Statistical thermodynamics of phase separation and ion partitioning in aqueous two-phase systems.

A general model for the phase behavior of polymer-polymer aqueous two-phase systems containing small amounts of added inorganic salts has been developed from statistical thermodynamics. The model is based on the solution theory of Hill and new electrolyte solution model based on Fluctuation Solution Theory. It includes the effect of polymer molecular weight with scaling expressions from the Renormalization Group theory of polymer solutions. The model has been used to calculate the phase diagram and the partitioning of salt for an aqueous two-phase system containing polyethylene glycol (MW = 8000) and dextran (MW = 28,700) with 0.1 mole/kg of added Na2SO4. The calculations have been compared to experimental results with good agreement.