Surface adsorption and aggregate formation of nonionic surfactants in a room temperature ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate (bmimPF6)

Surface adsorption and aggregation behavior of polyoxyethylene (POE)-type nonionic surfactants in a room temperature ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate (bmimPF6), were investigated by means of surface tension, 1H NMR, and dynamic light-scattering (DLS) measurements. The maximum surface excess in bmimPF6 is much lesser than those observed for aqueous solution, reflecting a much weaker solvophobicity exhibited by the surfactant hydrocarbon chain in the ionic liquid compared with the hydrophobicity experienced in water. Compared with another ionic liquid with different anion species, 1-butyl-3-methylimidazolium terafluoroborate (bmimBF4), the surfactant solution in bmimPF6 brings about higher critical micelle concentration (cmc), smaller micellar aggregation number, and weaker dependence of cmc on the surfactant hydrocarbon chain length. This indicates that the solvophobic interaction between surfactant hydrocarbon chains acts more weakly in bmimPF6 than in bmimBF4. The weaker solvophobic effect in bmimPF6 is also appreciable in the thermodynamic parameters for micelle formation derived from temperature dependence of cmc, and would be attributed to weaker attractive interaction between bmim cation and PF6- ion compared with that between bmim cation and BF4- ion. Present results give an insight into a better understanding of the importance of anion species for the properties of ionic liquids as a solvent to support a self-assembly of amphiphilic molecules.

Graphical abstractThe cmc of nonionic surfactants is higher and its hydrocarbon chain length dependence is weaker in bmimPF6 than in bmimBF4, indicating that the solvophobic interaction between hydrocarbon chains is affected by anion species of the ionic liquids.Figure optionsDownload full-size imageDownload high-quality image (63 K)Download as PowerPoint slideHighlights► The cmc of POE-type nonionic surfactants is higher in bmimPF6 than in bmimBF4. ► The cmc decrease with hydrocarbon chain length is weaker in bmimPF6 than in bmimBF4. ► The entropic contribution to ΔGm0 is smaller in bmimPF6 than in bmimBF4. ► Solvophobic interaction between hydrocarbon chains depends on anion species of the ionic liquids.