On the origin of hydrophilic interactions at alumina surfaces

Despite advances in understanding macroscopic wetting behavior, the atomistic mechanisms underlying hydrophilicity at solid-liquid interfaces remain only partially understood, particularly for chemically and structurally complex surfaces. Alumina, a widely used oxide in catalysis and surface coatings, exhibits such complexity due to the variability in surface termination and hydroxylation. In this study, we investigate the hydrophilicity of three crystallographic terminations of hydroxylated alumina-(0001), (1120), and (0112)-using molecular dynamics simulations based on neural network potential trained on density functional theory data. We quantify hydrophilicity through mean square displacement, density fluctuations, hydrogen bonding characteristics, vibrational density of states, the tetrahedral order parameter, and the contact angle. Our results reveal a clear hydrophilicity trend of (1120) < (0001) < (0112), with the relatively flat surface (1120) showing the least structured interfacial water and the corrugated surface (0112) showing the most pronounced confinement and hydrogen bonding. This trend is reflected in reduced water mobility, suppressed density fluctuations, stronger surface-to-water hydrogen bonds, and greater disruption of tetrahedral order near the (0112) interface. These findings demonstrate that atomic-scale surface roughness and the degree to which it can disrupt the structure of water are key to forming strong surface-to-water hydrogen bonds, thereby offering a microscopic rationale for hydrophilic behavior at oxide surfaces.