Salt-dependent Phase Behavior of Weak Polyelectrolyte Complexes

In mixtures of poly(diallyldimethylammonium chloride), a strong polycation, and poly(acrylic acid), a partially charged polyanion, within the pH range of 4.7–5.3, high-salt phase separation occurs. This phenomenon occurs above the critical salt concentration required for dissolving the coacervate formed at low salt concentrations, named the "upper critical salt concentration," and monomer concentrations above 0.1 M for each polymer. The transition from associative phase separation at low salt concentrations to a single solution and ultimately to segregative phase separation at high salt concentrations called the “lower critical salt concentration”, originates from the interplay between electrostatic interactions and the hydrophobicity of neutral PAA monomers in a high-salt solvent. Short-range ion pairing and counterion condensation with hydrophobic interactions between PAA neutral monomers and water is modeled through a Flory–Huggins χ parameter of around 0.6. Discussion will include Literature observations of a continuous transition with increasing salt concentration, without a homogeneous single-phase solution at intermediate salt concentration.