Two-Dimensional Second Virial Coefficients of Methane and Tetradeuteromethane on Graphitic Carbon

Specific two-dimensional second virial coefficients are obtained from low-coverage adsorption isotherms for CH4 and CD4 on graphitic carbon between 220° and 290°K. These data are analyzed in terms of both classical and quantum-statistical models employing a Lennard-Jones potential, and corrections for non-planarity of the adsorbed phase are also included. The sign of the quantity [ε(H)—ε(D)]/ε(D) cannot be unambiguously determined, but this quantity is somewhat smaller than in the corresponding three-dimensional case. The measured nonadditivity of pair potentials amounts to about 15% of the gas-phase energy, and is in good agreement with independent theoretical predictions.