The insight that was gained from these predictions is that mass and Lennard-Jones size parameter have opposing effects on the Soret coefficient. The lower mass of component 1 alone would result in negative thermodiffusion, whereas the smaller size parameter would trigger positive thermodiffusion. It thus depends on which effect is stronger and hence which type of model parameter ratio is larger. In the case of mixture B, the mass ratio ultimately dictates the negative sign. The τ parameters used in the model by Shukla and Firoozabadi 27 were originally determined for real systems, for which size and mass typically correlate. It is therefore possible that these parameters require a re-optimization if size and mass strongly diverge, as seems to hold for mixture B. Very recently, Hoang and Galliero 89 came to a similar conclusion on the lack of reliability of the Shukla-Firoozabadi model for the prediction of thermal diffusion factors for Lennard-Jones mixtures that are asymmetric in size and mass.