Multivalent Ion Transport through a Nanopore

Ion transport through nanopores finds increasing applications in biophysical and chemical processes. Distinct ion transport behaviors have been identified in sub 10 nm diameter nanopores as a result of complex interactions between ions and the nanopore surface. So far, most studies have been for monovalent electrolytes with little attention paid to the effects of multivalent ions. Here we report on the pronounced effects of high ion valency on the ion mobility inside narrow pores of a few nanometer diameters. Results show that ion mobilities are significantly suppressed at high electrolyte concentrations (>0.5 M), which is attributed to the formation of ion pairs inside the nanopore. Analyses suggest that a nanopore can help partially dehydrate ions and confine the electric field lines from an ion inside the nanopore, which enhances the interactions between ions and promotes the formation of ion pairs. This physical picture is further supported by the free energy landscapes demonstrating the diminished energy barrier for ion pair formation inside a nanopore.