Adsorption of Water on Silica

Abstract

SINCE the announcement¹ of anomalous water, its properties have been investigated by several workers^2,3, but little understanding of its structure has been achieved. In our laboratory recently we obtained water adsorption results on silica surfaces that have been treated so as to contain only freely vibrating hydroxyl groups or only hydrogen bonded hydroxyl groups in addition to the ever present siloxane linkages (unpublished). The adsorption of water on the freely vibrating hydroxyl group gives rise to a typical type III isotherm. Quantitative measurements of water adsorption on the hydrogen bonded groups indicated that up to a pressure of 13 torr the adsorption took place in two steps. The first step corresponded to the adsorption of one molecule of water for each pair of hydrogen bonded hydroxyls, while the second step showed that a further two molecules of water became adsorbed for each pair of H-bonded hydroxyls, presumably as a second layer. This evidence suggests that the results may have a bearing on the structure of anomalous water, which is prepared by the condensation of water in small capillaries from a water atmosphere in which p/p₀ is close to one. In view of this we have extended our measurements of water adsorption on the H-bonded surface hydroxyls out to p/p₀→1.