INFLUENCE OF DISTRIBUTION OF ATOMIC MASSES WITHIN MOLECULE ON THERMAL DIFFUSION .2. ISOTOPIC METHANE AND METHANE/ARGON MIXTURES

The temperature dependence of the thermal diffusion factor for several isotopic mixtures of methane and mixtures of argon and methane was studied in two swing separators between 80δK and 300δK. The isotopic mixtures showed a noticeable influence of the mass-distribution within the molecule, whereas a description with a Lennard-Jones (12,6) potential remained still possible. These mixtures could be classified roughly into two categories, one with a potential well depth of 190-196δK, the other with one of 223-225δK. The first consisted of mixtures of spherical top molecules with zero difference in the moments of inertia, the second of mixtures of molecules with different moments of inertia. From the comparison between theoretical and experimental results it is concluded that viscosity is not seriously affected by the presence of inelastic collisions, while diffusion and thermal diffusion would give smaller and larger ε{lunate}-values, respectively, if described by the Chapman-Enskog theory. For all studied mixtures of methane and argon the inversion temperature was found to be 137.5°K. A concentration reversion in these mixtures gave an unexpected small change of the thermal diffusion factor. Inelastic collision between argon and methane are probably much less important than those between two methane molecules. © 1968.