Viscosity and thermal conductivity from non-conformal potentials. I Theory and application to noble gases

The approximate non-conformal (ANC) theory provides effective spherical potentials for more than 40 pure real gases and predicts many of their binary mixtures in the case of the second volumetric virial coefficients B(T). Each substance is characterized by only three parameters; besides the usual minimum energy ε and its corresponding position r _m, a softness parameter s is proposed to take account of the form of a particular potential. In this work the ANC theory is extended to take account of viscosity η(T) and thermal conductivity λ(T) coefficients in dilute gases. Very simple expressions are derived to calculate the appropriate collision integral Ω as a function of temperature and in terms of the parameters ε, r _m and s, using a reference collision integral Ω₀(T). The theory is successfully applied to the noble gases Ne, Ar, Kr and Xe, with deviations from experiment for η(T) and λ(T) within experimental uncertainties. In the case of Ne, the ANC potential obtained here reproduces simultaneously η(T) and λ(T) as well as B(T) within experimental uncertainty, using a single set of parameters. A prediction for the radon gas is proposed.