Anomalous electrokinetics at hydrophobic surfaces: Effects of ion specificity and interfacial water structure

Abstract

We study the electrokinetic transport behavior of water molecules and ions in hydrophobic graphene nanochannels with variable surface charge densities as well as the interfacial water structure based on detailed molecular dynamics simulations. The interfacial water structure, described by the water density, hydrogen bonding, diffusion, distribution of the OH bond and dipole orientations, is strikingly influenced by the surface charge sign and density. We find anomalous electrokinetic effects which are related to the distribution of counterions close to the surface, ion-specific effects and the interfacial water structure. On a negatively charged graphene layer, the attraction of Na+ ions towards the surface enhances the interfacial friction. In contrast, if the surface is positively charged, high surface charge density triggers an anomalous enhancement of electroosmotic flow, accompanied by an abrupt change of the interfacial water structure. At high surface charge densities, the mobility of the interfacial water at the positively charged surfaces is suppressed more strongly compared to the negatively charged surface. Our results reveal new electrokinetic phenomena by the comparison of negatively and positively charged surfaces.