Electrokinetic transport in a water–chloride nanofilm in contact with a silica surface with discontinuous charged patches

Abstract

This paper reports on simulations of electrokinetic transport in an electrolyte nanofilm in contact with a negatively charged substrate with discontinuous counter-charged patches. All-atom molecular dynamics simulations of an aqueous solution of chloride placed on a silica slab were conducted. The response of the system is investigated as it is subjected to different axial electric fields and as the surface electrostatics is modified by implementing different positive charge densities on the patches. Chloride and water density profiles reveal that the charged patches cause an enrichment of hydrated chloride ions near the surface. Therefore, the system with patches proved to be successful in trapping ions. Moreover, the number of chloride ions trapped on the surface increases as higher charge densities are imposed on the patches. It is observed that higher axial electric fields remove chloride ions from the surface; however, the amount removed decreases as higher charge densities are imposed on the patches. Velocity profiles show a stagnant water layer near the patches. The water and ion accumulation suggest the formation of a Stern layer with thickness of ca. 0.4 nm on the patches. Higher chloride density near the surface and increased surface-water affinity induced by the presence of charged patches result in smaller electroosmotic velocities. The simulation results compare well with velocities calculated using a continuum approach. This study shows that in a charged electrolyte solution in contact with a substrate, axial and normal ion distributions and consequently electroosmotic velocities can be controlled by modifying the electrostatics of the substrate.