PNNL

Abstract

Molecular dynamics simulations of electrolyte solutions in contact with a neutral (100) goethite (a-FeOOH) surface were used to probe the structure of the electrical double layer at the mineral-water interface and gain insight into the adsorption properties of monovalent ions. Three electrolyte solutions were considered: NaCl, CsCl, and CsF. The electrolyte ions were chosen to cover a range of ionic sizes and affinities for the aqueous phase. The molecular dynamics simulations indicate the presence of a structured interfacial region resulting from the strong interaction of water with the mineral surface. The specific arrangement and preferred orientation of water that arise from this interaction create adsorption sites in the interfacial region, i.e., as far as 15 Å away from the surface, and hence give rise to a strong correlation between the water and ion distributions. The structure of the hydrated ion, its effect on the water arrangement at the interface, and the strength of the ion-water bond are found to be key factors that determine the location and extent of ion adsorption at the interface. Additionally, in all simulations, we find a build up of positive charges near the surface due to cation adsorption, which is compensated by an accumulation of anions in the next few Ångströms. This creates an excess of negative charges, which is in turn compensated by an excess of positive charges, and so on. Consequently, our simulations indicate the formation of an electrical double layer where the condensed layer extends over several hydration layers and the net charge in the diffuse layer oscillates with distance from the surface. As we modeled a neutral surface, the structure of the electrical double layer arises from the complex interplay of the interactions between the surface, water, and the electrolyte ions rather than from the need to neutralize a surface charge.