We could identify a limitation of the theoretical model by Shukla and Firoozabadi. Importantly, a detailed uncertainty analysis, which has, to the best of our knowledge, not been performed for the model yet, and complementing insights from a regression model, which leverages model parameters of the mixture chosen, helped us pinpointing the probable reason. The theoretical model is unable to account for opposing effects of mass and size on the Soret coefficient. A parameter that is mainly kinetic in nature plays here a major role in dictating the sign of the Soret coefficient and thus in the applicability of the analytical model. More elaborate models such as those by Artola et al. could be used instead, but this model requires, for example, more transport data.