Salt-Dependent Phase Re-entry of Weak Polyelectrolyte Complexes: from Associative to Segregative Liquid–Liquid Phase Separation

A high-salt phase-separation re-entry is observed in mixtures of poly(diallyldimethylammonium chloride), a strong polycation, and poly(acrylic acid) (PAA), a partially charged polyanion, within the pH range 4.7–5.3. This intriguing phenomenon exclusively occurs at salt concentrations exceeding the critical salt concentration required for dissolving the coacervate formed at low salt concentrations, here named the "upper critical salt concentration", and at monomer concentrations exceeding 0.1 M for each polymer. The transition from associative phase separation at low salt concentrations to a single solution and ultimately to segregative separation at high salt concentrations, called the "lower critical salt concentration", arises from the interplay between electrostatic interactions and the hydrophobicity of neutral PAA monomers in a high-salt solvent. To explain this transition, we use a theory combining short-range ion pairing and counterion condensation with long-range electrostatics using the random phase approximation and with hydrophobic interactions between PAA neutral monomers and water. The latter is modeled through a Flory–Huggins χ parameter of around 0.6. Literature observations of a continuous transition from associative to segregative phase transition with increasing salt concentration, without a homogeneous single-phase solution at intermediate salt concentration, are also predicted and discussed.