Inverse of Nanopore Ion Selectivity Due to Transport-Induced-Charge Phenomena

We report a new finding via molecular dynamics simulation that ion selectivity of a salt concentration biased nanopore can be inverted by locally induced charges as an electric field is imposed. The simulation is conducted with two identical ultrathin silica membranes separating two reservoirs containing potassium chloride aqueous solutions. Ion transport driven by an electric field through a nanopore on the silica membrane is investigated. The results indicate that the silica nanopore has a higher selectivity for potassium ions, even if the surface is uncharged. In contrast, when a concentration gradient in the same direction of the electric field is applied, negative charges caused by the local concentration difference between chloride ions and potassium ions are induced inside the nanopore. As a consequence of the negatively induced charges, the nanopore presents a higher selectivity for chloride ions, due to transport-induced-charge phenomena. The results of the molecular dynamics simulation are quantitatively examined, showing good agreement with analytical predictions.