Broadband dielectric spectroscopy of micelles and microemulsions formed in a hydrophilic ionic liquid: the relaxation mechanism and interior parameters

Abstract

Three striking dielectric relaxations located at 100 MHz, 1 GHz and 10 GHz respectively were observed both in binary mixtures of p-(1,1,3,3-tetramethylbutyl)phenoxypolyoxyethyleneglycol (TX-100) and 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF₄]), and in [bmim][BF₄]/TX-100/cyclohexane microemulsions in a wide frequency range. The mechanisms of the dielectric relaxations were proposed and the detailed characterization of low-frequency relaxation (around 100 MHz), which is identified as interface polarization, was performed theoretically. The dielectric parameters (relaxation time and dielectric increment) were obtained by fitting the dielectric spectra with the Debye plus cole–cole formula. The low-frequency relaxation mechanisms of both binary and ternary systems contribute to interfacial polarization, supported by the good agreement between relaxation times calculated by Maxwell–Wagner theory and the values obtained from experiment. Meanwhile, by analyzing the linear ionic liquid content dependence of dielectric increment of low-frequency relaxation, we find that this relaxation involves the dynamics of ionic liquids which located at the interface. Besides, the phase parameters, which reflect the interior properties of the binary and ternary systems, were calculated by Hanai theory, and their dependence on the variation of sample composition are properly explained; the distribution of cyclohexane in the dispersed phase is obtained quantitatively, which is of importance in the field of the microstructure of ionic liquid microemulsions.