Structure of the Electric Double Layer in Hydrothermal Systems. MolecularSimulation Approach and Interpretation of Experimental Results

摘要

When an aqueous electrolyte solution is put in contact with a solid surface thesolid-liquid interface develops a net electric polarization resulting from the formation of an electric double layer. While this phenomenon has been known for more than a century, giving rise to a variety of theories including the first one by Helmholtz followed by Gouy-Chapman (Israelachvili 1992), and Bockris-Devanathan-Miller (Bockris, Devanathan et al. 1963). The existence of the electric double layer has a strong impact on many chemical (electrochemical metal deposition, corrosion, catalysis), biochemical (ion channels), and geological (mineral dissolution/deposition and reactivity, crystallization) processes due to charge-shielding resulting from the movement of ions toward the interface to balance the charged surface. Typically, the theoretical investigation of the microscopic structure of the medium near the (charged or uncharged) interface has been done with simplified models, involving a structureless surface in contact with point (or hard-sphere) ions immersed in a dielectric continuum (Schmickler and Henderson 1986). Obviously, these models neglect important physicochemical aspects of the system, such as the solvent structure around the species in solution (solvation effects), as well as the reactive interaction between the solvent and the solid surface. In order to interpret the actual electrochemical behavior of ionic species in solution, their interaction with the solid surface and their mobility we need to develop molecular-based tools. These tools must take explicitly into account the discrete nature of all species in solution and the essential features to describe the solid surface and its interactions with the contact solution (ultimately, it will include the discrete nature of the surface). The main goal of our research is the analysis of the microscopic behavior of high-temperature aqueous electrolyte solutions in contact with metal oxide surfaces, to gain an improved understanding of the configurational behavior of the electric double layer. For that purpose we develop molecular dynamics protocols to characterize formation of the electric double layer through the determination of the profiles of species; concentrations, electric field, species diffusivity, and solvent polarization normal to the oxide surface.