Structural analysis of 1-ethyl-3-methylimidazolium bifluoride melt

doi:10.1016/S0168-583X(02)01400-3

Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms

Volume 199, January 2003, Pages 29-33

https://doi.org/10.1016/S0168-583X(02)01400-3 Get rights and content

Abstract

The structure of 1-ethyl-3-methylimidazolium bifluoride (EMImF · HF) melt has been analyzed at 333 K by a high-energy synchrotron X-ray diffraction method. The total correlation function of the EMImF · HF melt was similar to that of the solid state, indicating that not only the short range but also the intermediate-range ordering in the solid are partially preserved in the liquid state. The intra-molecular F–F correlation in the anions clearly appears in the total correlation function of the EMImF · HF melt, whereas prominent peaks are not observed in the case of a room temperature molten salt, 1-ethyl-3-methylimidazolium fluorohydrogenate.

Introduction

Recently, room temperature molten salts (RTMS) have received an attention for their interesting characteristics from the viewpoint of electrochemical and synthetic applications [1], [2], [3]. A series of alkylimidazolium fluorohydrogenates (RR^′ImF · 2.3HF, RR^′Im = 1-methylimidazolium (MIm), 1-ethyl-3-methylimidazolium (EMIm), 1-butyl-3-methylimidazolium (BMIm), 1-hexyl-3-methyl-imidazolium (HMIm)) have been synthesized [4] since the first discovery of highly conductive room temperature molten imidazolium salt, 1-ethyl-3-methylimidazolium fluorohydrogenate (EMImF · 2.3HF) (100 mS cm⁻¹ at 298 K) [5], [6]. The RR^′ImF · 2.3HF have relatively high conductivities and low viscosities compared with other RTMS. Infrared and NMR spectroscopic measurements suggest the existence of fluorohydrogenate anions such as H₂F₃⁻ and H₃F₄⁻, rapidly exchanging HF among them [4], [5], [6]. 1-Ethyl-3-methylimidazolium bifluoride (EMImF · HF) is crystallized from the HF-deficient melt obtained by thermal decomposition of the EMImF · 2.3HF melt. EMImF · HF is a solid with a melting point of 324 K having a unique layered structure containing cationic and anionic pillars (the anion is FHF⁻, D_∞h) [7].

Structural analysis of short-range structure of ionic species and intermediate-range order produced by the anions and cations of these salts is necessary to elucidate the mechanism to give high conductivity and low viscosity. As described above, the anions in these salts are oligomers formulated as H_nF_n+1⁻ and structural characterization of them in liquid state is also essential to the present study. The structures of the anions in the crystal KF·nHF, shown in Fig. 1 with the EMIm⁺ cation, are well-established [9]; FHF⁻ (n=1, D_∞h), H₂F₃⁻ (n=2, C_2v) and H₃F₄⁻ (n=3, D_3h). High-energy X-ray diffraction technique using synchrotron radiation is a powerful tool for the structural analysis of RR^′ImF-HF system containing complex ions of light elements, because the diffraction spectra, obtained up to high Q (=(4π/λ)sinθ, 2θ: scattering angle, λ: wavelength of photons) region, give high real space resolution in the distribution functions.

We have recently started high-energy X-ray diffraction measurements of RR^′ImF · 2.3HF melts in order to investigate the relationship between physical properties and their liquid structure [8]. The preliminary results suggest that the intermediate-range order of the cations and anions in the solid EMImF · HF are partially preserved in the RR^′ImF · 2.3HF melt. The purpose of the present study is to analyze the liquid structure of EMImF · HF melt to compare it with those of solid EMImF · HF and EMImF · 2.3HF melt in order to obtain preliminary insights of the liquid structure of highly conductive RTMS, RR^′ImF · 2.3HF.

Section snippets

Experimental techniques

The preparations of the samples were made as previously reported [5], [6], [7].

High-energy X-ray diffraction experiments were carried out using photon energy of 61.7 keV obtained with a Si(2 2 0) bent monochromator at SPring-8 high-energy X-ray diffraction beamline BL04B2 [10], [11]. Hygroscopic EMImF · HF was encapsulated in an airtight cell illustrated in Fig. 2 in a glove box of argon atmosphere with a gas purifier. A pair of Kapton film windows (25 μm thickness) for windows and an acryl

Results and discussion

Fig. 3 shows the calculated X-ray diffraction spectrum of the solid EMImF · HF (a), total structure factors, S(Q), of the EMImF · HF melt (b) and EMImF · 2.3HF melt (c). Fig. 4 shows the calculated total correlation functions, T(r), of the solid EMImF · HF (a), EMImF · HF melt (b) and EMImF · 2.3HF melt (c). The position of the first sharp diffraction peak (FSDP) of the EMImF · HF melt is the same as that of the EMImF · 2.3HF melt (Q=1.85 Å⁻¹). The solid EMImF · HF shows a layered structure in which the pillars

Conclusion

The liquid structure of the EMImF · HF melt was investigated by means of a high-energy synchrotron X-ray diffraction. The total structure factor, S(Q), of the EMImF · HF melt was similar to that of EMImF · 2.3HF. The coincidence between the positions of the (0 2 0) diffraction peaks of solid EMImF · HF and the FSDP in the EMImF · HF melt suggests the layered structure in solid EMImF · HF is partially preserved even in the liquid state.

Acknowledgments

The authors would like to thank to Sanko Chemical Industry, Co., Ltd., for their supply of EMImCl. The authors are indebted to Mr. A. Kajinami for his assistance in the measurement with temperature control. This work was supported by Grant in Aid for Scientific Research by the Japan Society for the Promotion of Science, Kansai Research Foundation for Technology Promotion, and Toyota High-tech Research Grant Program.

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