Determination of the pore-size distributions and pore-space interconnectivity of Vycor porous glass from adsorption-desorption hysteresis capillary condensation isotherms

Abstract

A model of a chaotic pore space in which cavities are randomly connected together by constrictions or windows is given. The behaviour of the system is described in terms of the cavity-size distribution, the window-size distribution, and the number of windows per cavity. A Bethe tree is used as the network of pore connections. The analysis of capillary condensation adsorption and desorption scanning curves is described, and the conclusion is drawn that desorption scanning curves are the more valuable in terms of the information that can be derived from them. This model is applied to the large number of scanning isotherms obtained by A. J. Brown (Ph. D. thesis, University of Bristol (1963)) for xenon on Vycor porous glass at 151 K. A method of correlating the results is given in which there is no need to separate the amount of capillary condensation from the amount of surface adsorption. The family of adsorption isotherms confirms the theory but some systematic differences remain. The family of desorption isotherms is used in a simple method that gives an estimate of the cavity-size distribution in terms of the Kelvin radius. The same isotherms are used in a more complicated method to give an estimate of the number of windows per cavity, together with a more precise cavity-size distribution and a window-size distribution. These results are then used to calculate the thickness of the surface adsorption. This enables the geometric pore sizes to be obtained from the Kelvin pore sizes. The cavity-size distribution thus obtained is in agreement with published values.