A NEW EXCLUDED-VOLUME THEORY AND ITS APPLICATION TO THE COEXISTENCE CURVES OF AQUEOUS POLYMER 2-PHASE SYSTEMS

A theoretical description of the coexistence curves of binary and some ternary systems has been developed using methods of statistical geometry. It is based on the concept that macroscopically any molecular species in a solution is distributed at random and that every system composition on the binodal is a geometrically saturated solution of each solute in the presence of the other. The link between the model and thermodynamics is also established. The model has been applied to aqueous polydisperse polymer two-phase systems, and it is shown that when the molecular weights of the two phase-forming components are rather disparate, only one parameter, the effective excluded volume, is needed to reproduce experimental coexistence curves for polyethylene glycol-Dextran-water systems. The current model has been compared with some other approaches (lattice solution theories, treatments based on the Setschenow equation) and the deficiencies of the Setschenow equation and the current model have been commented on.