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PubMed2006

Thermal diffusion of dextran in aqueous solutions in the absence and the presence of urea.

The Ludwig-Soret effect was studied for aqueous solutions of dextran in the temperature range 15 < T < 55 degrees C taking into account the effect of the addition of urea. In the absence of urea, the Soret coefficient S(T) changes sign; it is positive for T > 45.0 degrees C but negative for T < 45.0 degrees C. The positive sign of S(T) means that the dextran molecules migrate toward the cold side of the fluid; this behavior is typical for polymer solutions, whereas a negative sign indicates the macromolecules move toward the hot side. The addition of urea to the aqueous solution of dextran rises S(T) and reduces the inversion temperature. For 2 M urea the change in the sign of S(T) is observed at T = 29.7 degrees C and beyond that value S(T) is always positive in the studied temperature range. To rationalize these observations, it is assumed that the addition of urea leads to an opening of hydrogen bonds similar to that induced by an increase in temperature.

Dextrans↗
PubMed2005

Universal isotope effect in thermal diffusion of mixtures containing cyclohexane and cyclohexane-d12.

The Soret coefficients S(T) of the liquids acetone, benzene, benzene-d1, 1,3,5-benzene-d3, benzene-d5, benzene-13C6, benzene-d6, n-hexane, toluene, 1,2,3,4-tetrahydronaphthalene, isobutylbenzene, and 1,6-dibromohexane have been measured in protonated and perdeuterated cyclohexane by a transient holographic grating technique. It has been found that S(T) can be either positive or negative and even change its sign as a function of concentration. The isotope effect DeltaS(T)=-0.99 x 10(-3) K(-1), which is the change of S(T) after isotopic substitution of cyclohexane, neither depends on concentration nor on the nature of the mixing partner. Only in the case of the polar acetone is DeltaS(T) approximately 30% larger but still concentration independent. Based on the experimental findings, some general conclusions about the dependence of the Soret coefficient on molecular properties are drawn.

Journal Article↗