Change in Direction of Thermophoresis in Dilute Binary Molecular Solutions
The temperature dependence of thermal diffusion (thermophoresis) in dilute binary solutions is examined through the refinement of a previously developed hydrodynamic model. Specifically, the influence of thermal molecular motion in the liquid surrounding a dissolved solute on the interaction with that solute is considered. An effective interaction potential is calculated by averaging the standard dipole-dipole interaction potential over all possible positions of the solvent molecules using a Boltzmann factor. In this calculation, steric limitations to the approach of solvent molecules are considered. The resulting potential, which is proportional to absolute temperature, is used to calculate thermal diffusion coefficients for dilute binary mixtures. It is demonstrated that for certain values of the relevant molecular parameters, these systems demonstrate a change in the direction of thermophoresis. The results are consistent with reports regarding poly(N-isopropyl acrylamide) in monohydric alcohols, where the direction of thermophoresis changes with temperature, as well as with the chain length of the alcohol solvent at a given temperature. Our results predict that strong temperature dependence in thermophoresis is expected when solvent-solute interaction is weak and the hydrodynamic radius of the solvent is much smaller than that of the solute.
Semen N. Semenov and Martin Schimpf. "Change in Direction of Thermophoresis in Dilute Binary Molecular Solutions" Eighth International Meeting on Thermodiffusion. Bonn, Germany. Jun. 2008.
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