Effect of moisture on the surface free energy and acid–base properties of mineral oxides

Surface energetic properties of mineral oxides are important in many applications. Since oxide surfaces in practice have generally come in contact with water molecules, it is important to know how water coverage affects the surface properties. In this work, five oxide samples, namely MgO, Al2O3, TiO2, SnO2 and SiO2 are heat-treated to various extents, to produce different degrees of hydration, and characterized thereafter by inverse gas chromatography. Water contents of the treated samples are determined independently by Karl Fischer titration, and specific surface areas are measured by the BET method. The results show that in general as water coverage decreases, the Lifshitz–van der Waals component of the specific surface free energy (σSLW) increases, but the acid–base interaction potential (−ΔGAB) decreases. These attributes are more sensitive to changes in water coverage at lower coverages, where the surface is presumed to consist of patches of molecular water and unhydrated hydroxyl groups.