On the temperature dependence of thermal diffusion of liquid mixtures

Concentration- and temperature-dependent measurements of the Soret coefficient S_T have been performed for binary mixtures containing benzene and cyclohexane, both of different degrees of deuteration, and dibromohexane. They are discussed together with data on ethanol/water, toluene/n-hexane, and benzene in heptane, tridecane, and 2,2,4-trimethylpentane taken from the literature. S_T can be negative or positive and change its sign both as a function of concentration and temperature. For all systems the Soret coefficient can be expressed by the empirical function S_T = α(x)β(T)+S_Tⁱ, and for all but one (dibromohexane/cyclohexane) systems there exists a concentration where S_T becomes temperature invariant, corresponding to a zero of α(x). β(T) is of similar magnitude for all mixtures investigated. S_Tⁱ neither depends on temperature nor on concentration, and isotopic substitution changes only S_Tⁱ, but not α(x) or β(T). For benzene/cyclohexane S_Tⁱ is related to differences of molecular mass and moment of inertia, and α(x)β(T) is identified with the chemical contribution to the Soret coefficient.