Thermodynamics and Structure of Nonionic Block Copolymer Micelles in Ionic Liquids

Ionic liquids (ILs), organic salts that are fluid at ambient conditions, are a novel class of compounds with a combinatorially great chemical diversity and unique properties. [He & Alexandridis, Phys. Chem. Chem. Phys. 2015, 17 (28), 18238-18261; DOI: 10.1039/c5cp01620g] The very low volatility and high thermal and chemical stability that many ILs exhibit render them promising as solvents in diverse applications. [He & Alexandridis, Adv. Colloid Interface Sci. 2017, 244, 54-70; DOI: 10.1016/j.cis.2016.08.004] We are interested in the self-assembly properties in ionic liquids of block polymers, in particular those of the poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) family, commercially available as Poloxamers or Pluronics. [Bodratti et al., Adv. Colloid Interface Sci. 2017, 244, 132-163; DOI: 10.1016/j.cis.2016.09.003] [Bodratti & Alexandridis, Expert Opin. Drug Deliv. 2018, 15 (11), 1085-1104; DOI: 10.1080/17425247.2018.1529756] We consider here aqueous solutions of a relatively hydrophobic PEO-PPO-PEO block copolymer (Pluronic P123) with the representative ionic liquids ethylammonium nitrate (EAN) (protic IL) or 1-butyl-3-methylimidazolium tetrafluoroborate (BmimBF4) (aprotic IL). We report on the amphiphilic block copolymer micellization thermodynamics (obtained from isothermal titration calorimetry, ITC) and micelle structure (obtained from small-angle neutron scattering, SANS), and discuss the observed IL effects in the context of the underlying intermolecular interactions between PEO-PPO-PEO and IL, PEO-PPO-PEO and water, and IL and water. [He & Alexandridis, Polymers 2018, 10 (1), 32; DOI: 10.3390/polym10010032] [He et al., Colloids Surfaces A: Physicochem. Eng. Aspects 2018, 559, 159-168; DOI: 10.1016/j.colsurfa.2018.09.061] We further compare the effects of ionic liquid electrolytes to those of classic electrolytes.