Analyzing the concentration-dependent Soret coefficient minimum in salt solutions: an overview

Temperature gradients often cause the separation of the components in liquid mixtures by a process called thermodiffusion and quantified by the Soret coefficient. In recent years, the existence of minima in the Soret coefficient as a function of concentration has been investigated by experiments and simulations for various aqueous salt solutions. In this paper, we analyze the data of ten 1 : 1 electrolytes (lithium, sodium and potassium chloride, lithium, sodium and potassium iodide, potassium acetate, sodium and potassium thiocyanate and guanidinium chloride) in water, together with those of newly measured Soret coefficients for aqueous cesium iodide solutions. The latter were measured in the temperature range between 15 °C and 45 °C and concentrations between 0.5 and 3 moles per kg of the solvent using thermal diffusion forced Rayleigh scattering. We analyze the data by expressing the Soret coefficients as products of two factors, one purely thermodynamic factor and one being the ratio of two Onsager coefficients. It turns out that the ratio of Onsager coefficients is the main factor responsible for the non-monotonic behavior of the Soret coefficients, contrary to recent findings using computer simulations of binary Lennard-Jones mixtures. Moreover, for salts with the same anion, we find that the thermodynamic factors increase with increasing Pauling radii of the cations, while the Onsager ratios increase monotonically with the radii of the hydrated cations.