Thermodiffusion in binary liquids: the role of irreversibility.

We study thermal diffusion in binary mixtures in the framework of non-equilibrium thermodynamics. Our formal result displays the role of partial enthalpies hi and Onsager's generalized mobilities Ai. The ratio A1/A2 provides a measure for the irreversible character of thermal diffusion. Comparison with experimental data on benzene, cyclohexane, toluene and n-alkanes shows that irreversibility is essential for thermal diffusion, and in particular for the isotope effect.