Modulation of Solvent Properties Using Imidazolium-Based Ionic Liquids: Effects on the Thermodynamics of PEO–PPO–PEO Triblock Copolymer Aggregation

Differential scanning calorimetry, isothermal titration calorimetry, fluorescence spectroscopy, and dynamic light scattering were used to investigate the temperature- and concentration-induced aggregation of PEO-PPO-PEO triblock copolymers (L31, L81, L64, P123, and F127) in aqueous solutions of 1-butyl-3-methylimidazolium halides (C₄mimX, X = Cl^- or Br^-) and sodium chloride (NaCl), establishing a comparative energetic and structural framework for understanding how they modulate triblock copolymer self-assembly. The critical micellization temperature decreased in the presence of cosolutes following the order NaCl < C₄mimCl < C₄mimBr, while the critical micellar concentration increased with C₄mimX (C₄mimCl < C₄mimBr) and decreased with NaCl. Micellization was less endothermic with C₄mimX but more endothermic with NaCl. These trends are attributed to the solvation of the PPO block by C₄mimX (Cl^- > Br^-), which displaces water molecules and results in a higher partial molar enthalpy. This effect is modulated by the EO fraction of the copolymer, as revealed by principal component analysis. Such interactions play distinct roles in the desolvation of triblock copolymer unimers during micelle formation and in the transition from spherical to rod-like micelles. The results provide a holistic energetic understanding of the modulation of energy by ionic liquids during triblock copolymer aggregation.