Interfacial ion transfer and current limiting in neutral-carrier ion-selective membranes: A detailed numerical model

Ion selective membrane (ISM) electrodes are widely used for selective ion sensing applications. Recently, new modalities have emerged whose operation involves active electrical polarization of the media, and current theoretical models are unsuitable to predict their behavior beyond near-equilibrium conditions. Here, we apply numerical modeling of physicochemical transport in such systems to study mechanisms of interfacial ion transfer and their role in limiting processes. Importantly, our analysis suggest that membrane-phase complexation (MC) has strong merits as a replacement for interfacial complexation (IC) in theoretical treatments. For our purpose, we have developed a highly detailed model based on Nernst-Planck-Poisson (NPP) with kinetic reactions of first-order. The solutions were derived in terms of logarithmically transformed concentration variables, allowing us to input experimentally determined stability coefficients. A unique process, referred to here as the reaction boundary layer (RBL), is a characteristic of MC, and we found it could have a significant impact on current-voltage (I-V) characteristics and transfer selectivity. Together with other well-known processes such as electrically driven diffusion, the RBL dictates the limits of ISM electrical polarization. Using our model, we demonstrate that operating outside these limits results in ingress of interfering ion species and concurrent loss of transfer selectivity.