Dielectric relaxation spectroscopy for the binary system of 1-butyl-3-methylimidazolium hexafluorophosphate and ethanol: interactions and micro phase behavior

Abstract

The interaction between the ionic liquid (IL) 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF₆]) and ethanol and the micro phase behavior for the binary system have been investigated by dielectric relaxation spectroscopy (DRS) over a frequency range from 10 MHz to 20 GHz under constant temperature. The dielectric spectra with two relaxation processes were observed, and it could be satisfactorily fitted by an empirical equation including a non-Debye process at lower frequencies and a Debye process at higher frequencies. Detailed analysis indicates that either of the two relaxation processes contain the contributions of different polarization mechanisms, respectively. The low-frequency process, centered at about 1 GH is mainly due to the reorientation of the polar cation [bmim]⁺ and ion pairs formed between cation and anion of the IL and cooperative dynamics of the ethanol H-bond system; the high-frequency one, located at about 10 GHz is caused by the fast fluctuation of ion-pairs formed between cation and anion of the IL within the inter-ion distance and the rotation of singly H-bonded ethanol monomers at the ends of the ethanol chain. The ethanol concentration dependency of all the relaxation parameters shows a inflection point at the ethanol mass fraction of 20%, which is consistent with macro phase diagram. This inflection point is the cut-off point of one phase and two phase. The dielectric analysis indicates that a great micro-structure difference exists above and below 20%.