Thermophoresis and Thermal Diffusion

Motivation: It turns out that a simple nonequilibrium environment created by a temperature gradient can be used to monitor the reaction kinetics of large proteins with small substrate molecules. This is probably caused by a change in the hydration layer of the protein which is influenced by subtle conformations changes induced by the binding substrate molecule. The underlying effect is thermophoresis or thermal diffusion, which is the mass transport induced by a temperature gradient applied to a liquid mixture. The resulting separation of the components causes a concentration gradient parallel or antiparallel with respect to the temperature gradient. Beside the recently detected biophysical applications it is relevant in polymer characterization, and analysis of petrol reservoir characterization. Additionally the effect is also discussed as an accumulation mechanism for a hydrothermal emergence of life in the early stage of the earth and in the deep sea close to black smokers. The investigation of the thermophoresis and thermal diffusion in liquid mixtures is based on the determination of the transport coefficients D (mutual diffusion coefficient), DT (thermal diffusion coefficient) and ST (Soret coefficient). Experimentally we are determining these properties by Thermal Diffusion Forced Rayleigh Scattering (TDFRS) [1-3] and, hence, to provide a "thermophoretic scale", enabling a comparison with values obtained, by means of Molecular Dynamic (MD)-simulations and empirical parameters such as the log P, a partition coefficient describing the hydrophilicity of a compound. References: