Universal isotope effect in thermal diffusion of mixtures containing cyclohexane and cyclohexane-d12.

The Soret coefficients S(T) of the liquids acetone, benzene, benzene-d1, 1,3,5-benzene-d3, benzene-d5, benzene-13C6, benzene-d6, n-hexane, toluene, 1,2,3,4-tetrahydronaphthalene, isobutylbenzene, and 1,6-dibromohexane have been measured in protonated and perdeuterated cyclohexane by a transient holographic grating technique. It has been found that S(T) can be either positive or negative and even change its sign as a function of concentration. The isotope effect DeltaS(T)=-0.99 x 10(-3) K(-1), which is the change of S(T) after isotopic substitution of cyclohexane, neither depends on concentration nor on the nature of the mixing partner. Only in the case of the polar acetone is DeltaS(T) approximately 30% larger but still concentration independent. Based on the experimental findings, some general conclusions about the dependence of the Soret coefficient on molecular properties are drawn.