The role of molecular interactions in the change of sign of the Soret coefficient

The change of sign with composition in aqueous mixtures of associating fluids is analyzed by means of molecular-dynamics simulations. The results obtained are in quantitative agreement with the experimental data in water-ethanol and water-methanol solutions, which exhibit the mentioned change of sign. A subsequent theoretical analysis is addressed to establish a relationship between the dependence of the Soret coefficient with composition with the existence of large inter-species interactions. Although the change of sign of the Soret coefficient with composition is not only due to the so-called chemical contribution analyzed here, we discuss the role that these interactions play in such a change of sign.