THERMODYNAMICS OF AQUEOUS POLY(ETHYLENE GLYCOL)-DEXTRAN 2-PHASE SYSTEMS USING THE CONSISTENT OSMOTIC VIRIAL EQUATION

In this work we present a thermodynamic model for the prediction of the liquid-liquid phase behavior of aqueous poly(ethylene glycol) (PEG) - dextran two-phase systems. The model is based on the McMillan-Mayer solution theory (1945) and results in thermodynamically consistent expressions for the chemical potentials of the solutes derived from the osmotic virial equation (COVE). Applying the COVE, we have examined the predictability using a complete and reliable database of liquid-liquid equilibrium (LLE) and vapor-liquid equilibrium (VLE) data. As a result of this examination, we were able to demonstrate the essential influence of the molecular-weight distribution of polydisperse polymers on the LLE predictions. Accounting for the polydispersity in our calculations, the prediction of the compositions as well as the molecular-weight distributions in the coexisting phases is in good agreement with our experimental results, as illustrated for the system PEG 3000 + dextran 110000 + water at 293.15 K. It should be stressed, that these calculations axe true predictions, since the LLE were calculated using model parameters determined from VLE measurements alone.