\section*{Abstract}

Thermal diffusion is an intriguing phenomenon with an extensive list of possible applications. The effect is that in which a temperature gradient drives a mass flux, originally discovered by Ludwig and investigated further by Soret already in the late 19th century. Much research has gone into understanding this effect, and specifically into quantitatively predicting the coupling coefficient between heat and mass transport, still, no universally reliable model has so far been presented in the open literature.

Current developments within nano-engineering, bio-processing, hydrogen and \ce{CO2} storage and energy- and nuclear technology all warrant the development of an ability to accurately predict the effects of thermal diffusion in complex systems. Thus, this work will investigate more closely two approaches to modelling thermal diffusion.

The Enskog solutions to the Boltzmann equations for hard sphere systems have been shown to give accurate predictions in the low density regime. In this work, explicit evaluation of the collision integrals using Mie potentials is shown to produce highly accurate predictions of transport coefficients in the gas phase. Mie potential parameters obtained by regressing SAFT-type equations of state to equilibrium properties are revealed to give reliable predictions of transport coefficients when used with the Enskog solutions for Mie fluids. This provides a powerful link between the modelling of equilibrium and non-equilibrium properties using the same molecular interaction parameters and mixing rules.

The predictions obtained from the Enskog solutions for Mie fluids are compared to those obtained for hard spheres. It is found that the solutions for Mie fluids capture both temperature- and compositional effects that are not captured by the hard sphere solutions. This new implementation represents a significant improvement compared to the more commonly used solutions for hard spheres.

In a recent work, a specific model proposed by Kempers has been shown to give highly unreliable predictions of the Soret coefficient. The new implementation of the Enskog solutions for Mie fluids is used to investigate the underlying assumptions behind the Kempers model, and it is found that these assumptions likely do not hold. This analysis shows that future development of models for the Soret coefficient in the liquid phase should follow other approaches than that used by Kempers.