Three-Body Effects in Physical Adsorption

Third-order interaction data for argon, krypton, xenon, methane, and tetradeuteromethane on the highly graphitized carbon black P33(2700°) are analyzed by means of a two-dimensional model which specifically includes a term to account for three-body forces. The intermolecular pair potential in the gas phase is taken to have the Lennard-Jones (12, 6) form, and when the molecules are adsorbed in a monolayer we assume this potential to be modified by the addition of a repulsive three-body term proportional to r−3. For a given set of bulk-gas potential parameters the proportionality constant η in the r−3 term can be uniquely determined from the data, although η is found to be quite sensitive to the exact choice of bulk parameters. Three different theoretical formulas for η give only qualitative agreement with the empirical values. Surface areas determined from the data are insensitive both to the bulk-phase parameters and to the exact nature of the interaction model. Effective two-dimensional potential parameters for the (12, 6, 3) model differ by < 10% from the gas-phase parameters for all the systems studied, indicating that three-body effects in physical adsorption may be smaller than previously supposed.