Molecular weight fractionation and the self‐suppression of complex coacervation

A marked molecular weight fractionation accompanies the demixing or phase separation resulting from the complex coacervation of mixtures of aqueous solutions of salt‐free isoionic unfractionated gelatins with pI's of 5 and 9. Viscosity studies show that the fractionation is such that the concentrated phase tends to maintain constant, homogeneous composition. A second feature, seen in phase volume and concentration measurements, 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 assumes 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 χ, 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 molecular weight may act as different components, so that χP¹Q¹,P^kQ^k > 0, bringing about a separation of the system into three or more coexisting phases. The formation of several coexisting phases from a homologous polyelectrolyte system and the very marked requirement for phase homogeneity suggest that the phenomenon of complex coacervation is a very good model for some of the essential steps in the pre‐biologic organization of polymeric polyions.