  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.
