Thermodiffusion of aqueous solutions of various potassium salts

Thermophoresis or thermodiffusion has become an important tool to monitor protein-ligand binding as it is very sensitive to the nature of solute-water interactions. However, the microscopic mechanisms underlying thermodiffusion in protein systems are poorly understood at this time. One reason is the difficulty to separate the effects of the protein system of interest from the effects of buffers that are added to stabilize the proteins. Due to the buffers, typical protein solutions form multicomponent mixtures with several kinds of salt. To achieve a more fundamental understanding of thermodiffusion of proteins, it is therefore necessary to investigate solutions of buffer salts. For this work, the thermodiffusion of aqueous potassium salt solutions has been studied systematically. We use thermal diffusion forced Rayleigh scattering experiments in a temperature range from 15 °C to 45 °C to investigate the thermodiffusive properties of aqueous solutions of five potassium salts: potassium chloride, potassium bromide, potassium thiocyanate, potassium acetate, and potassium carbonate in a molality range between 1 mol/kg and 5 mol/kg. We compare the thermophoretic results with those obtained for non-ionic solutes and discuss the thermophoresis of the salts in the context of ion-specific solvation according to the Hofmeister series.