Realizing precise ion separation in polyamide nanofiltration membranes via stage control reactions

Thin-film composite nanofiltration membranes face a trade-off phenomenon between ion selectivity and permeability due to the structural constraints of single monomers during interfacial polymerization (IP). Inspired by homogeneous precipitation, we decouple disorderly competitive reactions of comonomers through using in situ–generated H⁺ during ultrafast IP processes as equilibrium-shifting inducers for the enamine reaction, thereby regulating the reaction sequence and relative amount of primary/secondary amine monomers. Combining the structural advantage of polyethyleneimine and piperazine monomers, the separation layer had a large free volume, high-density homogeneity, well-tuned nanopores, and tailored charge distribution. The staged-regulated membrane exhibited high water permeance and could even adapt to ion separation in ultrahigh-salt solutions (Mg/Li = 50, 20,000 parts per million), with a notable Mg²⁺/Li⁺ selectivity improvement of more than 1600% over that of the control, directly mixed dual aqueous monomer-prepared membrane. This stage control strategy for precise nanofiltration membrane synthesis can provide extensive flexibility in modulating the IP process for application-specific membrane structure design.