Role of Cation-π Interactions in the Phase Behavior and Thermodynamics of Complex Coacervates

Cation-π interactions have been suggested to play an important role in polyelectrolyte complex coacervation, but quantifying their contributions has proven difficult. To address this problem, we synthesized polycations and polyanions with welldefined aromatic content via post-polymerization modification of poly(N-acryloxy succinimide), and investigated the stability of both aromatic and non-aromatic complexes in the presence of inorganic salts with different expected cation-π interaction strengths. Optical turbidity and isothermal titration calorimetry experiments revealed that the stability and enthalpic favorability of complexation of the aromatic polyelectrolytes decreases with increasing cation-π interaction strength of the inorganic salt, as the salt cations occupy more of the aromatic sites and out-compete interactions with the polycation chains. “Asymmetric” mixing experiments, in which aromatic polycations were mixed with non-aromatic polyanions (and vice versa) revealed that cation-π interactions matter primarily when new cation-π interactions can form between the oppositely-charged chains. This work demonstrates that cation-π interactions between polyelectrolytes, and competition with free cations in solution, can play a significant role in polyelectrolyte complexation, and provides a convenient method for investigating cation-π interactions in a wide range of coacervate materials.