Studies of Water Adsorbed in Porous Vycor Glass

Properties of water adsorbed in porous Vycor glass and Vycor glass treated with hexamethyldisilazane (HMDS) have been studied by measuring NMR as a function of water content and temperature and observing penetration of water into the pores. Some of the observed properties are compared with those of methanol and ethanol. The NMR linewidth of water in original glass pores is proportional to the fraction of water in a surface monolayer on the glass. The melting point depression of the pore water is inversely proportional to the pore radius, and ice water interfacial tension is evaluated as 27.5 mN/m. When treated glass was immersed in water, liquid water could not penetrate the glass sample, which had more than 0.4/nm2 trimethylsilyl group substituted for surface hydroxyls; however, the alcohols could still easily penetrate the fully treated glass, which had 1.3/nm2 trimethylsilyl groups. Water adsorbed in weakly treated glass shows broadening of the NMR line; contrastingly, the alcoholic hydroxyls show narrowing of the lines. The freezing and melting point of pore water rise with surface treatments. These phenomena can be explained in terms of structure forming of adsorbed liquid on the surfaces and the interaction between the molecules in the structured layer and the subsequent layer.