We report experimental data showing the effect of temperature on polymer partitioning in the dextran (DEX500) -polyethylene glycol (PEG8000) -water system. Increasing temperature increases the concentration of PEG in the top phase and decreases the concentration of DEX in the bottom phase. A solution thermodynamic model based on local compositions with temperature-dependent parameters correlates the experimental data well, and an efficient Gibbs-free energy minimization algorithm for phase equilibrium calculations is described. The partial molar enthalpies and entropies of each of the solutes, calculated with the model, are negative and decrease in magnitude as temperature increases.

The role of polymer concentration, ionic strength, and salt type on the partitioning of proteins in aqueous two-phase systems is studied both experimentally and theoretically. Polymer-induced protein-protein interactions are also considered in terms of a perturbation theory, and are shown to not significantly affect the protein partition coefficient at moderate concentrations. A thermodynamic model combining the UNIQUAC and extended Debye-Hückel equations accounts for changes in salt type and salt concentration on protein partitioning with good accuracy, and calculated protein partition coefficients are in good agreement with experimental observations. Based on this detailed thermodynamic analysis, a simple model for the protein partition coefficient is developed and is shown to be useful for correlating measurements.

We consider the effects of the addition of poly(ethylene glycol) (PEG) of different molecular weights to aqueous two-phase system of PEG 8000 and dextran 500. The first purpose of this study was to determine the molecular weight partitioning of the polymers themselves so that, for example, aqueous two-phase separations using affinity ligands can be improved. The second purpose was to examine whether this molecular weight partitioning could be predicted by using solution thermodynamic models so that it would be possible to optimize affinity partitioning without extensive laboratory work. Experimentally, we find that, by increasing the PEG concentration of any molecular weight in the feed, the high molecular weight PEG concentration in the dextran-rich phase is reduced. This observation can be used to reduce the loss of expensive ligated PEG used in affinity partitioning. Further, there is generally good agreement between our experimental data and the predictions of a solution thermodynamic model.

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The partitioning of salts in aqueous two-phase systems formed from polyethylene glycol, dextran, and water produces an electrostatic potential difference between the phases which influences the partitioning of proteins and other biomolecules. In this paper we study the partitioning of, separately, NaCl, NaHaPO*, and NaHSO* in such systems, as well as the resulting electrostatic potential differences. Polymer concentrations were measured by liquid chromatography and ion concentrations by atomic absorption or inductively coupled emission spectroscopy. A combination of the UNIQUAC, Debye-Hückel, and Bronsted-Guggenheim equations, after accounting for differences in the standard states, resulted in a thermodynamic model that correlates the concentrations of all species (polymers, ions, and water) and allows calculation of the resulting electrostatic potential difference. At comparable concentrations, NaHSO* produces the largest electrostatic potential difference between the phases and NaCl the smallest.