On the microscopic origin of Soret coefficient minima in liquid mixtures

OR Gittus and F Bresme, PHYSICAL CHEMISTRY CHEMICAL PHYSICS, 25, 1606-1611 (2023).

DOI: 10.1039/d2cp04256h

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 at specific salt concentrations 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. Here, we report the existence of a minimum in liquid mixtures of non-polar liquids modelled as Lennard-Jones mixtures, demonstrating the generality of minima in the Soret coefficient. The minimum originates from a coincident minimum in the thermodynamic factor, and hence denotes a maximization of non- ideality mixing conditions. We rationalize the microscopic origin of this effect in terms of the atomic coordination structure of the mixtures.

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