Vibrational spectroscopic evaluation of hydrophilic or hydrophobic properties of oxide surfaces

M. Takeuchi, R. Kurosawa, J. Ryu and M. Matsuoka, J. Raman Spectroscopy, available online (2022).

Herein, we evaluated the hydrophilic or hydrophobic surface properties of various oxides using vibrational spectroscopy, such as FT-IR (mid-IR) and near-IR spectroscopy. Pristine SiO2 showed hydrophobic properties because H2O molecules weakly interacted with the hydroxyl groups via hydrogen bonding. Furthermore, the hydrophobic SiO2 adsorbed less toluene, as a hydrophobic probe molecule. In case of the alkyl-groups-modified SiO2, H2O molecules were barely adsorbed on the surface even in air, confirming the water-repellent property. The TiO2 and Al2O3 surfaces adsorbed larger amounts of H2O than did pristine SiO2, indicating hydrophilic properties, because the Ti4+ and Al3+ sites play an essential role in strong interactions with H2O molecules via coordinative adsorption. Furthermore, the hydrophilic TiO2 and Al2O3 showed a high affinity to toluene, indicating oleophilic (affinity to oil or non-polar compounds) properties, because the aromatic compounds were adsorbed on the cation sites by cation-π interaction. Moreover, MgO reacted with H2O molecules to yield Mg(OH)2, indicating a greater hydrophilicity than TiO2 or Al2O3. Based on the considerations, when evaluating the hydrophilic or hydrophobic properties of oxide surfaces, the hydroxyl groups and the cation sites on the surface must be considered.