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 th e 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 exp ressions for the chemical potentials of the solutes derived from the o smotic virial equation (COVE). Applying the COVE, we have examined the predictability using a complete and reliable database of liquid-liqui d equilibrium (LLE) and vapor-liquid equilibrium (VLE) data. As a resu lt of this examination, we were able to demonstrate the essential infl uence of the molecular-weight distribution of polydisperse polymers on the LLE predictions. Accounting for the polydispersity in our calcula tions, the prediction of the compositions as well as the molecular-wei ght distributions in the coexisting phases is in good agreement with o ur experimental results, as illustrated for the system PEG 3000 + dext ran 110000 + water at 293.15 K. It should be stressed, that these calc ulations axe true predictions, since the LLE were calculated using mod el parameters determined from VLE measurements alone.