Weak versus strong polyelectrolyte complexes: two-phase quantification shows chemistry-dependent ion uptake and...

Designing robust, salt- and pH-responsive polyelectrolyte complex (PEC)-based materials requires controlling the dense-phase composition, including polymer stoichiometry, ion uptake, and hydration, because these parameters ultimately govern mechanics and transport. Yet, these coupled parameters are rarely quantified across coexisting phases, especially under off-stoichiometric mixing. Here, at pH = 6.5 and 250 mM NaCl, we provide a phase-resolved comparison of a weak, charge-regulating system formed by poly(allylamine)/poly(acrylic acid) (PAH/PAA) and a strong, fixed-charge system formed by poly(diallyldimethylammonium)/poly(4-styrenesulfonate) (PDADMAC/PSS). Polymers and ions in both the dense PEC phase and supernatant were quantified using multinuclear NMR (¹H, ²³Na, ³⁵Cl), and bulk hydration was assessed gravimetrically. In both systems, the dense phase converges toward a preferred near-stoichiometric polymer composition, while off-stoichiometry is stored predominantly in the supernatant. Normalized two-phase balances show that most recovered polymer is found in the PEC phase (∼90% for PAH/PAA and ∼80% for PDADMAC/PSS at F^- = 0.5), whereas the large majority of the recovered Na⁺/Cl^- remains in the supernatant and only a small fraction is assigned to the PEC phase. PAH/PAA exhibits a composition-dependent increase in bulk liquid retention at high F^- and supernatant pH shifts consistent with proton exchange, suggesting that proton-coupled charge regulation contributes to charge compensation. In contrast, PDADMAC/PSS shows a small dense-phase ion fraction consistent with fixed-charge polymer pairing. Together, these results provide a transferable phase-composition framework based on stoichiometry, ion partitioning, and bulk hydration, for comparing these two selected PEC chemistries at the same preparation conditions, supporting rational PEC materials design.