FLUID PROPERTIES AND PHASE-EQUILIBRIA IN AQUEOUS PHASE BIOSEPARATIONS

Two non-equilibrium conditions have been identified in aqueous biphasi c systems: the partitioning of subcellular biological particles betwee n two polymer-containing phases, and the demixing of solutions of a sa lt and a polymer. In partitioning experiments, ribosomes and vesicles prepared from the bacterium Serratia marcescens or polystyrene latex m icrospheres were partitioned between aqueous phases consisting of solu tions of polyethylene glycol (PEG) and dextran having various molecula r masses. Fluid dynamic factors independent of chemical equilibria, su ch as interfacial area and phase separation rate, affected the partiti on coefficient of these particles. In demixing experiments, evidence w as 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 d ensity gradient of 10 to 200 mg/cm4. In these two examples, quasi-equi librium states are likely to affect thermodynamic data measurements th ought to be made at equilibrium.