The Heat of Immersion in Water and the Nature of the Surface Hydroxyl Group of Maghemite and Hematite

Abstract

Abstract The heats of immersion in water of maghemite (γ-Fe2O3) were measured in comparison to those of hematite (α-Fe2O3) prepared from the same maghemite by crystal transformation with heat treatment, and outgassing up to 200 °C. The heats of immersion and the heat of hydration of maghemite were lower than those of hematite throughout the measurement conditions. These differences were concluded to be due to the differences in bulk crystal structure, especially the density of the crystal. That is, the heat of hydration at the stage of physisorption depends on the surface hydroxyl group density, corresponding to the bulk crystal density. Also the surface hydroxyl groups are strongly affected by the polarization of the bonds in the bulk, this results in the difference of the heat of hydration at the stage of chemisorption (formation of the hydroxyl groups) and the acidity of the hydroxyl groups. This view is confirmed on the polymorphs of other metal oxides.