Electrokinetic transport of monovalent and divalent cations in silica nanochannels

Electrokinetic transport of aqueous electrolyte solutions in nanochannels and nanopores is considered important toward the understanding of fundamental ion transport in many biological systems, lab-on-chip, and organ-on-chip devices. Despite the overall importance of these systems and devices, detailed calculations showing velocity and concentration profiles for multi-component, multi-valent ionic species are limited. In this paper, molecular dynamics simulations were employed to compute velocity and concentration profiles for an electrolyte mixture containing sodium, magnesium, and chloride ions with water as the solvent in a ~7-nm-deep amorphous silica nanochannel. The results indicate that addition of trace quantities of divalent Mg2+ ions to monovalent (NaCl) electrolyte solutions while preserving overall system electroneutrality increases the maximum electroosmotic velocity of the solution by almost two times. Additionally, analyzing concentration profiles of individual ions revealed that Na+ was found to be preferentially attracted to the negatively charged silica wall in comparison with Mg2+ likely due to the hydrated divalent cation having a larger size compared to the hydrated monovalent cation.