Molecular origin of thermal diffusion in benzene + cyclohexane mixtures.

The isotope effect in thermal diffusion (Soret effect) of benzene+cyclohexane mixtures has been investigated by a holographic grating technique. The Soret coefficient can be split into additive contributions. One contribution, the isotope effect, stems from the differences of both mass and moment of inertia, and is independent of composition. An additional "chemical" contribution depends on concentration and even changes its sign at a benzene mole fraction x(benz) approximately 0.7. The mass effect is in agreement with molecular dynamics calculations: the heavier component migrates to the cold side.