Physical adsorption of krypton on graphite over a wide density range. A comparison of the surface excess of simple fluids on homogeneous surfaces

Abstract

The adsorption of krypton on graphitized carbon has been measured by a gravimetric method from 0.01 to 150 bar over a temperature range from 253 to 373 K (1.2T_c to 1.8T_c) corresponding to densities of the ideal gas up to the critical density ρ_c. The measurements yield the surface excess concentration Γ^σ of the adsorbed fluid. Along each isotherm Γ^σ is found to pass through a maximum at a density near 0.5ρ_c. At temperatures below 1.5T_c the surface excess becomes significantly greater than that expected for a single compressed monolayer in contact with fluid of uniform bulk density.

The surface excess isotherms of krypton are compared with similar results for argon and methane on the basis of reduced variables Γ^*=Γ^σσ², ρ^*=ρσ³ and T^*=kT/ε_s, where σ is the molecular diameter and ε_s is the well-depth of the gas–solid potential. The reduced isotherms of the rare gases agree within 2% over the experimental range of argon (ρ^* < 0.2, T^*= 0.26–0.28). The reduced adsorption of methane is lower than that of krypton near the maximum of the isotherms at reduced temperatures T^*= 0.20–0.25. This can be attributed to the fact that the gas–solid interaction parameter ε_s/k of methane is larger while its gas–gas interaction ε/k is lower than the corresponding parameters of krypton.