Influence of finite ion size and dielectric decrement on the ion current rectification in a single conical nanopore

Abstract

The ion current rectification (ICR) arising due to the transport of ionized liquids within a geometrically asymmetric nanopore is of great significance for the development of smart nanogadgets with unique working capabilities. Though the theoretical framework for the ICR is well developed, the influence of the finite size of ions on the ICR phenomena had not been addressed before. The ion steric repulsion due to finite ion size and dielectric decrement of the medium creates a counterion saturation. In this study, a modified electrokinetic model is adopted to describe the ICR phenomena of a single conical nanopore by considering the hydrated ions as finite-sized dielectric charged spheres. The Nernst–Planck equations for ion transport are modified to incorporate the short-range ion steric interactions modeled by the Boublik–Mansoori–Carnahan–Starling–Leland equation as well as Born force and dielectrophoretic force acting on the hydrated ions engender by the ion–solvent interactions. The counterion saturation attenuates the shielding effect of the surface charge of the nanopore leading to a larger ζ-potential and hence, a larger volume flux and reduced conduction. We find that the ion steric interactions and the dielectric decrement significantly influence the ICR phenomena as well as the ion selectivity of a conical nanopore, especially for moderate to high range of surface charge density, bulk concentration, and applied bias. We find that ICR varies linearly with temperature; however, the variation is found to be marginal. Our results show that the volume flux and the rectification factor of the conical nanopore can be suitably tuned by adding salt of larger counterion size or multivalent ions.