FLUID PROPERTIES AND PHASE-EQUILIBRIA IN AQUEOUS PHASE BIOSEPARATIONS

Two non-equilibrium conditions have been identified in aqueous biphasic systems: the partitioning of subcellular biological particles between two polymer-containing phases, and the demixing of solutions of a salt and a polymer. In partitioning experiments, ribosomes and vesicles prepared from the bacterium Serratia marcescens or polystyrene latex microspheres were partitioned between aqueous phases consisting of solutions of polyethylene glycol (PEG) and dextran having various molecular masses. Fluid dynamic factors independent of chemical equilibria, such as interfacial area and phase separation rate, affected the partition coefficient of these particles. In demixing experiments, evidence was found for a persisting gradient of PEG concentration in the upper, PEG-rich phase several days after the demixing of polyethylene glycol and sodium sulfate solutions. Optical analysis revealed a persisting density gradient of 10 to 200 mg/cm4. In these two examples, quasi-equilibrium states are likely to affect thermodynamic data measurements thought to be made at equilibrium.