Impact of charge homogeneity on ion selectivity in polyamide membranes

Ion-selective membranes, crucial for diverse applications such as water purification, brine disposal and resource recovery, rely heavily on the pore architecture and surface charge. Narrowing the pore size distribution (PSD) of the membrane is generally acknowledged to be essential for achieving higher ion selectivity. Here we challenge the conventional emphasis on PSD by introducing an alternative determinant—surface charge homogeneity—drawing inspiration from a counterintuitive relationship between PSD and ion selectivity observed in both commercial and laboratory-made polyamide nanofiltration membranes. By integrating multimodal atomic force microscopy technologies, we visually extracted nanoscale charge maps from three dimensions: surface potential, phase and functional groups. The metrological analysis methodology was originally developed to quantitatively describe the spatial charge distribution. It is demonstrated that nanoscale spatial charge homogeneity plays a crucial role in governing ion selectivity, surpassing the influence of PSD. Based on this perception, we devised the high-selective nanofiltration membranes and modules for the lithium–magnesium mixture separation by using a polyethyleneimine multivariate strategy to program polyamide membranes with stepwise-enhanced homogeneous distribution of electropositive-amine moieties. Our work unveils a unique charge homogeneity-dominated selectivity mechanism and demonstrates the feasibility of developing highly ion-selective membranes by facile nanocharge manipulation, surpassing the need for precise PSD control. The conventional focus on pore size distribution overlooks the role of surface charge homogeneity in ion separation by polymeric membranes. This study proposes a surface charge engineering strategy for fabricating highly ion-selective membranes.