Complexation of oppositely-charged polyelectrolytes in the presence of divalent ions

Polyelectrolyte solutions exhibit a demixing transition known as polyelectrolyte complexation (PEC). Recent interest in PEC has been spurred by its relevance in organizing biomolecules in intracellular environments, and by the advancement of theoretical treatments. A range of molecular parameters have been shown to influence the demixing transition, including charge patterning, polarity, stoichiometry, etc. We present a study on the effects of ion valency, which remains largely unexplored. We leverage our previous expertise in synthesizing homologous polyelectrolytes to investigate how the addition of divalent ions (Ca²⁺, Mg²⁺) affects their complexation behavior. Dye-labeling and ICP-MS measurements enable the construction of accurate complexation phase diagrams as well as quantification of ion partitioning. The two-phase window is found to be substantially narrower than the case of monovalent ions, while the ion and polymer partitioning remained similar. The results are rationalized by analyzing how ion valency reduces the ionic correlations, which enables the collapse of phase diagrams for both monovalent and divalent ions.