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 been explored extensively, both theoretically and experimentally. The purpose 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 that used recently for hydrogen, wherein the isotherm was carried to the vapor pressure at the higher pressures, and to Henry’s Law at the lowest pressures, with the intermediate range being described by the Dubinin–Radushkevich (DR) equation. All that is needed for this model is a knowledge of the vapor pressures of the adsorbates as a function of temperature, and the DR constant B. By comparison with measured values of B^−1/2 for the other inert gases, values of B^−1/2 of 440 cal/mol for Ne and of 2685 cal/mol for Rn were obtained. Using these values, calculations were carried out for Ne at temperatures of 7, 10, 15, 20, 27.2 (normal boiling point), 40, 77.4, 300, and 500 K, and for Rn at temperatures of 60, 77.4, 100, 150, 211.4 (normal boiling point), 300, and 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 Pascals), and the relative coverage range from 10 exp−15 to 10 exp 3 monolayers. Experimental data for the physical adsorption of Ne on a number of adsorbents are compared to the calculations and give reasonable agreement considering the simplicity of the model. No experimental data for Rn are available.