Micelle formation of nonionic surfactants in a room temperature ionic liquid, 1-butyl-3-methylimidazolium tetrafluoroborate: Surfactant chain length dependence of the critical micelle concentration

Micellization behavior was investigated for polyoxyethylene-type nonionic surfactants with varying chain length (CnEm) in a room temperature ionic liquid, 1-butyl-3-methylimidazolium tetrafluoroborate (bmimBF4). Critical micelle concentration (cmc) was determined from the variation of 1H NMR chemical shift with the surfactant concentration. The logarithmic value of cmc decreased linearly with the number of carbon atoms in the surfactant hydrocarbon chain, similarly to the case observed in aqueous surfactant solutions. However, the slope of the straight line is much smaller in bmimBF4 than in aqueous solution. Thermodynamic parameters for micelle formation estimated from the temperature dependence of cmc showed that the micellization in bmimBF4 is an entropy-driven process around room temperature. This behavior is also similar to the case in aqueous solution. However, the magnitude of the entropic contribution to the overall micellization free energy in bmimBF4 is much smaller compared with that in aqueous solution. These results suggest that the micellization in bmimBF4 proceeds through a mechanism similar to the hydrophobic interaction in aqueous surfactant solutions, although the solvophobic effect in bmimBF4 is much weaker than the hydrophobic effect.

Graphical abstractThe effect of hydrocarbon chain length on cmc of CnEm is much weaker in bmimBF4 than in aqueous solution, whereas that of polyoxyethylene chain length is slightly stronger in bmimBF4 than in aqueous solution.Figure optionsDownload full-size imageDownload high-quality image (76 K)Download as PowerPoint slideResearch highlights► POE-type nonionic surfactants form micelles in imidazolium-based IL, bmimBF4. ► The effect of hydrocarbon chain length on the cmc is much weaker than in aqueous systems. ► The effect of POE chain length on the cmc is slightly stronger than in aqueous systems. ► Solvophobic interactions between hydrocarbon chains induce surfactant self-assembly in bmimBF4.