Electrostatic-interaction-induced phase separation in solutions of flexible-chain polyelectrolytes

A model of a polyelectrolyte solution has been formulated on the basis of the formalism of the thermodynamic perturbation theory. Macromolecules have been described in terms of the model of a flexible chain with an excluded volume and a variable electrical charge. During construction of the thermodynamic perturbation theory, a set of three independent subsystems—polyelectrolyte macromolecules placed in a structureless charge background of counterions, counterions placed in a structureless charge background of macromolecules, and Coulomb gas ions of a low-molecular-mass salt—has been taken as the reference system. In the framework of this model, liquid-liquid phase separation due to strong correlation-induced attraction has been predicted. The behavior of the degree of ionization over a wide monomer concentration range, including the region of phase separation either in a salt-free solution or in the presence of univalent ions of a low-molecular-mass salt in the solution, has been studied. It has been shown that macromolecules in the coexisting phases should have different degrees of ionization. The occurrence of phase separation under normal conditions in the case when dimethylformamide is taken as a solvent and the nonoccurrence of this phase separation in the case of aqueous solutions of flexible-chain polyelectrolytes are predicted.