On the microscopic origin of Soret coefficient minima in liquid mixtures

TL;DR

This paper investigates the microscopic origins of Soret coefficient minima in liquid mixtures, demonstrating their generality across non-polar Lennard-Jones liquids and linking them to thermodynamic and structural factors.

Contribution

It reveals the microscopic mechanism behind Soret coefficient minima and shows their occurrence in non-aqueous Lennard-Jones mixtures, extending understanding beyond aqueous solutions.

Findings

Soret coefficient minima are present in Lennard-Jones mixtures.

The minima are linked to a minimum in the thermodynamic factor.

Microscopic origin explained via atomic coordination structure.

Abstract

Temperature gradients induce mass separation in mixtures in a process called thermodiffusion and quantified by the Soret coefficient. The existence of minima in the Soret coefficient of aqueous solutions was controversial until fairly recently, where a combination of experiments and simulations provided evidence for the existence of this physical phenomenon. However, the physical origin of the minima and more importantly its generality, e.g. in non-aqueous liquid mixtures, is still an outstanding question. We report the existence of a minimum in liquid mixtures of non-polar liquids modelled as Lennard-Jones mixtures, demonstrating the generality of this phenomenon. The Soret coefficient minimum originates from a coincident minimum in the thermodynamic factor, and hence denotes a maximimzation of non-ideality mixing conditions. We explain the microscopic origin of this effect in terms of the atomic coordination structure of the mixtures.