Three-Body Effects in Physical Adsorption

Abstract

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.