Divalent Cation Effects in Poly (diallyl dimethylammonium)-Poly (styrene sulfonate) Complexes*

Ion-specific effects and the Hoffmeister series serve as central platforms for understanding physiochemical and biological phenomena observed in surfactants, enzyme activity, amino acids, and polyelectrolyte complexes (PECs). The assembly and dynamics of PECs and polyelectrolyte multilayers (PEMs) are influenced by water content, pH, and salt concentration. However, the influence of divalent salts on the assembly of polyelectrolyte complexes remains unclear. This work showcases that divalent chloride salts (MgCl₂ and CaCl₂ following the Hoffmeister series) directly impact the glass transition temperature and the ion-ion interactions PECs. Here, alkali earth divalent ions (Mg²⁺, Ca²⁺) are introduced into poly(diallyldimethylammonium chloride)/ polystyrene sulfonate (PDADMA/PSS) PECs during assembly. These PECs are studied for the cations’ influence on physicochemical properties (thermal transition, polymer composition) at varying salt concentrations (0.10 M, 0.15 M, and 0.20 M). Modulated differential scanning calorimetry (MDSC) experiments are performed and demonstrated that PECs synthesized with CaCl₂ have a significantly higher glass transition temperature when compared to PECs synthesized with MgCl₂. Neutron activation analysis (NAA) and nuclear magnetic resonance (NMR) spectroscopy demonstrated that this difference was due to the strong ion-specific effect influencing the ratio of intrinsic and extrinsic ion pairings in the system. Furthermore, this study demonstrates a universal linear relationship between the thermal transition and the number of water molecules surrounding oppositely charged polyelectrolyte–polyelectrolyte intrinsic ion pairs. This study proffers another bridge between divalent cation behavior on polymer assembly properties and its transition to industrial applications such as controlled drug delivery, sensors, and water purification.