Electrostatic-modulated assembly of charged COF nanosheets into high-performance nanofiltration-based desalination membranes

Ionic covalent organic framework (iCOF) nanosheet membranes hold great potential in nanofiltration-based desalination by offering superior chargeability for ion exclusion. However, controllable assembly of iCOF nanosheets into high-performance membranes remains challenging due to the electrostatic repulsion between them. We propose an electrostatic-modulation strategy to reduce the chargeability of iCOF nanosheets by modulating the dielectric properties of the solvent environment, which enables the fabrication of dense laminated charged COF membranes (CCOFMs). By decreasing the solvent dielectric constant from 78.5 (water) to 24.3 (ethanol), the zeta potential of sulfonated COF nanosheets is compressed from −35.8 mV to −3.76 mV. The reduction in chargeability weakens the electrostatic interaction energy between iCOF nanosheets by approximately 500 kJ mol −1 and thereby promoting the formation of a denser laminated structure during subsequent vacuum-assisted assembly. The denser structure of CCOFMs leads to narrowed effective pore size from 0.678 nm to 0.476 nm to enhance ion exclusion by coupling steric hindrance and electrostatic effect. The optimized CCOFM display outstanding rejections of ∼96.3%, ∼95.0%, and ∼80.0% for Na 2 SO 4 , MgSO 4 , and NaCl, respectively, surpassing the benchmark COF membranes. The dense structure also renders CCOFM outstanding stability by maintaining Na 2 SO 4 rejection above 94.7% for over 1000 h, which enables the purification of organic saline wastewater with Na 2 SO 4 rejection of ∼97.3% and tetracycline rejection of ∼99.9%. The electrostatic-modulated assembly of iCOF nanosheets enlightens the precise construction of charged laminated membranes for ion-related separations.