Phase separation in polyelectrolyte systems. I. Complex coacervates of gelatin

The complex coacervation of mixtures of gelatins of different isoionic pH (pI = 5.0, 9.0) was studied in a test of the applicability of the Voorn-Overbeek thermodynamic analysis of the coacervation process. The compositions of the coexisting phases at equilibrium and the total amounts of gelatin in each phase were determined as functions of the equilibration temperature and initial mixing concentration, C_T. Concentrations ranged from 0 to 1% gelatin and temperatures from 10 to 50°. A parameter, θ, was defined to describe the intensity of the coacervation. In general it was found that θ increased as the temperature was decreased. In isotherms above the gelation temperature (t ≥ 40°) θ decreased as the total gelatin concentration increased. At t < 40° plots of θ vs. C_T showed a minimum which shifted to lower values of C_T as t was decreased until, at 10°, the minimum disappeared and θ increased with increasing C_T. These data could not be fitted in with requirements and assumptions of the Voorn-Overbeek theory. A change in the model of the reacting system has been suggested in which electrostatic aggregates are formed in the initial mixture even at t ≥ 40°. The coacervation equilibrium is supposed to exist between dilute solutions of large, nearly neutral polyion aggregates and the concentrated phase of intertwined but not specifically aggregated random coils. Coacervation in the region where gelation is significant (t < 40°) is more complicated and must be treated separately.