Porous silica-water interactions .1. Structural and dimensional changes induced by water adsorption

Hydration of a porous silica glass was studied during thermal cycles involving dehydroxylation. Silanol content was deduced by thermogravimetry. Differential scanning calorimetry and H-1 NMR measurements of the silica hydration layer have been found coherent with a water layer bonded to the solid surface, itself covered with liquid-like water. The first hydration layer is non-uniform and involves essentially unconnected water molecules. The amount of water in this layer corresponds roughly to the amount of silanol hydroxyl groups. However water starts clustering before its bonding to silanols is complete. This fact is especially obvious on thermally pretreated samples, because isolated silanols and water have a weak affinity for each other. The first hydration layer on a pristine silica sample displays specific water structures, whose concentration decreases on a thermally pretreated sample. These species are assigned to water molecules bridging silanols that are weakly interacting on a fully dehydrated silica and condense in the range 400-600 degrees C. (C) 1997 Elsevier Science B.V.