The role of confinement and surface charge on electrical and diffusio-osmotic conductivity of electrolyte solutions from <i>ab initio</i> methods

Abstract

We perform extensive ab initio molecular dynamics simulations to compute transport properties of a KCl solution at high concentration using Green–Kubo relations based on Onsager’s system of linear equations. Our results show an increase in the electrical conductivity under confinement, with a further enhancement in the case of a positive charge on graphene. The presence of surface charges also determines the direction of electro-osmotic flow and of the diffusio-osmotic electrical current in the nanofluidic systems. Structural analysis reveals that in the case of positively charged graphene sheets, chloride ions accumulate at the surface. This promotes a spatial separation of anions and cations, thereby reducing their correlations and leading to an increased electrical conductivity in this system. We show how accounting for electronic structure by means of ab initio molecular dynamics is essential in the calculation of the transport properties. While quantitative results can be obtained for the diagonal terms of the Onsager transport matrix, only a qualitative impact can be gauged for the off-diagonal terms due to limitations in sampling. Nevertheless, our results reveal that surface charges significantly alter ionic conductivity and determine the direction of the electro-osmotic flow and of the diffusio-osmotic current, thereby advancing the understanding of transport at the nanoscale.