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SynopsisA marked molecular weight fractionat ion accompanies the demixing or phase separatiou resulting from the complex coacervation of mixtiires of aqueous solutions of salt-free isoionic unfractionated gelatins with PI'S of 5 and 9. Viscosity stiidies show that the fractionation is such that the concentrated phase tends to maintain constant, homogeneons composition. A second featnre, seen in phase volitme and concentration measrirements, is a marked self-suppression of coacervation intensity with increasing mixing Concentration. These data were interpreted in terms of a dilute-phase aggregate model which assiimes nearly equal electrostatic free energies of mixing in dilute aggregate and concentrated random phase. The driving force for phase separation is the entropy increase upon formation of the random phase but demixing also depends upon the polymer-solvent interaction parameter X, in the same fashion as in simple coacervation. The dilute-phase aggregate model indicates that the sharp molecular weight selection takes place in the aggregate formation step and explains the self-suppression. Phase equilibria studies utilizing fractionated, paucidisperse high molecular weight gelatins, emphasize the requirement for concentrated phase homogeneity and indicate that aggregates of different moleciilar weight may act as different components, so that X p q 1 , p k Q k > 0, bringing about a separation of the system into three or more coexisting phases.The formation of several coexisting phases from a homolagons polyelectrolyte system and t,he very marked requirement for phase homogeneity suggest that the phenomenon of complex coacervation is a very good model f& some of t,he essent.in1 steps in the pre-1)iologin organisat,ion of polymeric polyions.When aqueous solutions containing gelatins of different, isoionic point,s are mixed at, a pH hetween the isoionic points of the gelatins, a phase separat,ion may occur in which two liquid phases are formed.' This liquid-liquid phase demixing or complex coacervation is particularly interesting since both phases are rich in the same solvent, apd both types of gelatin appear in each phase. The demixing parameters have been studied in some detail'-3 from the point of view of the theoretical treatment of the coacervation problem developed by V~o r n~-~ and Voorn and Overbeek'O and based on the earlier data of Bungenberg de .Jong.llThe gelatin-gelatin coacervation studies'-3 showed the phase equilibria to deviate from the Voorn-Overbeek'O predictions in several importmt,

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