Pore structure analysis by water vapor adsorption: I. t-Curves for water vapor

New methods of pore structure analysis have made a complete analysis of the pore systems of solids possible. Nitrogen adsorption-desorption isotherms have been used in the analysis to date. There are many important adsorbents that have part of their pore systems, often the major part, inaccessible to nitrogen but accessible to water vapor. The methods used for complete analysis are equally applicable for both adsorbates, but a prerequisite to the use of water vapor is to have t-curves, i.e., curves that give the statistical thickness of the film adsorbed on nonporous adsorbents as a function of the relative pressure. Several t-curves were published for nitrogen, but the first t-curves for water vapor are those presented in this paper. The thickness of the adsorbed film at a given relative pressure depends on the heat of adsorption; consequently, the t-curve to be used is that which matches the heat of adsorption of water on the adsorbent to be analyzed. The C constant of the BET equation is an adequate measure of the heat of adsorption for the purpose of the analysis. Several t-curves are offered, depending on the C constants.