CALCULATED PHYSICAL ADSORPTION-ISOTHERMS OF NEON AND RADON ON A HETEROGENEOUS SURFACE

The physical adsorption isotherms on a heterogeneous surface of neon ( Ne) have received limited attention, and those of radon (Rn) virtually none. The other inert gases, helium, argon, krypton, and xenon have b een explored extensively, both theoretically and experimentally. The p urpose of the present article is to calculate the physical adsorption isotherms of Ne and Rn on a heterogeneous surface over wide ranges of temperature, pressure, and coverage. The model used is the same as tha t used recently for hydrogen, wherein the isotherm was carried to the vapor pressure at the higher pressures, and to Henry's Law at the lowe st pressures, with the intermediate range being described by the Dubin in-Radushkevich (DR) equation. All that is needed for this model is a knowledge of the vapor pressures of the adsorbates as a function of te mperature, and the DR constant B. By comparison with measured values o f B-1/2 for the other inert gases, values of B (-1/2) of 440 cal/mol f or Ne and of 2685 cal/mol for Rn were obtained. Using these values, ca lculations were carried out for Ne at temperatures of 7, 10, 15, 20, 2 7.2 (normal boiling point), 40, 77.4, 300, and 500 K, and for Rn at te mperatures of 60, 77.4, 100, 150, 211. (normal boiling point), 300, an d 500 K, In all, the pressure range extended from 10 exp-18 to 10 exp 7 Torr (about 10 exp-16 to 10 exp 9 Rascals), and the relative coverag e range from 10 exp-15 to 10 exp 3 monolayers. Experimental data for t he physical adsorption of Ne on a number of adsorbents are compared to the calculations and give reasonable agreement considering the simpli city of the model, No experimental data for Rn are available. (C) 1997 American Vacuum Society.