Intrinsic Nature of Interfacial Interactions between Ionic Liquids and Rutile TiO₂ Single Crystal Surfaces Studied by in Situ Contact Angle Measurement in a Vacuum

In situ contact angle measurements of ionic liquids (ILs) on well-defined rutile TiO₂ single crystal surfaces with various treatments were performed in a vacuum by a sessile drop method. The contact angles of ILs on the clean TiO₂ surfaces prepared in vacuum were significantly smaller than on those contaminated with airborne hydrocarbon. As a result, there was revealed a different correlation in Zisman's plots of the hydrophobic and hydrophilic ILs, which was further confirmed by their different wettability change on the hydrophilized TiO₂ surface with UV light treatment in air. In addition, the critical surface tension on the vacuum-annealed TiO₂ surface was lower than on the initial clean surface. These results will be qualitatively discussed by considering the polar and apolar interactions at the interface between ILs and TiO₂.