Electrochemical investigation of Donnan exclusion mechanisms in molecular layer-by-layer membranes

Membrane-based separations play a central role in water purification, energy generation, and resource recovery, and their performance is strongly influenced by ion transport through steric, dielectric, and electrostatic interactions. Among these, Donnan exclusion is particularly important for selective ion transport; however, discrepancies between theoretical predictions and experimental results have hindered a comprehensive understanding of this mechanism. Here, we use molecular layer-by-layer assembly to incorporate a carboxylate-bearing diamine, 3,5-diaminobenzoic acid, into polyamide thin films, thereby enabling precise control over fixed-charge density without altering solution pH. Electrochemical impedance spectroscopy measurements demonstrate that variations in membrane resistance directly reflect the impact of Donnan exclusion. To the best of our knowledge, this work provides the first demonstration that the chemically controlled modulation of polyamide charge density can explicitly manifest this mechanism through resistance analysis. While the results reproduce the qualitative trends predicted by the established Donnan/non-Donnan transport theory, they also reveal quantitative deviations that underscore the limitations of its simplifying assumptions.