Nonaqueous ionic liquid microemulsions of 1-butyl-3-methylimidazolium tetrafluoroborate, toluene and ethanol

Abstract

The phase behavior of the ternary system consisting of the hydrophilic ionic liquid (IL) 1-butyl-3-methylimidazolium tetrafluoroborate (bmimBF₄), toluene and ethanol was investigated. Single-phase microemulsion and multiphase regions were observed in the ternary phase diagram. To the best of our knowledge, this is the first report on nonaqueous IL microemulsions prepared in the absence of traditional surfactants. The microstructures and structural transitions of the microemulsion system were investigated by cyclic voltammetry, pulse field gradient spin-echo nuclear magnetic resonance and electrical conductivity measurements. The microemulsions exhibit IL-in-toluene (IL/O), bicontinuous (BC) and toluene-in-IL (O/IL) microstructures, similarly to traditional surfactant-based microemulsions. The three kinds of microstructures were confirmed from freeze-fracture electron microscopy observations. A pearl necklace-like structure was observed in the BC microemulsion. The polarity of bmimBF₄ domains in the IL/O microemulsion was investigated by UV-visible absorption spectroscopy using methyl orange as a probe. It was found that the polarity was lower than that of the bulk bmimBF₄, and it increased with increasing bmimBF₄ content at a constant toluene–ethanol ratio.