Thermodynamics of Poloxamer Block Copolymers in Ionic Liquid Solutions
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Due to useful properties such as high thermal stability, non-flammability, non-volatility, and high ionic conductivity, ionic liquids can be used as solvent media in many chemical products and processes. Poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) block copolymers (commercially available as Poloxamers or Pluronics) can self-assemble in selective solvents, including ionic liquids, as a result of their amphiphilic nature. PEO-PPO-PEO molecules can assemble in solvents to form micelles above a certain critical micellizaion concentration (CMC). We consider here solutions of a relatively hydrophobic PEO-PPO-PEO block copolymer (Pluronic P123, EO 20 -PO 69 -EO 20 ) in the ionic liquid ethylammonium nitrate (EAN) and its mixtures with water. We report on the onset of micelle formation (CMC) and on micellization thermodynamics, and discuss the underlying interactions between P123 and EAN, P123 and water, and EAN and water. Future, we compare the effects of ethylammonium nitrate on aqueous Pluronic P123 micellization to those of ammonium nitrate. Finally, we compare the effects of the protic ionic liquid EAN to the effects of the aprotic imidazolium ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim][BF 4 ]). Isothermal titration calorimetry (ITC) has been used to determine CMC and micellization enthalpy. Other thermodynamic parameters including Gibbs free energy, entropy, and heat capacity of micellization can be calculated from ITC results.