Study of the soret effect in monosaccharide solutions.

We investigated the thermal diffusion behavior of aqueous solutions of monosaccharides with the infrared thermal diffusion forced Rayleigh scattering (IR-TDFRS) setup. As monosaccharides, we studied the three aldohexoses glucose, galactose, and mannose and the two ketohexoses sorbose and fructose. All sugars have the same molecular weight, but their structures differ as well as some physical properties such as viscosity, density, thermal expansion coefficient, and optical rotation. Additionally, we measured the viscosity and the optical rotation of the monosaccharide solutions in the investigated temperature and concentration range. While there is a clear correlation between the structure and the thermal diffusion behavior for alkanes, the situation is much more complicated for the monosaccharides. Nevertheless, as in the case of the alkanes, we find a correlation between the thermal diffusion coefficient with the ratio of the thermal expansion coefficient and the kinematic viscosity. We discuss the physical principles, which connect the thermal diffusion behavior with other thermophysical properties and the structure of the different sugars.