Dense and Highly Wrinkled Polyamide Nanofiltration Membrane via Synergistic PEI – TiO ₂ Interlayer for Enhanced Selectivity

To address the limited rejection efficiency of conventional strongly negatively charged nanofiltration (NF) membranes toward divalent cations, this study proposes a co‐deposition strategy using polyethyleneimine (PEI) and titanium dioxide (TiO 2 ). A positively charged PEI–TiO 2 interlayer was constructed on a poly(p‐phenylene terephthalamide) (PPTA) substrate to achieve high‐efficiency rejection of divalent cations, while simultaneously promoting the formation of a highly cross‐linked and structurally dense polyamide (PA) layer, thereby significantly enhancing the membrane's separation performance. The resulting PA/PEI–TiO 2 /PPTA membrane exhibited excellent permeance and solute rejection, with a Na 2 SO 4 rejection rate of 96.3%. Remarkably, the incorporation of the PEI–TiO 2 interlayer effectively modulated the structure of the PA layer, leading to a denser cross‐linked network. Consequently, the MgSO 4 rejection rate increased significantly from 77.2% to 90.9%, achieving effective synergistic rejection of both Na + and Mg 2+ . This work demonstrates the synergistic role of the PEI–TiO 2 composite interlayer in charge regulation and structural enhancement, providing a feasible strategy and theoretical foundation for designing NF membranes with stable multilayer structures and high rejection performance.