Evaluation of the adsorption states of H2O on oxide surfaces by vibrational absorption: near- and mid-infrared spectroscopy

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.