Mica–Polyethylenimine Membranes for High-Efficiency pH-Responsive Nanofiltration Enabled by a Strongly Coupled Charge Flip

Abstract

The pH-responsive two-dimensional nanofiltration membranes enable tunable separation, yet existing membranes suffer from weak charge gating. Herein, we prepared a mica–polyethylenimine membrane using a “coupled charge flip” strategy. The membrane exhibits a strong charge flip (zeta potential from +46.1 to −36.4 mV) across pH 2–12, originating from the synergy between polyethylenimine (zeta potential from +35.2 to +2.5 mV) and mica (zeta potential from −3.1 to −55.7 mV), yielding a charge-flip amplitude (82.5 mV) 41% greater than that of the best performing bipolar pH-responsive membranes. This enables superior pH-dependent molecular gating (e.g., methyl orange) with rejection ranging from 96.98 to 5.03% (pH 2–12) and a gating ratio of 19.28, which is 1.76 times that of the best reported pH-responsive membranes, while maintaining a water permeance of 155 L m–2 h–1 bar–1 and stable cycling over eight pH switches. This work provides a new framework for designing pH-responsive nanofiltration membranes with high gating ratios.