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 and Onsager's generalized mobilities. The mobility ratio provides a measure for the irreversible character of thermal diffusion. Comparison with experimental data on benzene, cyclohexane, toluene and alkanes shows that irreversibility is essential for thermal diffusion, and in particular for the isotope effect.