Interfacial Partitioning Kinetics Govern Nitrate-Chloride Selectivity in Ion-Selective Membranes

Ion partitioning is an important component of ion-selective membrane transport, but its kinetic contribution to ion selectivity has received limited attention because of limited quantitative analytical tools. Here, experiments, density functional theory calculations, and the kinetic barrier network model were combined to elucidate the contribution of interfacial partitioning kinetics to the high NO3-/Cl- selectivity of a custom membrane (101M). Under single-salt conditions, Cl- exhibited faster partitioning into the membrane and also faster overall transmembrane transport than NO3-. Under mixed-salt conditions, however, the apparent barrier for Cl- partitioning into the membrane exceeded that of NO3- by 11.7 kJ·mol-1, reversing the overall transmembranetransport preference and producing an NO3-/Cl- selectivity of up to 150. Cross-ion-exchange experiments independently supported this reversal and clarified its mechanistic origin. The apparent partitioning barrier depended on the identities of both the incoming ion and the resident counterion occupying the membrane sites, with the resident-ion effect associated with differences in ion-site binding energy. These results identify interfacial partitioning kinetics as a dominant determinant of competitive ion transport and a critical design parameter for highly selective ion-separation membranes.