Impedance Spectroscopic Study on Room Temperature Ionic Liquid-Water Mixtures

Hiroshi Abea, Masami Aonoa and Yukihiro Yoshimurab

aDepartment of Materials Science and Engineering, National Defense Academy, Yokosuka, Kanagawa 239-8686, Japan
bDepartment of Applied Chemistry, National Defense Academy, Yokosuka, Kanagawa 239-8686, Japan

J. Chem. Chem. Eng. 6 (2012) 383-390.

Abstract
AC impedance spectroscopy in pure room temperature ionic liquids (RTILs) and RTIL-water mixture was measured at the temperature of range from 30 oC down to -30 oC. The cations of RTILs are N,N-diethyl-N-methyl-N-(2-methoxyethyl) ammonium ([DEME]), 1-ethyl-3-methylimidazolium ([C2mim]) and 1-butyl-3-methylimidazolium ([C4mim]), the anions are tetrafluoroborate ([BF4]) and bis(trifluoromethanesulfonyl)imide ([TFSI]). In all pure RTILs, there are two kinds of local minima in real part of the AC impedance Zreal. By adding water to [DEME][BF4] (0 mol% < x < 94 mol%) at room temperature, the local minimum value at higher frequency decreased remarkably at the fixed frequency with increasing water concentration. Above 94 mol% H2O, a quite different profile of the AC impedance spectroscopy was obtained. In addition to Zreal, temperature dependence of an imaginary part of the impedance Zimag had an isosbestic point below 94 mol%. The isosbestic point disappeared above 94 mol%. The isosbestic point in Zimag reveals an interaction between [DEME][BF4] and H2O.

Fig. 1
Real part of AC impedance spectroscopy pure water at room temperature. UP, DIS and CW represent ultrapure water, distilled water and city water, respectively.


Fig. 2
The real part of AC impedance spectroscopy in pure RTILs at room temperature.
Fig. 3.
Temperature dependence of the real part of the AC impedance in pure [DEME][BF4].
Fig. 4.
Zreal as a function of water concentration at 20 oC (a) below 80 mol% and (b) above 80 mol%.
Fig. 5.
Temperature dependence of (a) Zreal at 92.3 mol%; (b) Zimag at 92.3 mol%; (c) Zreal at 94.9 mol% and (d) Zimag at 94.9 mol%.
Fig. 6.
Unstable factor using coaxial electrodes. The factor is estimated by the temperature dependence of Zreal in pure [DEME][BF4].
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Last Modified: May 1, 2012