On physical adsorption: VIII. Monolayer adsorption of argon and nitrogen on graphitized carbon

Abstract

Low-pressure adsorption isotherms of argon and nitrogen at 77.8°K. and 90.1°K. on graphitized carbon black, P-33, are reported. It is found that the adsorbed monolayer behaves like a two-dimensional ideal gas at the lowest pressures measured; and that its behavior at slightly higher pressures can be described by a two-dimensional van der Waals equation. The critical temperature of the adsorbed monolayer is estimated from the van der Waals constants. Above about 50% coverage the adsorption isotherm has the form of the Langmuir equation; which is interpreted as the adsorption of a mobile film on localized sites, where the sites are superposed on the low-energy solid substrate by the packing of the adsorbate. The isosteric heats of adsorption are calculated as a function of the fraction of the surface covered, and are found to follow the same general pattern as that reported by Amberg, Spencer, and Beebe (4) for krypton adsorbed on an identical substrate.