Phase Transitions of Adsorbates. Part 2. Vapour Pressure and Extension Isotherms of the Porous-Glass plus Water System Below 0C,

The vapour pressure and extension isotherms of the water-porous 96 % silica glass determined at + 1.5, – 6.4, – 14.6, – 25.4 and – 35.3°C. Isotherms obtained at these temperatures coincided if the amount of adsorbed water, a, was plotted against the relative pressure with reference to that of undercooled water. This demonstrates that the isothermally adsorbed water is in a liquid-like state and confirms that the drastic reduction of a with declining temperature is due to a decrease of the maximum relative pressure that can be realized experimentally. In contrast to previous studies, a minimum in the extension isotherm at low concentration was found, indicating that adjacent hydrogen-bonded silanol groups of the glass may not be the site of water adsorption as has been assumed in the past. Examination of the pertinent literature suggests that a minimum of the extension isotherm is obtained on the adsorption of most substances if the experimental temperature is not higher than the bulk boiling point. The contraction effect and the secondary hysteresis at low coverages are assumed to be caused by microporosity.