Experimental study of the thermal diffusion behaviour of mixtures consisting of simple and chain-like molecules

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We plan systematic measurements on the thermal diffusion behaviour (Soret effect) in mixtures of rigid and chain-like molecules with and without specific interactions. The goal is to gain a better microscopic understanding of the thermal diffusive process. The data will be obtained by means of a holographic grating technique the so-called thermal diffusion forced Rayleigh scattering (TDFRS). We want to study three groups of low molecular weight mixtures: Rigid molecules, chain-like molecules and molecules with specific interactions. All experiments will be executed in close collaboration with the theoretical group of Prof. Dr. Florian Müller-Plathe (IU Bremen), see his part in this tandem application. First, we want to gain insight in the influence of mass and the moment of inertia on the Soret coefficient by studying stiff low molecular weight molecules, e.g. tetrahedral or ring shaped. The focus should be on systems with no specific interactions as hydrogen bonds. Deuterated solvents should be used in order to study the influence of mass and moment of inertia. The concentration and temperature dependence of the systems should also be studied. Furthermore the influence of conformational changes should be studied. Secondly, we want to study whether the independence of the thermal diffusion coefficient DT on the chain length, which has been observed for polymer solutions is still valid for oligomer solutions and for well-defined chain like molecules. These are better defined than oligomers, which often differ in end group and structure. Systematic measurements are also planned for oligomers to check whether it is possible to find a general expression for the Soret coefficient ST which describes oligomers of different chain length. Finally, we want to extend our study to systems with specific interactions as hydrogen bonds. Our main focus is on aqueous solutions. We want to investigate the influence of the mass, the solvent quality and specific interactions, such as hydrogen bonds on the thermal diffusive process. One particular system of interest is ethylene glycol (EG) in ethanol/water. For poly(ethylene glycol), one observes a sign change of the Soret coefficient as function of solvent composition. Systematic studies of the molecular weight, concentration dependence and the study in the deuterated solvent mixture should give insight in the microscopicmechanism of the thermal diffusion process. Computer simulations will accompany the experiments in order to understand the process at the mi- croscopic level.