Second virial coefficients of the CH4/CF4 system and intermolecular potentials

This work presents a careful consideration of the second virial coefficients for methane, tetrafluoromethane, and their binary mixtures. This system is characterized by the abnormal property that the virial coefficients of the mixture are not in between the virial coefficients of the pure components. In searching for an explanation for this phenomenon the potential parameters of the Lennard-Jones (n-6) and (n-7) families were determined from the temperature dependence of the second virial coefficients of the pure gases and their mixture. The study is based upon published data for the system as well as our own new measurements. It was concluded that the reason for this unusual behaviour is simply that the pure gases and their mixture do not follow the same intermolecular potential model. This was confirmed by two facts: first, they do not follow the law of corresponding states; and, second, the depth of the potential for the mixture and its zero are almost the geometric and arithmetic means of the corresponding parameters of the pure components, all obtained according to their respective best potential model. An application of the obtained parameters to calculate the viscosity coefficients indicated, however, that they can not describe both properties simultaneously.