An Atomistic Description of the γ-Alumina/Water Interface Revealed by Ab lnitio Molecular Dynamics

We report ab initio molecular dynamics (AIMD) simulations of the (100) and (110) gamma-A1203/water interfaces at 300 K, using two sets of supercell models for each surface and two time lengths of simulation (10 and 40,ps). We first show that the effect of liquid water on the vibrational frequencies of hydroxyl groups at the interface varies according to the type of surface. This trend is explained by two key parameters affecting the interaction of both surfaces with water: the nature of the OH groups (i.e., mu(1)-OH, mu(1)-H2O, mu(2)-OH, and mu(3)-OH) and H-bond network among surface OH groups. The hydroxylated (110) surface favors the local structuration of water at the interface and the solution of its mu(1)-OH and mu(1)-H2O groups by water similarly as in bulk liquid water. By contrast, on the (100) surface, a stronger H-bond network among mu(1)-OH and mu(1)-H2O groups reduces the water/surface interaction. We illustrate also how the interfacial interacting sites are spatially organized on the surfaces by two-dimensional maps of O-H distances. On both surfaces, the interfacial water layer orientation is predominantly H-up-H-down. For long AIMD simulation time, Grotthuss-like mechanisms are identified on the (110) surface.