Aggregation Behavior of a Fluorinated Surfactant in 1-Butyl-3-methylimidazolium Bis(trifluoromethylsulfonyl)imide Ionic Liquid

The cationic fluorinated surfactant, FC-4, unlike other surfactants, forms micelles in the room temperature ionic liquid, 1-butyl-3-methylimidazolium bis(trifluoromethlsulfonyl)imide (bmimTf(2)N). Surface tension, freeze-fracture transmission electron microscopy, F-19 NMR, H-1 NMR, and Fourier transform infrared measurements revealed that (i) the FC-4 cation forms an ion pair with the Tf2N anion, (ii) the ion pairs undergo association to form premicellar aggregates,and (iii) the premicellar aggregates transform into micelles at the critical micelle concentration (CMC). The thermodynamic parameters for micelle formation derived from the temperature dependence of the CMC demonstrated that the solvophobic interaction between the solvophobic tails of the surfactant molecules is rather weak in bmimTf(2)N compared with other ionic liquids, in accordance with the observation that surfactants do not readily form micelles in bmimTf(2)N. The fact that FC-4 forms micelles in such an inconvenient solvent is attributed to the ion-pair formation between the surfactant cation and the ionic liquid anion.