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 experimenta lly. 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 condensat ion, which is related to the appearance of hysteresis. In the region corres ponding to the equilibrium state of films, the film thickness was determine d mainly by the molecular and structural components of the disjoining press ure and the Laplacian pressure drop. For the studied samples of porous glas ses, noticeable differences in the adsorption isotherms were observed, whic h are caused by both the film curvature and the variation in the structural components of the disjoining pressure. The latter fact was explained by th e effect of temperature and alkaline treatments of glasses in the course of their preparation. Based on the obtained values of parameters and taking i nto account possible temperature variations in the course of experiment, we estimated the limiting thickness of a films. These values and the disjoini ng pressure isotherms for water on glass plotted from the data on polymolec ular adsorption are adequately consistent with similar results obtained ear lier by other researchers for the flat surfaces of quartz and glass.