubility of certain small-molecule monomers in water while enhancing that of chemically analogous polymers. Here, we exploit WHA-mediated Hofmeister hydration control to regulate polyamide-forming interfacial polymerization (IP), using the piperazine (PIP) and trimesoyl chloride (TMC) nanofiltration system as the primary model. Introducing WHAs into the aqueous phase reduces the affinity of amine monomers for water and accelerates partitioning into the reaction front in the oil phase, remarkably reinforcing IP self-limitation. Simultaneously, interfacial WHA enrichment lowers interfacial tension and, together with accelerated amine delivery, potentially intensifies transport-driven interfacial instabilities. WHAs also lower interfacial water availability, attenuating acyl chloride hydrolysis. These synergistic effects yield hollow-ridged polyamide selective layers with reduced thickness and high crosslinking density, delivering enhanced water permeance and salt rejection. Crucially, module-scale fabrication of high-performance membranes demonstrates that WHA-mediated Hofmeister hydration control is directly compatible with established desalination pipelines.