Thermal diffusion measurements and simulations of binary mixtures of spherical molecules

Thermal diffusion forced Rayleigh scattering measurements on binary mixtures of carbon tetrabromide (CBr4), tetraethylsilane, and di-tert-butylsilane in carbon tetrachloride (CCl4) are reported at different temperatures and concentrations. The Soret coefficient of CBr4 in CCl4 is positive and S-T of both silanes in CCl4 is negative, which implies that the heavier component always moves to the cold side. This is the expected behavior for unpolar simple molecules. Both silanes have the same mass so the influence of the difference in shape and moment of inertia could be studied. For all three systems, S-T decreases with decreasing CCl4 concentration. The results are discussed in the framework of thermodynamic theories and the Hildebrand parameter concept. Additionally, the Soret coefficients for both silane/CCl4 systems were determined by nonequilibrium molecular-dynamics calculations. The simulations predict the correct direction of the thermophoretic motion and reflect the stronger drive toward the warm side for di-tert-butylsilane compared to the more symmetric tetraethylsilane. The values deviate systematically between 9% and 18% from the experimental values. (c) 2007 American Institute of Physics.