Heat of Immersion of Titanium Dioxide in Water. II. The Effect of the Crystallinity of Anatase

Two kinds of anatase samples which have very large specific surface areas and low crystallinities were prepared, and the interaction between the solid surface and the water molecule was investigated by measuring the water content, the heat of immersion in water, and the infrared spectrum of each sample. Two characteristic features were found in these samples: first, the plot of the heat of immersion vs. the pretreatment temperature did not reveal such a maximum as had been found previously with the crystalline anatase, and second, the heat of surface hydration as calculated from the plot of the heat of immersion vs. the water content was found to be remarkably smaller than with the crystalline anatase. These facts have led us to conclude that, with the anatase samples of a low crystallinity, not only does the rehydration reaction of a surface oxide structure produced at an elevated temperature proceed more easily on immersion in water, but also the heat of rehydration is smaller, than in a sample of a high crystallinity. Moreover, it was found from an infrared spectroscopic investigation that the water molecules physisorbed on the surfaces of large-area samples could be desorbed more gradually than those on small-area samples and remained small in quantity up to near 300°C in vacuo.