Abstract

The interactions of H₂O molecules with various oxide surfaces were investigated using vibrational absorption spectroscopies, Fourier transform infrared (FT-IR), near infrared (NIR) and an analysis of the adsorption isotherms with regard to the intermolecular hydrogen bonds for H₂O molecules. The combination bands of the fundamental vibration modes for H₂O molecules in the NIR region provided very useful information on their intermolecular hydrogen bonds. The physicochemical properties of the solid surfaces such as hydrophilicity or hydrophobicity were also investigated as they relate to the viewpoint of the distribution of “hydrogen-bonded H₂O” and the “hydrogen bond-free H₂O”. It was found that SiO₂ surfaces adsorb smaller amounts of H₂O and hydrocarbon molecules as compared to TiO₂ or Al₂O₃, showing hydrophobicity. The smaller amounts of H₂O molecules adsorbed on the SiO₂ surfaces enabled them to spread out to form H₂O thin layers due to the small interaction between the H₂O molecules and SiO₂ surface as well as the small contribution of the intermolecular hydrogen bonds. These results clearly indicate that the hydrophobic SiO₂ surfaces show high wettability against the H₂O molecules. On the other hand, larger amounts of H₂O molecules and hydrocarbons were found to adsorb not only on the surface hydroxyl groups but also on the cationic (Ti⁴⁺ or Al³⁺) sites of the TiO₂ or Al₂O₃ surfaces, suggesting that TiO₂ or Al₂O₃ surfaces show both hydrophilic and oleophilic properties. Moreover, larger amounts of H₂O molecules were found to form aggregated bulky H₂O networks on TiO₂ or Al₂O₃ surfaces due to the large contribution of the intermolecular hydrogen bonds of the H₂O molecules. The results revealed that TiO₂ or Al₂O₃ surfaces, on which large amounts of H₂O and hydrocarbons can adsorb, show low wettability against H₂O molecules. The hydrophilic or hydrophobic properties of oxide surfaces are indicated by the direct interaction between the guest molecules and host solid surfaces and these interactions can be analysed by vibrational absorption (FT-IR and NIR) and adsorption isotherm measurements at the molecular level. Moreover, we discuss the wettability of solid surfaces as determined by the contact angle of the H₂O droplets and by the macroscopic interaction of H₂O with oxide surfaces on which not only H₂O but also hydrocarbon molecules are adsorbed in air until saturation.

© 2009 IM Publications LLP

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