The interaction between two oppositely charged polyelectrolytes

Mixtures of a weak polybase (polyethylenimine) and a weak polyacid acrylamide-acrylic acid copolymer in aqueous solutions at several ionic strengths and polymer concentrations are studied potentiometrically. When the concentrations of the polyethylenimine and acrylamide-acrylic acid copolymer charges are not too different, phase separation into two liquid phases (“complex coacervation”) is observed. In the pH region where no phase separation occurs, potentiometric titrations are performed on mixtures of both polymers. From the titrations of polyethylenimine solutions, acrylamide-acrylic acid copolymer solutions, and the mixtures, the free energy of interaction has been evaluated according to the theory of Litan. The dependence of the free energy of interaction on pH, polymer concentrations, and ionic strength is explained quantitatively with a model of cooperative electrostatic physical association.