Polymeric surfactant micelle structure modulated by ionic liquids

The very low volatility, high thermal and chemical stability, and broad range of intermolecular interactions that many ionic liquids (ILs) exhibit render them promising as solvents or additives in diverse applications. We are interested in the self-assembly in ionic liquids of amphiphiles, in particular those of the poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) block polymer family, commercially available as Poloxamers or Pluronics. We consider here aqueous solutions of a relatively hydrophobic PEO-PPO-PEO block copolymer (Pluronic P123, EO20PO70EO20) with the representative ionic liquids ethylammonium nitrate (EAN) (protic IL) or 1-butyl-3-methylimidazolium tetrafluoroborate (BmimBF(4)) (aprotic IL). We further compare the effects of ionic liquid electrolytes to those of the classic electrolyte ammonium nitrate (NH4NO3). We report on the block copolymer micelle structure ascertained from analysis of small-angle neutron scattering (SANS) data, and discuss the observed IL effects in the context of the underlying intermolecular interactions. Opposite effects on micelle structure are observed upon addition of EAN or NH4NO3 compared to BmimBF(4): an increase in the micelle core radius and association number, and a decrease in the micelle shell thickness indicating dehydration upon addition of EAN or NH4NO3; a decrease in the micelle core radius and association number, and an increase in the shell thickness indicating increased solvation upon addition of BmimBF(4). A temperature increase leads to similar effects on the micelle structure as the addition of the kosmotropic electrolytes EAN and NH4NO3. This study for the first time reports PEO-PPO-PEO micelle structure in IL-water mixtures, and utilizes it to proxy the operating intermolecular interactions and the role that ILs play in the self-assembly process. (C) 2021 Elsevier B.V. All rights reserved.