Microstructures of micellar aggregations formed within 1-butyl-3-methylimidazolium type ionic liquids

Nonionic surfactant Triton X-100 was shown to aggregate and form micellar aggregation in ionic liquids (ILs), 1-butyl-3-methylimidazolium tetrafluoroborate (bmimBF(4)) and 1-butyl-3-methylimidazolium hexafluorophosphate (bmimPF(6)). The surface tension measurements revealed that the dissolution of Triton X-100 in ILs depressed the surface tension in a manner analogous to aqueous solutions, and a relatively higher critical micellar concentration (CMC) was obtained compared to that of water. Freeze-fracture transmission electron microscopy (FFTEM) shows that the micelles have an irregular droplet shape, which is larger than that formed in water. The micellar droplets preferred to assemble into larger clusters. (1)H NMR and two-dimensional rotating frame nuclear Overhauser effect (NOE) experiments (2D ROESY) show that the addition of Triton X-100 destroyed the ion pairs of pure ILs due to the electrostatic interaction between the positively charged imidazolium cation of ILs and the electronegative oxygen atoms of oxyethylene (OE) units of Triton X-100. The electrostatic interaction behaves similar to hydrogen bond that occurred between the OE units of nonionic surfactants and water molecules in aqueous micelles and cooperates with solvatophobicity, leading to the formation of IL micelles. The 2D ROESY analysis reveals that the microstructures of Triton X-100-based micelles in ILs are not regular spherical, which accords with the FFTEM image. Similar to the aqueous micellar systems, the hydrophobic interaction or solvatophobicity was found to drive the formation of micelles.