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

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.