Driving forces for phase separation and partitioning in aqueous two-phase systems.

A set of simple analytical equations, derived from the Flory-Huggins theory, are used to identify the dominant driving forces for phase separation and solute (e.g., protein) partitioning, in the absence and presence of added electrolyte, in every general class of aqueous two-phase systems. The resulting model appears to capture the basic nature of two-phase systems and all trends observed experimentally. Case studies are used to identify fundamental differences in and the magnitudes of enthalpic and entropic contributions to partitioning in polymer-polymer (e.g., PEG-dextran), polymer-salt, and thermoseparating polymer-water (e.g., UCON-water) two-phase systems. The model therefore provides practitioners with a better understanding of partition systems, and industry with a simple, fundamental tool for selecting an appropriate two-phase system for a particular separation.