Polyamide Membranes with Tunable Surface Charge Induced by Dipole–Dipole Interaction for Selective Ion Separation

Nanofiltration (NF) has the potential to achieve precise ion–ion separation at the subnanometer scale, which is necessary for resource recovery and a circular water economy. Fabricating NF membranes for selective ion separation is highly desirable but represents a substantial technical challenge. Dipole–dipole interaction is a mechanism of intermolecular attractions between polar molecules with a dipole moment due to uneven charge distribution, but such an interaction has not been leveraged to tune membrane structure and selectivity. Herein, we propose a novel strategy to achieve tunable surface charge of polyamide membrane by introducing polar solvent with a large dipole moment during interfacial polymerization, in which the dipole–dipole interaction with acyl chloride groups of trimesoyl chloride (TMC) can successfully intervene in the amidation reaction to alter the density of surface carboxyl groups in the polyamide selective layer. As a result, the prepared positively charged (PEI-TMC)-NH2 and negatively charged (PEI-TMC)-COOH composite membranes, which show similarly high water permeance, demonstrate highly selective separations of cations and anions in engineering applications, respectively. Our findings, for the first time, confirm that solvent-induced dipole–dipole interactions are able to alter the charge type and density of polyamide membranes and achieve tunable surface charge for selective and efficient ion separation.