Carlos F. P. MIRANDA1, Luís M. N. B. F. SANTOS1
1CIQUP, Institute of Molecular Sciences (IMS)- Department of Chemistry and Biochemistry, Faculty of Science, University of Porto, Porto, Portugal
This work presents an extensive study of the electrical conductivity of imidazolium ionic liquids. The study is composed by the evaluation of the effect of two structural features: the nature of the anion and the length of the alkyl chain of the cation. For the anion effect, the electrical conductivities of four ILs based on the 1-butyl-3-methylimidazolium cation [C4C1im]+ and combined with the bis(trifluoromethylsulfonyl)imide (NTf2), trifluoromethanesulfonate (OTf), hexafluorophosphate (PF6−) and tetrafluoroborate (BF4−) anions. The study of the effect of the alkyl chain was focused on the homologue series [CnC1im][OTf] (n = 2, 4, 6, 8, 10 and 12).
The measurements were performed between 283 and 333 K using a multi-frequency impedance methodology and were based on the scanning of resistance, R, and reactance, X, from 20 Hz to 500 kHz. The electrical conductivity of the ionic liquids was derived from the extrapolation of resistance to infinite frequency. The utilization of this analytical procedure in combination with the multi-frequency scanning allows the mitigation of polarization effects . The experimental scanning showed that the structural effects were not only observed on the magnitude of the electrical conductivity but also on the shape of the R and X spectrums.
The Vogel-Fulcher-Tammann (VTF) equation was used to fit the conductivity – temperature data, σ(T) as well as to derive the pre-exponential coefficient and energy of barrier. Concerning the anion effect the following trend of electrical conductivity was found, NTf2 ~ BF4− > OTf > PF6−. The results show that the electrical conductivity is dependent of anion shape and interaction potential between the ions, which are well described by the pre-exponential coefficient and energy of barrier, respectively. Additionally, it was found that conductivity-temperature dependence could be strong enough to change the order of conductivity in the ILs. Regarding the cation effect, the electrical conductivity was found to decrease with the increase of the alkyl chain as it was expected. Furthermore, it was observed that the obtained electrical conductivity data and VTF coefficients may presented evidence of the nanostructuration of the ionic liquids .
This work was supported by the Fundacão para a Ciência e Tecnologia (FCT) through the FCT/MCTES (PIDDAC)) to CIQUP, Faculty of Science, University of Porto (Project UIDB/00081/2020),
IMS-Institute of Molecular Sciences (LA/P/0056/2020)). Carlos F. P. Miranda is grateful to FCT for his research grant (Reference: 2020.05717.BD).
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