Ab Initio Molecular Dynamics Study of the AlOOH Boehmite/Water Interface: Role of Steps in Interfacial Grotthus Proton Transfers

The investigation of metal oxide/water interfaces at the molecular level represents a fundamental issue for the understanding of chemical, physical, and biological processes involved in several fields such as erosion, heterogeneous catalysis, prebiotic chemistry, corrosion, hygiene, or biocompatibility. In this context we have studied the mineral (101) γ-AlOOH (boehmite) surface/water interface by means of density functional theory based molecular dynamics (DFT-MD). Boehmite (101) is a stepped surface, covered with monocoordinated (μ₁) OH groups placed at the step edge and dicoordinated (μ₂) OH groups placed along the terraces. At the surface, the respective concentrations of different OH species are found as 0.48 μ₂-OH + 0.26 μ₁-OH₂ + 0.24 μ₁-OH + 0.02 μ₂-OH₂. We show that the interfacial water molecules are somehow frozen in specific orientations, with 60% having one proton pointing to the surface (water is a H-bond donor to the surface) and 40% with one proton directed away from the surface (water is a H-bond acceptor with the surface). The effect of the surface on the water organization is lost at 6 Å from the surface, where liquid bulk is fully recovered. Proton transfers are observed at the interface between μ₁ and μ₂ species involving a Grotthus mechanism between distant μ₁/μ₂ groups. A bridge of interfacial water molecules has been found to assist this proton transfer. A pK value of 1.4 is calculated for this acid–base reaction, where μ₂-HOH is found to be a stronger acid than μ₁-HOH. These results represent a first step toward the understanding of the increased reactivity of defective surfaces in the presence of explicit solvent, using a first-principle representation of the full interface.