Thermal Diffusion of Dextran in Aqueous Solutions in the Absence and the Presence of Urea

Abstract

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.

Thermal Diffusion of Oligosaccharide Solutions: The Role of Chain Length and Structure

“…Previously, Sugaya et al 24 observed that the polysaccharide dextran accumulates at the warm side for temperatures below 45 • C. This shows that in the case of aqueous solutions the contribution due to specific interactions can be more important than the mass difference, especially at low temperatures, where the hydrogen bonds play an important role. Figure 2).…”

“…A similar trend has been also found in many systems, for instance, in aqueous suspensions of polystyrene spheres for different sizes of particles, 25 in water and in water/ethanol solutions of polyethylene oxide, 37 and in aqueous solutions of dextran. 24 At low temperatures (T=15-20 • C) D T is practically independent of the mass concentration, while the differences become higher when increasing both the temperature and the sugar concentration. In other words, the higher the sugar concentration, the less pronounced is the increase of D T with temperature (cf.…”

Temperature Dependence of Soret Coefficient in Aqueous and Nonaqueous Solutions of Pullulan

“…A similar behavior of S T was reported in another polysaccharide solution, dextran in water, which was the first reported polysaccharide system for which the thermal diffusion has been studied and which shows a sign change with temperatur. 24 Dextran is mainly composed of α- The sign change temperatures of the aqueous solutions of pullulan and dextran are at 41.7 and 45.0 • C, which is much higher than the temperature, T = 4 • C, with the largest density of water.…”

“…[16][17][18][19] Some aqueous solutions of polymers show negative Soret coefficients, which correspond to a migration of the polymers to the warm side. [20][21][22][23][24][25] It has been revealed that a sign of S T of the polymer in solution can occur as function of the solution temperature as well as of the solvent composition. [26][27][28][29][30] For liquid mixtures composed of small molecules thermally induced sign change of S T have been studied by several researchers [31][32][33][34] , sometime those sign changes could be correlated with structural changes in the liquid mixtures.…”

“…The reason is probably that the addition of urea destroys the hydrogen bonding network. 24 Thermally induced sign changes of the Soret coefficient can also be found in biological polymers, proteins, DNA and polysaccharide.…”