Ion separations remain a central challenge in modern chemical processes with broad implications for energy and environmental technologies. The difficulty in separating these species arises from the nearly indistinguishable physicochemical properties of many ions: their hydrodynamic radii often differ by only minute amounts, and similar charges further complicate selectivity. So far, efforts to overcome these challenges have primarily focused on enhancing separation efficiency, either by narrowing pore size distributions to sharpen size-based discrimination or by introducing chargeable functional groups to the membrane to improve charge-based separations.
Now, Dan Lu and co-workers show that membrane selectivity is not dictated solely by pore size or charge density, but also nanoscale spatial charge homogeneity, which can, at times, play an even greater role in ion selectivity than pore structure and charge density. Using multimodal atomic force microscopy combined with complementary spectroscopic techniques, the team mapped nanoscale charge variations against surface potential, phase and functional groups, thereby disclosing the impact of nanoscale spatial charge uniformity on membrane ion selectivity. Comparisons between commercial benchmarks (NF90 and NF270) and laboratory-made polyamide membranes revealed that uneven charge distributions disrupt ion partitioning at the membrane–solution interface, allowing non-target ions to leak through charge-deficient regions, ultimately compromising selectivity.
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