Abstract

In this work, we study electroosmosis in a multivalent electrolyte solution. Specifically, we study the response of an electroosmotic system as the concentration of a divalent cation is varied in a water solution of sodium chloride confined in a nanochannel and as axial electric fields are applied to the system. It has been observed in recent experiments by Cevheri and Yoda,1 that the electroosmotic velocity decreases as the concentration of a divalent cation increases in a monovalent solution in a microchannel. In this study, we simulate a silica nanochannel with height of 7.0 nm. with a sodium chloride solution confined inside including variable quantities of Mg++. The velocity profiles show an opposite trend to the observed by Cevheri and Yoda.1 In this atomistic study, the electroosmotic velocity in the Electrical Double Layer (EDL) increases as a higher percentage of divalent cations are added to the solution. We propose an explanation of the observed phenomena related to the fact that the divalent ions are much more effective dragging water molecules so the electroosmotic velocity increases where the proportion of Mg++ is higher; that is, in the EDL. Thus, by adding divalent ions to a monovalent electrolyte, it is possible to control electroosmosis under nanoconfinement where molecular effects are significant.