Temperature, Molecular Weight, and Concentration Dependences of Thermal Diffusion for Ethylene Glycol Oligomers and Crown Ethers

The Soret coefficient S T of ethylene glycol oligomers (EGOs) and crown ethers (CEs) in water were obtained by thermal diffusion forced Rayleigh scattering by changing the temperature, molecular weight, and concentration. The effect of a hydroxyl group on the EGOs and the effect of the cyclic structure of CEs on the thermal diffusion were determined systematically by changing the molecular weights of the EGOs and CEs. For dilute aqueous solutions, EGOs and CEs, except EG, show positive S T values that decrease with increasing temperature, which is similar to the results of previous studies on mixtures of water and organic solvents. The temperature dependence of S T changes its behavior from negative to positive with decreasing number of repeating units of EGOs. This behavior is related to the increase in the number density of the hydroxyl group. The S T values of EG show two different concentration regions, namely, the low concentration (0 2 wt%) and high concentration (2 100 wt%) regions.