| Summary: | A theoretical and experimental study of the phase behavior of aqueous salt-polymer two-phase systems has been done. A statistical mechanical model has been developed for the chemical potential of every component in the salt-polymer-water system. The model incorporates the effect of short-range forces by use of the isothermal-isobaric osmotic pressure expansion of Hill. The effect of long-range forces such as electrostatic interactions is incorporated with a non-primitive electrolyte model based on the work of Pailthrope et al. and on Kirkwood-Buff theory. The effect of polymer-polymer and polymer-salt interactions is represented in the model by Hill osmotic virial coefficients. The polymer molecular weight dependence of the second virial coefficients is predicted with the results of polymer scaling laws. An isopiestic experiment has been developed to measure the thermodynamic activity data required to evaluate the model parameters. Six different aqueous mixtures of polyethylene glycol 1000 and 8000 and MgSO₄, Na₂SO₄, and Na₂CO₃ were studied at 25°C, 1 ATM. Phase diagrams for these six systems were calculated from the model and compared to experiment with good agreement between them.
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