Polymolecular adsorption of water by porous glasses and the disjoining pressure of adsorption films

Adsorption of water vapors by porous glasses characterized by various pore radii and relatively narrow pore size distributions was studied experimentally. It was established that completely reversible polymolecular adsorption takes place within a wide range of relative vapor pressures. At high vapor pressures, polymolecular adsorption was accompanied by capillary condensation, which is related to the appearance of hysteresis. In the region corresponding to the equilibrium state of films, the film thickness was determined mainly by the molecular and structural components of the disjoining pressure and the Laplacian pressure drop. For the studied samples of porous glasses, noticeable differences in the adsorption isotherms were observed, which are caused by both the film curvature and the variation in the structural components of the disjoining pressure. The latter fact was explained by the effect of temperature and alkaline treatments of glasses in the course of their preparation. Based on the obtained values of parameters and taking into account possible temperature variations in the course of experiment, we estimated the limiting thickness of a films. These values and the disjoining pressure isotherms for water on glass plotted from the data on polymolecular adsorption are adequately consistent with similar results obtained earlier by other researchers for the flat surfaces of quartz and glass.