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) = alpha( x) beta(T)+ S(T)(i), and for all but one (dibromohexane/cyclohexane) systems there exists a concentration where S(T) becomes temperature invariant, corresponding to a zero of alpha(x). beta(T) is of similar magnitude for all mixtures investigated. S(T)(i) neither depends on temperature nor on concentration, and isotopic substitution changes only S(T)(i), but not alpha(x) or beta(T). For benzene/cyclohexane S(T)(i) is related to differences of molecular mass and moment of inertia, and alpha(x) beta(T) is identified with the chemical contribution to the Soret coefficient.