Dual-charged composite nanofiltration membrane enabled by negatively charged CS/SBE-β-CD interlayer for enhanced ion separation.

To improve the permeability-selectivity balance in polyamide nanofiltration membranes, a dual-charged composite membrane (PES-CS/SBE-β-CD-PA) was fabricated via interfacial polymerization on a self-assembled CS/SBE-β-CD interlayer designed to regulate the interfacial availability and transport of PEI monomers during polyamide formation, yielding a selective layer with a lower apparent cross-sectional thickness (approximately 173 nm) and more uniform morphology. The negative potential of the interlayer-terminated surface before IP (-9.8 mV), the positive potential of the final PA surface (15.63 mV), and the sequential fabrication process support the designed negative-interlayer/positive-PA configuration. The optimized membrane exhibits a molecular weight cut-off of 445 Da, an average pore size of 0.51 nm, and MgCl2 and MgSO4 rejections of 94.00% and 91.34%, respectively, while maintaining a water permeance of 15.20 L m-2·h-1·bar-1. The morphology, pore-size, and charge results support a separation mechanism combining steric/hydration effects with charge-dependent Donnan partitioning. In addition, the membrane shows improved antifouling properties and stable separation performance during 72 h continuous filtration under the tested conditions. This work presents an effective interfacial-regulation strategy for constructing nanofiltration membranes with balanced separation and antifouling performance.