Origin of the Hydrophobic Behaviour of Hydrophilic CeO₂

Abstract

The nature of the hydrophobicity found in rare‐earth oxides is intriguing. The CeO2 (100) surface, despite its strongly hydrophilic nature, exhibits hydrophobic behaviour when immersed in water. In order to understand this puzzling and counter‐intuitive effect we performed a detailed analysis of the water structure and dynamics. We report here an ab‐initio molecular dynamics simulation (AIMD) study which demonstrates that the first water layer, in immediate contact with the hydroxylated CeO2 surface, is responsible for the effect behaving as a hydrophobic interface with respect to the rest of the liquid water. The hydrophobicity is manifested in several ways: a considerable diffusion enhancement of the confined liquid water as compared with bulk water at the same thermodynamic condition, a weak adhesion energy and few H‐bonds above the hydrophobic water layer, which may also sustain a water droplet. These findings introduce a new concept in water/rare‐earth oxide interfaces: hydrophobicity mediated by specific water patterns on a hydrophilic surface.