Molecular simulation of the magnetite-water interface

This paper reports molecular dynamics simulations of the magnetite (001)-water interface, both in pure water and in the presence of a 2.3 molal solution of NaClO4. The simulations are carried out using a potential model designed to allow the protonation states of the surface functional groups to evolve dynamically through the molecular dynamics trajectory. The primary structural quantities investigated are the populations of the surface functional groups, the distribution of electrolyte in the solution, and the surface hydrogen bonding relationships. The surface protonation states are dominated by extensive hydrolysis of interfacial water molecules, giving rise to a dipolar surface dominated by FeOH2+-OH2-OH- arrangements. Triply coordinated, more deeply buried, surface sites are inert, probably due to the relative lack of solvent in their vicinity. The electrolyte distribution is oscillatory, arranging preferentially in layers defined by the solvating water molecules. The presence of electrolyte has a negligible effect on the protonation states of the surface functional groups. Steady-state behavior is obtained for the protonation states of the surface functional groups and hydrogen-bonding network. Although the overall structure of the electrolyte distribution is fairly well established, the electrolyte distribution has not fully equilibrated, as evidenced by the asymmetry in the distribution from the top to the bottom of the slab.