Change in Surface Property and Structure of Porous Silica Particles by Hydrothermal Treatments.

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Abstract
Various hydrothermal treatments have been carried out for mesoporous silica particles. The hydrothermal treatments are classified into two categories according to the phase in which reactions progresses. In liquid phase, reactions were initiated by immesrsing silica particles in various solutions, whereas in gas phase, silica particles were exposed to water vapor under high pressure by means of autoclave. By measuring increases in immersional heat and H-bond type silanol density, progress of the rehydration of surface siloxane is quantified in both methods. Particulary in liquid phase, rehydroxylation was significantly promoted by the addition of basic substances and also at high temperature. On the other hand, in gas phase, nitrogen and water adsorptions change geometric structure of silica particles in addition to rehydroxylation. This is because dissolution of silica surface, redeposition of the solute and consequently the rehydration occurred in the process of hydrothermal treatments in gas phase. The dissolution progressed significantly with temperature higher than 160°C

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