Thermal Diffusion in Binary Gas Mixtures and Intermolecular Forces

The available binary thermal diffusion data for a number of gas systems as a function of temperature and composition have been critically examined and interpreted using the kinetic theory of Chapman and Enskog. In the computation of the thermal diffusion factor ${\alpha }_T$ two familiar central intermolecular potentials, viz., Lennard-Jones (12-6) and modified Buckingham exp-six have been considered. The theoretical expressions for ${\alpha }_T$ employed are of known and tolerable accuracy so that the uncertainty associated on this account in many early interpretations is avoided in this study. The scope of work is confined mostly to inert gas mixtures as the kinetic theory is rigorously developed so far only for such systems. This comprehensive effort brings to light many interesting points. A general deficiency is realized in the available ${\alpha }_T$ data. Even for some systems where data exist as a function of composition, it is found that the composition range which is theoretically interesting is not investigated. This study also leads to the unpleasant prospect that some of the available data may be in error. Thus, many indications are provided for planning future work on ${\alpha }_T$ measurements. Further, the L-J (12-6) potential is found to be inadequate for representing the force field of these molecules and especially for such systems where He is one of the two components. To be specific, the choice of a repulsive index as twelve is found to be a bit large. The exp-six potential has succeeded in general to explain the ${\alpha }_T$ data of many of the systems discussed in this article.