Abstract
Li-bis(trifluoromethylsulfonyl)imide based ionic liquids with either butyl-trimethylammonium or N,N-dimethyl-N-(2-(propionyloxy)-ethyl)butan-1-ammonium as the anion were studied using proton and fluorine relaxometry as well as using field-gradient diffusometry to gain separate access to cation and anion dynamics in these compounds. The transport parameters obtained for these ionic liquids are compared with the estimates based on the conductivity data from literature and from the present work. The impact of cation size on correlation effects, the latter parameterized in terms of various Haven ratios, is mapped out.
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Author contributions: All the authors have accepted responsibility for the entire content of this submitted manuscript and approved submission.
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Research funding: We thank the Deutsche Forschungsgemeinschaft for funding this work under project No. 396060266.
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Conflict of interest statement: The authors declare no conflicts of interest regarding this article.
Appendix: Radio-frequency electric impedance experiments on BTMA–TFSI
For BTMA–TFSI low-temperature electrical impedance data are obviously not available. Therefore, we carried out coaxial high-frequency reflectometry as well as low-frequency impedance measurements and present our results in Figure 6 in the modulus format. The complex electrical modulus is M* = M′ + iM" = 1/ε* = iωε 0/σ* where σ* designates the complex conductivity, ε* the complex dielectric constant, and ε 0 the permittivity of free space. From the maxima of M" conductivity relaxation times for the liquid and for the crystalline phase were determined and added to Figure 5. The σ 0 data obtained in this work are given in Table 1. For the liquid, the dc conductivities determined from the data are included in Figure 4 as well. From microwave experiments for BTMA–TFSI it was reported that σ 0 = 1.72 mS cm−1 at 25 °C [21].
The imaginary part of the electrical modulus M" spectra of BTMA–TFSI is shown. For 310–240 K the data are recorded upon cooling in steps of 10 K. After the sample had crystallized, a few additional modulus spectra were taken. As an example, here we show an M" spectrum acquired at 260 K. Clearly, as compared to the liquid, in the crystal a much slower conductivity response (lower peak frequency and larger peak amplitude) is observed.
Compilation of the dc-conductivities measured in the present work. The first four entries correspond to the crystallized sample, the others refer to the liquid state.
| T (K) | 245 | 250 | 255 | 260 | 260 | 270 | 280 | 290 | 300 |
|---|---|---|---|---|---|---|---|---|---|
| log10[σ 0/(S cm−1)] | −11.7 | −11.1 | −10.3 | −9.45 | −3.84 | −3.46 | −3.15 | −2.91 | −2.69 |
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