Non-Monotonic Dependence of Ion Selectivity on Interfacial Polymerization Time in Nanofiltration Membranes

Polyamide nanofiltration (NF) membranes offer promising opportunities for resource recovery because their sub-nanometer selective layers provide intrinsic ion−ion selectivity. However, achieving sufficient monovalent/divalent separation, such as Li+/Mg2+, remains challenging because the polyamide (PA) structure formed during interfacial polymerization (IP) is heterogeneous and difficult to control. As a time-dependent process, IP enables reaction time to regulate the PA layer structure and ion transport. Here, we systematically investigate IP reaction times from 10 s to 5 min in polyamide NF membranes and show that monovalent/divalent ion selectivity exhibits a non-monotonic dependence on the reaction time, reaching a maximum separation factor of 30.5 at 0.5 min. Shorter reaction times increased water permeance and reduced ion rejection due to incomplete polymerization, whereas prolonged reaction decreased water permeance and MgCl2 rejection while increasing LiCl rejection. Membrane characterization revealed that these trends arise from coupled changes in the PA network density and charge distribution. The 0.5 min reaction produced a thin (32 nm) and densely cross-linked PA, favoring water transport and steric exclusion of Mg2+. In contrast, the 5 min reaction produced a substantially thicker (117 nm) and more open PA layer containing carboxylic groups throughout its depth, contributing to stronger electrostatic exclusion of LiCl. These findings establish the IP reaction time as an important parameter for tuning the structure and charge of PA membranes, enabling improved control of monovalent/divalent ion selectivity.