Stabilization of polyelectrolyte complexes via cation-π interactions

We investigate the impact of cation-π and π-π interactions on the phase behavior and thermodynamics of polyelectrolyte complexation in a series of well-defined polyelectrolytes with aromatic (benzyl) and non-aromatic (cyclohexanemethyl) sidechains. The phase behavior of the complexes is quantified using optical turbidity in the presence of salts with different expected cation-π interaction strengths. We find that for the non-aromatic polymers, the measured salt resistances are independent of salt choice. For the aromatic polymers, on the other hand, the salt resistance increases as the salt is changed from KCl to NaCl to LiCl, reflecting decreasing ability of the inorganic salt to compete with and break apart inter-chain cation-π interactions. The thermodynamics driving this behavior are further quantified via isothermal titration calorimetry, and reveal that the enthalpy of complexation is significantly higher in the aromatic systems with weakly interacting salts. These results suggest that cation-π interactions can be an important driver of polyelectrolyte complexation, and should be accounted for when analyzing and interpreting complexation experiments on other aromatic polyelectrolytes.