Depiction of a non-aqueous ionic liquid surfactant-free microemulsion by dielectric relaxation spectroscopy

Abstract

A dielectric relaxation was observed at approximately 700 MHz in the surfactant-free microemulsion which is composed of 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]), ethanol and toluene. The measured frequency was between 1 MHz and 3 GHz. Two inflection points (22.39 and 31.09 wt%) on the curve of the dielectric parameters vs weight fraction of IL give the phase boundaries between IL/O and B.C., B.C. and O/IL sub-regions. The permittivity of the frequency at the beginning of the relaxation phenomenon and the maximum of the derivative of dc conductivity both give 31.09 wt% as the percolation threshold. Both frequency dependences of permittivity and loss angle provide the critical exponent u = 0.72, which suggest that a static percolation occurs in the microemulsion near the critical threshold. This conclusion is also supported by the scaling dependence of dc conductivity on IL weight fraction. The results of phase parameters obtained by Hanai theory show that: the dielectric relaxation of this system is proved to be caused by the interfacial polarization, which indicates that dispersed particles indeed exist in this surfactant-free microemulsion; in the IL/O sub-region, disperse particle not only contains IL and TX-100, but also includes oil molecule; and for the O/IL sub-region the continuous phase should be the IL/ethanol binary solvent rather than the pure IL; the main reasons for the occurring of the static percolation in this system may be the large volume fraction of dispersed particle and the existent of IL molecule. The substitution of ethanol for surfactant causes change in interface properties but no change in percolation mechanism, indicating that ethanol indeed distributes at the interface and molecular details at the interface do not determine the mechanism of percolation