Effect of salt concentration on the formation and phase composition of weak and strong polyelectrolyte complexes

Salts play a central role in governing interactions in aqueous systems. In polyelectrolyte complexes (PECs), both salt concentration and salt type can influence phase composition, ion partitioning and, consequently, material properties. Here, we compare the supernatant and PEC-phase compositions of a weak, pH-dependent, polyelectrolyte system poly(allylamine)/poly(acrylic acid) (PAH/PAA), and a strong, fully ionized, polyelectrolyte system, poly(diallyldimethylammonium)/poly(4-styrenesulfonate) (PDADMAC/PSS) at charge stoichiometry and pH 6.5 across a range of KBr concentrations. The distribution of polymers, counterions originating from the polyelectrolyte stocks (Na⁺ and Cl^-), and added KBr ions (K⁺ and Br^-) in both the dense PEC phase and supernatant was quantified using multinuclear NMR (¹H, ²³Na, ³⁵Cl, ³⁹K and ⁸¹Br), while bulk hydration was assessed gravimetrically. Varying KBr concentration therefore affected both the total salt concentration and the relative ratios of native and added ions in the studied systems. For both systems, almost all polymers were found in the PEC, and the polymer concentration in the PEC remained constant as a function of KBr concentration. By contrast, ion partitioning and hydration were more sensitive to salt conditions. PAH/PAA showed clear anion exchange, with Cl^- decreasing and Br^- increasing in the PEC phase, whereas PDADMAC/PSS showed a larger reconstructed contribution from added-salt ions. Overall, KBr primarily affected ion distribution and hydration rather than the polymer content of the PEC phase. Together, these insights provide a fundamental understanding of how salt concentration affects phase composition, ion partitioning, and hydration in stoichiometric PECs, and help explain how salt selection may influence PEC-based material properties.