1,10-Diaza-18-crown-6-derived ion-recognition polyamide nanofiltration for efficient NaCl/Na2SO4 separation

Efficient separation of NaCl and Na 2 SO 4 is essential for resource recovery from hypersaline wastewater. Herein, a crown ether-functionalized polyamide nanofiltration membrane (denoted as DA18C6-PA) was fabricated via interfacial polymerization (IP) using 1,10-diaza-18-crown-6 (DA18C6) as the sole aqueous-phase amine monomer and trimesoyl chloride (TMC) as the organic-phase monomer. The IP conditions were systematically optimized, and the separation performance of the resulting membrane toward Cl − and SO 4 2− ions in mixed salt solutions containing Na + , Li + , K + , and Mg 2+ was comprehensively evaluated. Density functional theory (DFT) calculations were further performed to elucidate the conformational transformation of the aza-crown ether during IP and its correlation with cation recognition. Compared with the conventional PIP-PA membrane, the DA18C6-PA membrane exhibited a more compact and uniform selective layer with enhanced surface electronegativity, leading to improved water permeance and superior SO 4 2− rejection. More importantly, IP-induced conformational distortion of the aza-crown ether reduced the crown ether cavity size, thereby enhancing the geometric matching and coordination stability toward Na + over Li + , K + , and Mg 2+ . Benefiting from the favorable geometric and coordination environment provided by the distorted aza-crown ether cavity for Na + -specific recognition, the DA18C6-PA membrane achieved an exceptionally high NaCl/Na 2 SO 4 separation factor of 176.2. This work establishes a molecular design strategy for integrating host–guest recognition into polyamide nanofiltration membranes via conformational regulation of reactive aza-crown ethers, offering new mechanistic insights into precise ion-selective separation.