Intermolecular Force and Transport Coefficients

Abstract

THE Chapman–Enskog theory of non-uniform gases expresses the transport properties in terms of a set of collision integrals¹ which depend on the law of molecular interaction. These integrals have been evaluated for various molecular models and the computed values of the transport coefficients have been compared with the experimentally observed values by Chapman and Cowling. Hirschfelder, Bird and Spotz², Winter³ and others have shown that the Lennard-Jones 12 : 6 potential energy function is more in accord with the observed properties of gases than any other model, but the values of the force constants ɛ and r₀ employed by them were deduced from viscosity data. It is well known that the coefficient of thermal diffusion is far more sensitive to the type of molecular interaction than the three elementary gas coefficients. We have, therefore, utilized the data on thermal diffusion, and in particular the variation of the thermal separation ratio R_T of argon isotopes with temperature, to calculate the force constant ɛ.