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.
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