Dynamic regulation of interfacial Polymerization: Mechanistic insights into the role of salt-responsive substrate in overcoming nanofiltration trade-off

Overcoming the trade-off between permeability and selectivity remains a key challenge in thin-film composite (TFC) nanofiltration (NF) membrane development. Herein, we propose an interfacial polymerization (IP) regulation strategy by grafting a salt-responsive zwitterionic material onto the substrate and using NaHCO₃ as the aqueous alkali. The grafted substrate exhibited higher adsorption capacity and stronger interaction toward the aqueous-phase monomer piperazine (PIP), enabling sustained but restricted PIP release during IP. Simultaneously, NaHCO₃ triggered a reduction in substrate pore volume and generated in-situ CO₂, whose enhanced interfacial accumulation further restricted PIP diffusion and pushed the diffusion-governed IP process further from equilibrium. This enhanced the PIP crosslinking degree, yielding a polyamide (PA) layer with reduced thickness, smaller pores, and higher surface charge density. Meanwhile, CO₂-induced ridge-and-valley structures enlarged the filtration area of the PA layer, and the formation of gutter structures at the PA-substrate interface eliminated resistance from pore misalignment. Consequently, the resulting membrane achieved simultaneous enhancements in permeability and selectivity, with long-term stable water permeance of 35.4 L m⁻² h⁻¹ bar⁻¹ and Na₂SO₄ rejection of 99.4 %. This work presents a novel strategy for dynamically regulating interfacial reactions using stimuli-responsive materials, providing new insights into membrane fabrication process design.