Quasielastic neutron scattering characterization of the relaxation processes in a room temperature ionic liquid

Triolo, Alessandro; Russina, Olga; Arrighi, Valeria; Jurányi, Fanni; Janssen, Stefan; Gordon, Christopher P.

doi:10.1063/1.1613637

Cited by 125 publications

(135 citation statements)

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“…Wide-angle X-ray scattering results on the [C 4 mim][I] room temperature ionic liquids show prominent peaks in the residual radial distribution curve [118], indicating certain periodical arrangements of the iodide anions. The existence of different rotamers and local structures has also been found by interpretation of results from optical heterodynedetected Raman-induced Kerr-effect spectroscopy (OHD-RIKES) [139], from neutron scattering and diffraction experiments versus temperature [111][112][113]140], from Coherent anti-Stokes Raman Scattering (CARS) [141] and by theoretical molecular dynamics calculations [43,47,[142][143][144]. The local structures may also lead to other unique properties of ionic liquids; for example, if magnetic anions are aligned in RTILs, novel magnetic liquids will be created [145,146].…”

Section: Conformational Equilibria In Liquids Versus Temperaturementioning

confidence: 99%

“…Although other techniques such as infrared spectroscopy, X-ray, and neutron diffraction studies have been used to study these ions in the liquid or solid state or at surfaces [6,52,[109][110][111][112][113][114][115][116][117][118][119][120], a real gain in our understanding came with the combination of crystal structure solution, Raman spectroscopy and ab-initio DFT calculations [108,121]. We concentrate the story on the instructive example of the l-butyl-3-methylimidazolium cation, [C 4 mim] þ (see Fig.…”

Section: Brief Introduction To Ab-initio Model Calculationsmentioning

confidence: 99%

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Raman Spectroscopy and Ab-Initio Model Calculations on Ionic Liquids

2007

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Summary.A review of the recent developments in the study and understanding of room temperature ionic liquids are given. An intimate picture of how and why these liquids are not crystals at ambient conditions is attempted, based on evidence from crystallographical results combined with vibrational spectroscopy and ab-initio molecular orbital calculations. A discussion is given, based mainly on some recent FT-Raman spectroscopic results on the model ionic liquid system of 1-butyl-3-methylimidazolium ([C 4 mim][X]) salts. The rotational isomerism of the [C 4 mim] þ cation is described: the presence of anti and gauche conformers that has been elucidated in remarkable papers by Hamaguchi et al. Such presence of a conformational equilibrium seems to be a general feature of the room temperature liquids. Then the ''localized structure features'' that apparently exist in ionic liquids are described. It is hoped that the structural resolving power of Raman spectroscopy will be appreciated by the reader. It is of remarkable use on crystals of known different conformations and on the corresponding liquids, especially in combination with modern quantum mechanics calculations. It is hoped that these interdisciplinary methods will be applied to many more systems in the future. A few examples will be discussed.

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Section: Conformational Equilibria In Liquids Versus Temperaturementioning

confidence: 99%

Section: Brief Introduction To Ab-initio Model Calculationsmentioning

confidence: 99%

Raman Spectroscopy and Ab-Initio Model Calculations on Ionic Liquids

2007

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“…In order to clarify the above mentioned complex interplay one may study a plethora of physical properties, either with experiments [10][11][12][13][14][15][16][17][18] or simulations. 19 However, it is reasonably expected that the combination of both approaches will give a deeper insight [20][21][22][23] as on the one hand, simulations are able to decompose a physical property into contributions associated specifically with the cations, the anions and the coupling of both.…”

Section: Introductionmentioning

confidence: 99%

The influence of polarizability on the dielectric spectrum of the ionic liquid 1-ethyl-3-methylimidazolium triflate

Schröder

Sonnleitner

Buchner

et al. 2011

Phys. Chem. Chem. Phys.

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This work reports for the first time the computational, frequency-dependent dielectric spectrum of the polarizable molecular ionic liquid 1-ethyl-3-methylimidazolium triflate as well as its experimental analogue. In the frequency range from 500 MHz up to 20 GHz the agreement between the computational and the experimental spectrum is quantitative. For higher frequencies up to 10 THz the agreement is still remarkably good. The experimental asymptotic limit e N is 2.3. The difference in the computational value of 1.9 comes solely from the neglect of polarizability of the hydrogen atoms. For reasons of efficiency the simulations are based on the Lagrangian algorithm for the Drude oscillator model which cannot handle polarizable hydrogens. In the computational analysis the complete spectrum of the generalized dielectric constant P Ã 0 ðnÞ is splitted into its translational and non-translational components, called dielectric conductivity W 0 (n) and dielectric permittivity e(n). For 1-ethyl-3-methylimidazolium triflate both components contribute with equal weight and overlap in the complete frequency range. The inclusion of polarization forces, however, is quite different for the two components: the collective non-translational dynamics is accelerated and hence the dielectric permittivity is shifted to higher frequencies. The low frequency region of the dielectric conductivity is also affected while its high frequency part remains almost unchanged. Inductive effects are not only visible at high frequencies but also contribute in the sub-GHz region. The computational peak found in this region correlates with the experimental OKE-spectrum. It may be interpreted as the correlation between the induced dipole moment of the cations and the local electric field exerted by the anionic cage.

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“…The butyl chain for the imidazolium ring shows the minimum in the melting temperatures [13,15], but most important, bmim based RTIL are known to be easily supercooled. In particular, a quasielastic neutron scattering study of 1-butyl-3-methylimidazolium PF 6 (bmim-pf 6 ) showed the existence of two relaxation processes, the slower one showing the characteristics typical of the glass transition dynamics [17].Among all the available cations that form ionic liquids, we chose to study the family of bmim derivatives due to their property of being easily supercooled. In this work we study the dynamics of a series of RTIL based on the same bmim cation and different anions: chloride (cl), hexafluorophosphate (pf6), trifluoromethane sulfonate imide (msf), bis(trifluoromethane sulphonate)imide (bmsf) in the temperature range from 400 K down to 10 K. Differential scanning calorimetry (DSC) is used to detect phase transitions of the samples and to extract the calorimetric glass transition temperature T g .…”

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confidence: 99%

Evidence of secondary relaxations in the dielectric spectra of ionic liquids

2006

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We investigated the dynamics of a series of room temperature ionic liquids based on the same 1-butyl-3-methylimidazolium (BMIM) cation and different anions by means of broadband dielectric spectroscopy covering 15 decades in frequency, and in the temperature range from 400 K down to 10 K. A dc conductivity is observed in these systems above Tg with a typical ionic conductor behavior. Below, two relaxation processes appear, with the same characteristics as the secondary relaxations typically observed in glasses. The activation energy of the secondary processes and their dependence on the anion are different. The fast relaxation process is attributed to conformational changes in the butyl group of the BMIM cation and the slower process shows the characteristics of a genuine JG relaxation.The glass transition involves a dramatic slowing down of the structural relaxation in supercooled liquids from the ps time scale towards diverging times which ultimately brings the liquid into the glassy state. The only technique that can follow the evolution of the dynamics of the system in this huge time scale window is dielectric spectroscopy, and studies covering the full dynamic range (over 15 decades) of some paradigmatic glass formers are available [1][2][3]. Dielectric relaxation showed that secondary relaxations appear at frequencies higher than the main relaxation process, the importance of these processes being already pointed out more than three decades ago by Johary and Goldstein (JG) [4]. They proposed that these processes appear as a consequence of the glassy state, and they demonstrated that one type of process, commonly termed now "JG β relaxation", occurs in liquids of simple rigid molecules and does not involve intramolecular motion. The fundamental origin of secondary relaxations in supercooled liquids is a matter of current attention and dispute, with much effort being devoted to clarifying its true nature [5][6][7][8].Secondary relaxations have been studied in many systems, but the search of these processes in new types of materials should help to obtain a better understanding of their origin. A particular insight was obtained recently with binary systems, showing a continuous transformation between a high frequency wing and a JG relaxation when the composition of the mixture is changed [9]. Ionic systems, formed by anions and cations have been studied to a very low extent. The most common ionic materials are salts, like the NaCl, which have usually melting temperatures around 800• C. An exception is calcium potassium nitrate Ca 0.4 K 0.6 (NO 3 ) 1.4 (CKN), with a rather low T g at 333 K [10,11]. This so called molten salt was shown to exhibit a secondary mechanical relaxation, but it was never clearly observed in dielectric spectroscopy [4,12]. So the existence of the secondary relaxations in systems with free charges, and in particular, in purely ionic systems is an open question.A new class of chemicals has been discovered in the last decades, the room temperature ionic liquids (RTIL). They are molten salt...

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Quasielastic neutron scattering characterization of the relaxation processes in a room temperature ionic liquid

Cited by 125 publications

References 41 publications

Raman Spectroscopy and Ab-Initio Model Calculations on Ionic Liquids

Raman Spectroscopy and Ab-Initio Model Calculations on Ionic Liquids

The influence of polarizability on the dielectric spectrum of the ionic liquid 1-ethyl-3-methylimidazolium triflate

Evidence of secondary relaxations in the dielectric spectra of ionic liquids

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