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

• Applicant: Privatdozentin Dr. Simone Wiegand
• Subject Area: Preparatory and Physical Chemistry of Polymers
• Term: from 2004 to 2011
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 5441916

• Final Report Year: 2011

Final Report Abstract

In multicomponent fluids a diffusive mass flow can be caused not only by a concentration but also by a temperature gradient. This so-called Soret effect (thermodiffusion) is even nowadays still poorly understood. In this project we investigated simple non-polar and polar molecules with a spherical or chain-like structure. Reliable data could be received by systematic and meaningful variation of experimental parameters. Hence, conclusions regarding the underlying mechanism could be derrived. As experimental techniques a holographic method and a thermal lens set-up operating in the infrared have been developed. Refractive index contrast factors, in particular the temperature dependence have been accurately determined by an interferometer. We found for simple non-polar spherical molecules that the heavier molecules enrich in the cold region. We experimentally observed this behavior for three different molecules dissolved in tetra carbon chloride. The same trend could be confirmed by reverse non-equilibrium molecular dynamic simulation. In the next step we showed by systematic studies of linear and branched alkanes, that there is a strong tendency for anisotropic molecules to enrich at the warm side, even if their mass is larger. This tendency disappears gradually with an increasing degree of branching. For the most spherical heptane we observe the normal, which means the molecule with the higher mass enriches at the cold side. We observed the same trend for different organic ring compounds as solvent. Whereas it is surprising that the determined Soret coefficient for o- and p−-xylene show the largest difference although their structure is very similar. This observation extends also to studies of polar mixtures. The chemistry of the molecules, which is illustrated by the outer layers in the cartoon, plays an important role and can even lead to sign changes. Measurements of aqueous mixtures showed that a sign change of the Soret coefficient can be related to a structural change in the liquid. The chemical by a single molecule but also the contribution to Soret effect by interactions between the molecules is still an open question. Nevertheless we found several empirical correlations for non-polar and polar systems. For instance is there a clear correlation between the thermal diffusion coefficient and the ratio of the thermal expansion coefficient and the kinematic viscosity for alkanes as well as for sugar solutions. In non-polar mixtures we could assign to each component a heat affinity, which allows the calculation of the Soret coefficient of the mixture and could be related to the heat of evaporisation, density, mass and the asymmetry of the pure component and it was even possible to find an empirical correlation between the properties of the pure components and the Soret coefficients of the mixture. An additional interesting observation is, that the rigidity of the chain-like molecules is related to position of the plateau value of the thermal diffusion coefficient, DT . For more flexible molecules the plateau value is reached earlier. The same observation has also been made in molecular dynamic simulations, but there is still no microscopic understanding so far.

Publications

• (2006). Study of the thermal diffusion behavior of alkane/benzene mixtures by thermal diffusion forced rayleigh scattering experiments and lattice model calculations. J. Phys. Chem. B 110(51): 26215-26224
  P. Polyakov, J. Luettmer-Strathmann, et al.
  
• (2007). Ludwig-Soret effect of poly(N-isopropylacrylamide): Temperature dependence study in monohydric alcohols. Macromolecules 40(5): 1638-1642
  R. Kita, P. Polyakov, et al.
  
• (2007). Thermal diffusion measurements and simulations of binary mixtures of spherical molecules. J. Chem. Phys. 127(1): 014502
  P. Polyakov, M. Zhang, et al.
  
• (2008). Reverse Nonequilibrium Molecular Dynamics Calculation of the Soret Coefficient in Liquid Heptane/Benzene Mixtures. J. Phys. Chem. B 112(47): 14999-15004
  P. Polyakov, F. Müller-Plathe, et al.
  
• (2008). Systematic study of the thermal diffusion in associated mixtures. J. Chem. Phys. 128(3): 034505
  P. Polyakov and S. Wiegand
  
• (2008). Thermal diffusion and molecular diffusion values for some alkane mixtures: A comparison between thermogravitational column and thermal diffusion forced Rayleigh scattering. J. Phys. Chem. B 112(28): 8340-8345
  P. Blanco, P. Polyakov, et al.
  
• (2009). Investigation of the Soret effect in aqueous and non-aqueous mixtures by the thermal lens technique. Phys. Chem. Chem. Phys. 11: 864-871
  P. Polyakov and S. Wiegand
  
• (2009). Study of the Soret Effect in Hydrocarbon Chain/Aromatic Compound Mixtures. J. Phys. Chem. B 113(40): 13308-13312
  P. Polyakov, E. Rossinsky, et al.
  
• (2010). Study of the Soret Effect in Monosaccharide Solutions. J. Phys. Chem. B 114(8): 2807-2813
  P. Blanco and S. Wiegand
  
• (2010). Thermal Diffusion of Oligosaccharide Solutions: The Role of Chain Length and Structure. J. Phys Chem. B 114(33): 10740-10747
  P. Blanco, H. Kriegs, et al.