Molecular Statics Calculations of Acid-Base Reactions on Magnetite(001)

Abstract

Gas phase proton affinities and acidities are calculated for surface oxygen sites on the hydroxylated magnetite (001) surface. The binding energies of protons are not consistent with the higher acidity observed for the magnetite surface relative to the hematite and goethite surfaces. The pH of zero charge of magnetite, calculated using a linear free energy relationship between the gas-phase and solution pKas of the iron hydrolysis species, is 9.6. It is therefore proposed that the higher acidity of magnetite is either due to solvent effects or the complex electronic structure of magnetite. We also find that the relaxation of the surface in response to protonation and deprotonation is highly collective, often involving additional acid-base reactions at adjacent sites. These collective effects are as important or more important than, for example, the charge and number of the coordinating metal ions at a given oxide surface site. While this behavior may be represented in a mean field model, we suggest that explicit recognition of these collective interactions may have benefits in aqueous surface chemistry.