Surface adsorption and aggregate formation of nonionic surfactants in a room temperature ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate (bmimPF 6)

Abstract

Surface adsorption and aggregation behavior of polyoxyethylene (POE)-type nonionic surfactants in a room temperature ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate (bmimPF 6), were investigated by means of surface tension, 1H NMR, and dynamic light-scattering (DLS) measurements. The maximum surface excess in bmimPF 6 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 (bmimBF 4), the surfactant solution in bmimPF 6 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 bmimPF 6 than in bmimBF 4. The weaker solvophobic effect in bmimPF 6 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 PF 6 - ion compared with that between bmim cation and BF 4 - 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.