Physical adsorption of argon, krypton and nitrogen on iron and pyrex at very low pressures

Abstract

Physisorption of argon, krypton, and nitrogen on Pyrex and iron was studied by a static technique in a constant volume system, in the temperature range 77.4 to 90.2°K and in the pressure range 10⁻⁹ to 10⁻⁴ torr. Analyses via Electron Spectroscopy for Chemical Analysis (ESCA), Scanning Electron Microscopy (SEM), and Neutron Activation Analysis (NAA) were made to characterize the solid sample surfaces. Both Pyrex and iron surfaces were degreased with ethylene dichloride. The iron surface was further cleaned using the Diversey process. Both surfaces were given a mild bake in vacuum. A modified Dubinin-Radushkevich-Polanyi (D-R-P) theory was used for interpreting the experimental results. This theory assumes that the adsorbed phase is not necessarily liquid-like and is a function of adsorption temperature. Comparisons were made between the modified and the original theories and it was found that the modified theory had a wider applicability and gave better data correlation for all five systems in the present study. It was found that the Dubinin-Radushkevich (D-R) parameter B, and the limiting (maximum) adsorption energy, (π/4)^½ (B), ^-½ of all systems were primarily dependent on the adsorbate. A correlation between the limiting mean adsorption energy and the polarizability of adsorbate was made. From the D-R parameter, N_m, it was found that the effect of the Diversity process on iron was to increase the surface area.

Isotherms of the argon/Pyrex system showed that Henry's Law behavior was reached at low coverages and high temperatures. Isosteric heats of adsorption were calculated for all systems. Isosteric heat versus coverage curves indicated that the Pyrex surface was heterogeneous and the iron surface was less heterogeneous. The magnitude of isosteric heats also indicated that all three gases were physisorbed on the Pyrex and iron.

Work function measurements were taken simultaneously with the adsorption isotherm measurements. The results were marginal but confirmed that nitrogen was physisorbed on iron. Water contact angles on Pyrex and iron surfaces were also measured. It was found that the contact angle was very sensitive to the surface contamination and that ordinary laboratory air may contaminate the sample surface very rapidly. A finite water contact angle on Pyrex was found instead of a zero contact angle as is generally used in many works.