TEMPERATURE-DEPENDENT ADSORPTION OF HYDROGEN, DEUTERIUM, AND NEON ON POROUS VYCOR GLASS

Adsorption isotherms of H-2, D-2, and Ne have been measured in the tem perature range from 15 K to the corresponding critical points in sampl es of porous Vycor glass. From the Brunauer-Emmett-Teller theory the s urface layer coverages are determined. These are found to be temperatu re dependent. A model-independent approach allows us to fit the data f or coverages ranging from submonolayer to thin film, below capillary c ondensation, for each adsorbate at all temperatures with a temperature -independent curve. This characteristic curve represents the distribut ion of adsorption sites versus the adsorption potential. In the interm ediate coverage range, the isotherms exhibit the modified Frenkel-Hals ey-Hill (FHH) behavior. The adsorption saturates for low-adsorption po tentials. The characteristic curve is a useful universal curve since i t is roughly the same for the three species investigated. We examine t he relative strengths of the surface potentials and densities of the t wo isotopic modifications of hydrogen and of the more classical Ne ads orbed on porous Vycor glass. The characteristic adsorption curve is co mpared with results from two models for the adsorbate: Dubinin's isoth erm for microporous solids and its extension to rough surfaces which p laces importance on the porosity of the surface, and Halsey's model, w hich is an extension of the FHH isotherm that takes into account the l ong-range variations of substrate adsorption potential.