Pullulan and dextran: uncommon composition dependent Flory-Huggins interaction parameters of their aqueous solutions.

Vapor pressure measurements were performed for aqueous solutions of pullulan ( M w 280 kg/mol) and dextran ( M w 60 and 2100 kg/mol, respectively) at 25, 37.5, and 50 degrees C. The Flory-Huggins interaction parameters obtained from these measurements, plus information on dilute solutions taken from the literature, show that water is a better solvent for pullulan than for dextran. Furthermore, they evince uncommon composition dependencies, including the concurrent appearance of two extrema, a minimum at moderate polymer concentration and a maximum at high polymer concentration. To model these findings, a previously established approach, subdividing the mixing process into two clearly separable steps, was generalized to account for specific interactions between water and polysaccharide segments. Three adjustable parameters suffice to describe the results quantitatively; according to their numerical values, the reasons for the solubility of polysaccharides in water, as compared with that of synthetic polymers in organic solvents, differ in a principal manner. In the former case, the main driving force comes from the first step (contact formation between the components), whereas it is the second step (conformational relaxation) that is advantageous in the latter case.

Liquid/gas and liquid/liquid phase equilibria of the system water/bovine serum albumin.

The thermodynamic behavior of the system H2O/BSA was studied at 25 °C within the entire composition range: vapor pressure measurements via head space sampling gas chromatography demonstrate that the attainment of equilibria takes more than one week. A miscibility gap was detected via turbidity and the coexisting phases were analyzed. At 6 °C the two phase region extends from ca. 34 to 40 wt % BSA; it shrinks upon heating. The polymer rich phase is locally ordered, as can be seen under the optical microscope using crossed polarizers. The Flory-Huggins theory turns out to be inappropriate for the modeling of experimental results. A phenomenological expression is employed which uses three adjustable parameters and describes the vapor pressures quantitatively; it also forecasts the existence of a miscibility gap.

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.