Two aqueous phase systems are obtained when water is added to small amounts of incompatible but water soluble polymers. This phenomenon has been applied to the industrial separation and purification of biomaterials, including plant and animal cells, virus, proteins and nucleic acids. This work aims to investigate theories and models for describing the thermodynamic behaviour of such complex systems. The first problem to face to is the description of the substratum, namely the multicomponent liquid liquid equilibrium for water-polymer-polymer and water-polymer-electrolyte systems. The paper deals also with the description of the protein distribution among the two liquid phases. A critical review focuses on the problem involved in modelling aqueous two phase systems and indicates the more interesting models available in the literature. A new model, based on the perturbed hard chain theory, is developed and results are reported for systems containing polymers and systems containing electrolytes. Good results are obtained for polyethylenglycol-water and for water-electrolyte systems.
