The explicit dependence of the thermal diffusion factor with respect to composition and interaction parameters for the van der Waals binary mixture is obtained in the framework of the mean-field kinetic variational theory and in the scheme of Scott and van Konynenburg. Here, we present a numerical study where the global behavior of the thermal diffusion factor is described in terms of molecular masses, sizes, and interaction parameters, along the phase diagram of this model mixture. This

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... al study allows us to understand what molecular parameters modify the sign of the thermal diffusion factor. In addition, a comparison is made between the thermal diffusion factor coming from the van der Waals mixture and from the hard-sphere mixture. © 1997 American Institute of Physics. ͓S0021-9606͑97͒51019-1͔ ph ͑relative to the local center of mass velocity, iϭ1,2), under the condition of no external forces and mechanical equilibrium (ٌ pϭ0), is 17,18 Here, we will confine ourselves to binary mixtures. In Eqs. ͑1͒, is the mass density, T is the temperature, D i j CM are the mutual diffusion coefficients, and D i T are the thermal diffusion coefficients. Equations ͑1͒ have been written in such a way that all the gradients occurring therein are independent. The choice of the component L is arbitrary, and although it is not explicitly stated, the D i j CM and the D i T will depend upon the choice of L. 17, 18 As mentioned, the property more often used to describe thermal diffusion in a binary mixture is the TDF, ␣ 12, which is defined at stationary states as with ␣ 12 ϩ␣ 21 ϭ0. The TDF is invariant to the change of reference system ͑center-of-mass or center-of-volume͒. a͒ On leave from Instituto de Fisica, UG. 8204