Superhydrophilicity of $\alpha$-Alumina Surfaces Results from Tight Binding of Interfacial Waters to Specific Aluminols

Understanding the microscopic driving force of water wetting is challenging and important for design of materials. In this work, we investigate, using classical molecular dynamics simulations, the water/$\alpha$-alumina (0001) and ($11\overline{2}0$) interfaces chosen for their chemical and physical differences. There is only one type of aluminol group on the nominally flat (0001) surface but three types on the microscopically rougher ($11\overline{2}0$) surface. We find that both surfaces are completely wet, consistent with contact angles of zero. Moreover, the work required to remove water from a nanoscale volume at the interface is larger for the (0001) surface than the ($11\overline{2}0$) surface, suggesting that the (0001) surface is more hydrophilic. In addition, translational and rotational dynamics of interfacial water molecules are slower than that in bulk water, suggesting tight binding to the surface. Interfacial waters show two major polar orientations, either pointing to or away from the solid surface. In the former case, waters donate strong hydrogen bonds to the surface, while in the latter they accept relatively weak ones from aluminol groups. The strength of hydrogen bonds is estimated using their lifetime and geometry. We found that for all aluminols, water-to-aluminol hydrogen bonds are stronger and have longer lifetimes than the aluminol-to-water ones. One exception is the long lifetime of the \ce{Al3OH}-water hydrogen bonds on the ($11\overline{2}0$) surface, due to geometric constraints. Interactions between surfaces and interfacial waters promote a templating effect whereby the latter are aligned in a pattern that follows the underlying lattice of the mineral surface.