TEMPERATURE-DEPENDENT ADSORPTION OF NITROGEN ON POROUS VYCOR GLASS

Adsorption isotherms of N-2 have been measured in the temperature rang e from 77 to 120 K in samples of porous vycor glass. From the Brunauer -Emmett-Teller theory the surface layer coverages are determined. Thes e are found to be temperature dependent. When adsorption-isotherm cove rage data are expressed as a function of the adsorption potential delt a mu, the result is roughly temperature independent for coverages rang ing from submonolayer to thin film, below capillary condensation. This characteristic curve, which represents the distribution of adsorption sites vs the adsorption potential, is compared with results from two models for the adsorbate: Dubinin's isotherm for microporous solids an d its extension to rough surfaces, which places importance on the poro sity of the surface, and Halsey's extension of the Frankel-Halsey-Hill isotherm, which takes into account the long-range variations of the s ubstrate adsorption potential. The impact of this work on the interpre tation of N-2 adsorption data in terms of a surface area is discussed.