Graphical AbstractVerifiedCharge Homogeneity Redefines Ion Selectivity in Polyamide Membranes
Ion-selective membranes are critical for water purification, resource recovery, and energy storage. Polyamide (PA) nanofiltration (NF) membranes are the gold standard, with size sieving and charge repulsion as fundamental mechanisms. However, the role of surface charge distribution has been overlooked. A groundbreaking study in Nature Water reveals that nanoscale spatial charge homogeneity, not pore size distribution, is the key determinant of ion selectivity. Using multimodal atomic force microscopy (AFM) techniques, including Kelvin probe force microscopy (KPFM) and electrostatic force microscopy (EFM), the authors mapped surface charge of commercial NF270 and NF90 membranes. NF270, despite looser pores, exhibited a Cl−/SO4^2− selectivity nearly tenfold higher than NF90, correlating with a more homogeneous charge distribution. AFM-infrared spectroscopy (AFM-IR) showed uniform carboxyl group (–COOH) distribution on NF270, while NF90 had patchy, structure-dependent distribution, creating defects for ion leakage. Molecular dynamics simulations confirmed SO4^2− ions preferentially localize in regions lacking –COOH. The authors translated this insight into a polyethyleneimine multivariate (PEI-MTV) strategy to program homogeneous positive charge on PA membranes for cation separation. Comparing grafting routes, EDC/NHS-catalyzed amidation yielded the most homogeneous charge distribution and highest charge density, achieving exceptional Li+/Mg2+ separation performance. This work establishes a transformative design principle for highly selective membranes via nano-charge manipulation, bypassing precise pore size control.
