Ionic liquids (ILs) are a new class of organic solvents that are stable over a large range of temperatures and have negligible vapor pressures.[1] Owing to their molecular structure, associating a cation and an anion, their physicochemical properties can be easily modulated by changing one of the ions. Ionic liquids are now widely used in organic synthesis and chemical separations due to their high solvation ability and their tuneable nature. [2,3] Almost unexplored, surfactant organization in ILs could open new research directions towards micellar catalysis into IL media, solvation enhancement for apolar entities and lyotropic properties. Indeed, surfactant organization occurs in some nonaqueous solvents [4,5] and is largely used in aqueous systems for drug vectorization, catalysis, gelification and other applications.[5] Recently, microemulsions have been obtained in IL-oil mixtures [6] and only premicellar aggregation has been detected in pure ILs. [7,8] However, no clear demonstration of the existence of a micellar phase in ILs has been reported yet.Here, we describe the behavior of a series of pure alkyl poly-(oxyethyleneglycol) ethers in 1-butyl-3-methyl-imidazolium (bmim) ILs with various counter ions [BF 4 À , PF 6 À and Tf 2 N À , that is, bis(trifluoromethylsulfonyl)amide; Figure 1]. The choice of nonionic surfactants, denoted C n E m (n = 12-16; m = 4-8), was justified to avoid the exchange of counter ions with the solvent. For high concentrations of surfactant (ca. 1 mol L À1 ), the mixtures C n E m /bmimBF 4 were generally solid at room temperature with a cloud point at high temperature (see Supporting Information). Herein, all experiments were performed at 25 8C with concentrations lower than 1 mol L
À1, at which the mixtures were all limpid and homogeneous.Surface tension measurements were performed to probe the aggregation behavior of the surfactants in bmimBF 4 . For all the selected surfactants, the surface tension of the C n E m /bmimBF 4 solutions decreased when the surfactant concentration increased (Figure 2). This indicated their adsorption at the air/solution interface. The Szyszowski-Langmuir adsorption equation [9] fitted well this decrease, leading to an estimation of the area per molecule of the surfactant at the air/bmimBF 4 interface, that occurred to be comparable or lower than the ones found at the air/water interface (Table 1). Those differences between bmimBF 4 and water may be related to a change in the organization and/or solvation state of the adsorbed surfactants. In bmimBF 4 like in water, the molecular area of the surfactant decreased with increasing alkyl chain length or decreasing number of oxyethylene groups.This initial decrease of the surface tension is followed by an abrupt change in the slope of the surface tension versus C (Figure 3; semi-log plot). After this breaking point, the surface tension of the solutions remains more or less constant. Such a behavior suggested the formation of micelles within the ILs, where the break point corresponds to a critical micelle...
