Reaction enthalpy from the binding of multivalent cations to anionic polyelectrolytes in dilute solutions

Dilute solutions of sodium poly(styrene sulfonate) (NaPSS) in the presence of Al³⁺, Ca²⁺, and Ba²⁺ were analysed by means of isothermal titration calorimetry (ITC) in order to investigate the heat effect of bond formation between those cations and the anionic SO₃^- residues of NaPSS. The selection of the cations was guided by the solution behavior of the corresponding PSS salts from a preceding study [M. Hansch et al., J. Chem. Phys. 148(1), 014901 (2018)], where bonds between Ba²⁺ and anionic PSS showed an increasing solubility with decreasing temperature and Al³⁺ exhibited the inverse trend. Unlike to Al³⁺ and Ba²⁺, Ca²⁺ is expected to behave as a purely electrostatically interacting bivalent cation and was thus included in the present study. Results from ITC satisfactorily succeeded to explain the temperature-dependent solution behavior of the salts with Al³⁺ and Ba²⁺ and confirmed the non-specific behavior of Ca²⁺. Additional ITC experiments with salts of Ca²⁺ and Ba²⁺ and sodium poly(acrylate) complemented the results on PSS by data from a chemically different polyanion. Availability of these joint sets of polyanion-cation combinations not only offers the chance to identify common features and subtle differences in the solution behavior of polyelectrolytes in the presence of multi-valent cations but also points to a new class of responsive materials.