Self-consistent Field Analysis of Segregative Aqueous Dextran-Polyethylene Glycol Solutions: (1) Bulk Phase Diagrams.

Quasi-ternary dextran-water-polyethylene glycol (D-W-PEG) systems can undergo a segregative phase transition from a homogeneous to two-phase state with one phase enriched in dextran, the other in PEG, and both phases typically dominated by the spectator-like water component. The relation between the driving forces and the resulting phase behavior for such aqueous two-phase systems (ATPSs) is systematically studied with a Scheutjens-Fleer self-consistent field (SF-SCF) theory. We consider both the repulsive interactions between the polymeric species (the major driving force) and the solvent quality disparity (the minor driving force). In a narrow parameter range, when the major driving force is very weak, a closed-loop two-phase region with two critical points may be found. However, in more realistic parameter settings, an ATPS has an'open' binodal with a single critical point. We report on the volume ratio, the composition of the phases, (structural) parameters of the interface, such as the width, and the interfacial tension, as a function of the (average) water fraction in the system. We show that the volume-management strategy influences the phase diagram when polymers are polydisperse. By fitting a set of experimental phase diagrams, SF-SCF model parameters for D-W-PEG systems are established, which will be used in future to underpin experiments for these systems.