In Situ Constructed Cu-IL Complex Interlayers for High-Permeance Nanofiltration Membranes with Enhanced Sulfate Separation

High-permeance nanofiltration (NF) membranes with efficient sulfate separation are highly desirable for water purification and ion separation, but their performance is often constrained by the insufficient control of polyamide (PA) formation during interfacial polymerization. Herein, a copper–ionic liquid complex interlayer was constructed in situ on a polysulfone substrate through a coordination-assisted assembly strategy to regulate PA formation and improve NF separation performance. The introduction of the Cu-IL interlayer altered the interfacial polymerization environment, leading to the formation of a thinner and smoother PA selective layer with a modified surface elemental composition. Compared with the pristine NF membrane, the optimized NF/Cu-IL-2 membrane exhibited a reduced PA-layer thickness from 114 to 75   nm, a decreased surface roughness from 9.1 to 7.1   nm, and a lower O/N ratio from 1.22 to 1.13. Benefiting from the reduced transport resistance and improved water affinity, NF/Cu-IL-2 achieved a water permeance of 15.8   L   m -2 h -1 bar -1 , which was 85% higher than that of the pristine NF membrane, while maintaining a Na 2 SO 4 rejection above 97%. Notably, the effect of Cu-IL loading was non-monotonic, and further increasing the interlayer loading did not continuously reduce the PA-layer thickness or improve water permeance, indicating the existence of an appropriate Cu-IL loading range. Moreover, NF/Cu-IL-2 maintained relatively stable water permeance and Na 2 SO 4 rejection during a continuous 144   h filtration test under the tested chlorine-free conditions. These results demonstrate that coordination-assisted Cu-IL interlayer engineering is an effective strategy for improving water transport while maintaining sulfate rejection under the investigated single-salt conditions.