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Relaxation dynamics in pyrrolidinium based ionic liquids: The role of the anion conformers
Accepted Manuscript Relaxation dynamics in pyrrolidinium based ionic liquids: The role of the anion conformers
O. Palumbo, F. Trequattrini, G.B. Appetecchi, L. Conte, A. Paolone PII: DOI: Reference:
S0167-7322(17)32750-2 doi: 10.1016/j.molliq.2017.08.017 MOLLIQ 7725
To appear in:
Journal of Molecular Liquids
Received date: Revised date: Accepted date:
26 June 2017 2 August 2017 5 August 2017
Please cite this article as: O. Palumbo, F. Trequattrini, G.B. Appetecchi, L. Conte, A. Paolone , Relaxation dynamics in pyrrolidinium based ionic liquids: The role of the anion conformers, Journal of Molecular Liquids (2017), doi: 10.1016/j.molliq.2017.08.017
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ACCEPTED MANUSCRIPT Relaxation dynamics in pyrrolidinium based ionic liquids: the role of the anion conformers. O. Palumbo1,*, F. Trequattrini1,2, G. B. Appetecchi3, L. Conte4 and A. Paolone1 CNR-ISC, U.O.S. La Sapienza, Piazzale A. Moro 5, 00185 Roma, Italy
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Physics Department, Sapienza University of Rome, Piazzale A. Moro 5, 00185 Roma, Italy Materials and Physicochemical Processes Laboratory (SSPT-PROMAS-MATPRO), Via
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3ENEA,
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School of Engineering, University of Padua, via Marzolo 9, 35131 Padova, Italy
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Anguillarese 301, 00123 Roma, Italy
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Abstract
We report density functional theory (DFT) calculations and low-frequency mechanical spectroscopy results on two ionic liquids having the same N-butyl-N-methyl-pyrrolidinium cation (PYR14), but anions,
(nonafluorobutanesulfonyl)(toluenesulfonyl)imide
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different
(IMT4)
and
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1,3 hexafluoropropane-disulfonylimide (IM3), belonging to the per(fluoroalkylsulfonyl)imide family. For both samples, a relaxation process is observed in their supercooled liquid phase, which can be described by a hopping model between non-equivalent configurations. In agreement with previous results, the relaxation is attributed to the ions motion. The comparison among calculations of the anion conformers population and the analysis of the experimental data points out that the
*Corresponding author: Oriele Palumbo, CNR-ISC, Piazzale A. Moro 5, I-00185 Rome, Italy e-mail: [email protected] Fax: +39-06- 49694323 Tel.: +39-06-49914400 1
ACCEPTED MANUSCRIPT non-equivalent configurations between which the ion motion occurs involve a different configuration for the anion.
Keywords
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1. Ionic liquids; 2. Conformers; 3. Ion dynamics
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ACCEPTED MANUSCRIPT Introduction In recent years, ionic liquids (ILs) have attracted great interest for both fundamental and applicative research due to their peculiar properties such us a wide liquid phase temperature range with a low melting point as they are classified as salts with melting point below 100 °C. Moreover, they
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usually show negligible vapor pressure, chemical and electrochemical stabilities up to high
reactions and synthesis but also for energy applications [1-3].
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temperatures, and high ionic conductivity, which make them attractive not only for catalytic
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These properties can be obtained by means of a proper choice of bulky and asymmetric organic
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cations, like imidazolium, pyrrolidinium, piperidinium, ammonium or alkyl phosphonium, and organic/inorganic anions, like hexafluorophosphate, tetra-fluoroborate, triflate, dicyanamide,
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tetracyanamethanide or bis(trifluoromethanesulfonyl)imide (TFSI) [1, 2]. All the properties of ILs are the consequence of competitive microscopic interaction forces, the balance of which can
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generate a wide range of results, allowing the possibility of tuning the properties of the liquids by
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assembling different combinations of cation and anion or by changing the length and the type of the side chains of the composing ions [1, 3]. A better physical understanding of the ions interactions
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and dynamics within the liquids would be highly useful also for practical applications.
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Many of the ions composing the ILs have geometric isomers, which can coexist in the liquid state according to the Boltzmann distribution [4-7] and whose configuration affects the physical and chemical properties of the ILs both in the liquid and solid phase. In particular, for ammonium and pyrrolidinium based ILs, we showed that the relative concentration of the anion conformers, and its variation as a function of temperature and pressure [6-7], is strictly related to the cation and to the length of its alkyl chain [5, 8]. Moreover, the suppression of the crystallization (and, therefore, the widening of the liquidus range) in mixed systems, composed by two ionic liquid sharing a common
3
ACCEPTED MANUSCRIPT TFSI anion and cations of the same family but differing in the length of the alkyl chain, is attributed to the competition between the two conformers of the TFSI anions [8-9]. Furthermore, by means of combined mechanical spectroscopy measurements and DFT calculation, we recently highlighted a possible role played by the anion conformers in the ion dynamics [10, 11].
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In particular, low-frequency mechanical spectroscopy experiments allowed the determination of the mechanical modulus and of its variation during the main phase transitions occurring by varying the
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temperature, in both liquid and solid state for two ionic liquids sharing the same TFSI anion but
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with different cations, 1-butyl-1-methylpyrrolidinium (PYR14) and 1-allyl-3-H-imidazolium (Allyl). A thermally activated relaxation process was reported in the liquid phase of both ILs and it was
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successfully analyzed in terms of a modified Debye model. The obtained parameters suggested the attribution of the observed relaxation peak to the ions motion, which can be described by a hopping
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model between non-equivalent configurations. DFT results on the possible conformers of the ions indicated that these configurations can be likely identified by the two anion conformers. Indeed,
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previous combined nuclear magnetic resonance and rheology experiments with ab initio calculations [12] in pyrrolidinium-based ILs provided measurements of the diffusion and selfdiffusion coefficients and suggested that any relative motion of two oppositely charged ions within
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the bulk liquid cannot just consist of simple “sliding” movements, but must involve rather complex
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intramolecular rearrangements. Moreover, the occurrence of translational motion by means of hopping processes and rearrangements of molecular network has also been suggested for various ILs [13].
In order to further ascertain the role of the anion configurations in the ion dynamics, in the present work we extended the low-frequency mechanical spectroscopy experiments to other pyrrolidinium based ILs, having the same PYR14 cation and different anions all belonging to the per(fluoroalkylsulfonyl)imide family, but having different asymmetry. In particular we studied N-butyl-N-methyl-pyrrolidinium (nonafluorobutanesulfonyl)(toluenesulfonyl)imide (PYR14-IMT4), 4
ACCEPTED MANUSCRIPT and N-butyl-N-methyl-pyrrolidinium 1,3hexafluoropropane-disulfonylimide (PYR14-IM3). Among the ILs with a per(fluoroalkylsulfonyl)imide anion, PYR14-TFSI has been firstly widely studied for its potential use as electrolyte in lithium batteries, for which it is important to enlarge the liquidus range by shifting the melting temperature to lower values. Later on, several combinations of cations belonging to the pyrrolidinium family and anions of the per(fluoroalkylsulfonyl)imide family were
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also studied to achieve this goal [3, 12, 14-16]. In particular, calorimetric measurements [3, 15-16]
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showed that an increase in the difference between the cation and the anion sizes induces a lowering of the melting transition temperature that ends in a very difficult crystallization, as observed in
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(trifluoromethanesulfonyl)(nonafluorobutanesulfonyl)imide
N-butyl-N-methyl-pyrrolidinium
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(PYR14-IM14), for which only a glass transition is detected around 190 K despite the repeated thermal cycles carried out at low rates and at low temperatures. A similar behavior has been
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reported also for PYR14-IMT4 [3], indicating that the unfavorable packing of strongly asymmetric anions such as IM14 or IMT4 remarkably lowers the cation-anion lattice energy and, by slowing
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down the kinetics, hinders the crystallization of the ionic liquid materials [15], which is detectable
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only at very low temperature rate [15].
The anions selected for the present study present different geometries (Figure 1), thus a different
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number and distribution of conformer is expected; however, to our knowledge a systematic
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conformational analysis is still lacking for IMT4 and IM3 anions. To fill this gap, in the present work density functional calculations have been extended to these two anions providing for the first time information about their possible conformers and their relative minimum energy state. In particular, with the aim of verifying the effective involvement of the anion conformers in the diffusive process, the IMT4 and IM3 anions were selected since for the former we expected a high number of possible conformer configurations, due to its long alkyl chain, while for the latter we expected a lower number of configurations.
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Figure 1. Geometries of the anion composing the studied samples and of their low energy
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conformers.
It is worth noting that also the PYR14 cation presents different conformers [17] but, in our previous
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work [10], the comparison of the DMA spectra measured on ILs having the same TFSI anion but
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different cations [10] suggested that the cation conformers were not involved in the relaxation process. The presently obtained results further confirm the central role of the anion conformational
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structures in the intramolecular rearrangements involved in the dynamics of the studied ILs and
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support the idea that the cation conformers are not directly involved in the process.
Computational Preliminary force field calculations were performed using the Spartan software [18, 19] to find the possible geometries of the ions composing the ILs; possible duplicates were eliminated during this procedure. Starting from these inputs, we performed DFT calculations by the same software to find the stable points on the potential energy surface (PES) and to calculate the energy of each 6
ACCEPTED MANUSCRIPT conformer. DFT calculation were performed by the B3LYP hybrid density functional theory methods, adopting the 6-31G** basis set. This particular choice of basis set and theory was extensively used in the literature in order to locate stable points on the PES of the ions composing
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the ILs [4-5, 20-22].
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Experimental
The investigated samples were synthesized by a procedure developed at ENEA and described in
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detail elsewhere [14-15]. Their geometries are shown in Figure 1.
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Dynamic mechanical analysis was carried out using a PerkinElmer DMA 8000 instrument, following a procedure already used by us in previous works [10-11]; the samples, which are all in
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the liquid phase at room temperature, were laid out into a stainless steel Material Pocket, supplied by PerkinElmer, with dimensions of 30.0 mm by 14.0 mm by 0.5 mm; the pocket, which is scored
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in the mid-point, was then folded in half and crimped closed to form a sandwich. Flexural vibration
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measurements were performed in the three-point bending configuration. The storage modulus, M, and the elastic energy dissipation, tan δ, were measured in an inert argon atmosphere, at frequencies
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of 1 Hz and 10 Hz, during cooling/heating scan at 4 Kmin-1, between 150 and 330 K.
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It should be pointed out that, with this setup, the stress applied on the sample is not a pure shear stress but, due to the spatial isotropy of liquids, the mechanical modulus presently measured is a combination of both the shear and the bulk modulus [10, 23-24]. The experimental setup provides the opportunity of measuring during the same run the mechanical response function of the sample by changing both the frequency and temperature, in a large T range. Moreover, it allows the measurement of the modulus both in the liquid and solid phase and during the phase transitions, thus allowing the application of theoretical models usually adopted for the
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ACCEPTED MANUSCRIPT analysis of mechanical spectra of solids to the whole measured spectrum, also including contributions to the spectra coming from the non-solid phases. In case species can move between two configurations with a relaxation rate -1 by means of thermal activation in a standard anelastic solid [25], the elastic energy dissipation presents a maximum
given by:
1
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tan T
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when the Debye relaxation condition, = 1, is satisfied. For a single relaxation time, , tanis
(1)
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where is the angular vibration frequency and the relaxation strength is proportional to the
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concentration of the relaxing species, to the elastic modulus and to the change in the local distortion. α is the Fuoss–Kirkwood width parameter and is equal to 1 for a single time Debye
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relaxation; α < 1 produces broadened peaks with respect to Debye ones. For classical Arrhenius processes = 0eW/kT, where W is the activation energy, whilst assuming for
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the relaxation time a Vogel-Fulcher-Tamman type (VFT) temperature dependence: [ ⁄ (
)]
(2)
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where W is the activation energy and T0 is the VFT temperature (i.e., the temperature at which the
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configurational entropy of the system is equal to zero). The empirical VFT formula has been largely used to describe the temperature dependence of several physical properties of ionic liquids above the glass transition, like the conductivity and the inverse of the viscosity [1], as observed in many others glass-forming liquids. If relaxation occurs between two equivalent sites, the relaxation strength in eq. (1) decreases with increasing T. Instead, in the case of hopping between two non-equivalent configurations with energy separation E, the relaxation strength, which is proportional to the product of the respective populations in the two configurations, becomes [26]: 8
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c E sech 2 T 2kT
(3)
Considering equations 1 and 3, a more general expression for tan δ is then given by: c 1 T cosh E 2kT
(4)
2
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tan
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Results and discussion
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The computational results of the present study show that the anion IM3 does not possess conformers. On the contrary, for the IMT4 ion the conformational analysis indicates that this ion
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possesses 51 conformers, with the higher energy conformers spanning the entire energy range up to ~24 kJ/mol. However, calculations pointed out the occurrence of two low energy configurations
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named as tt and tg, whose geometry is reported in Fig.1: the tt is the most stable, while at an energy 15 meV (1.31 kJ/mol) higher we found the tg conformer. Both conformers display a trans
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configuration of the S-N-S bonds. However, the lowest energy conformer shows an all-trans
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configuration of the alkyl chain, while the conformer of higher energy shows a gauche-trans configuration of the part of the chain more distant from sulfur atom.
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Figure 2 displays the DMA spectra (modulus, M, and tan δ) of PYR14-IMT4 measured on cooling at 4 Kmin-1. The storage modulus is reported as relative variation with respect to the value at 290 K
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(M/M290 K − 1) because it is not possible to separate the contribute of the ILs from that of the pocket. However, as reported elsewhere [10] the curves of both M and tan δ measured for the empty pocket are flat in the whole temperature range of the measurements and have to be considered as a background. The tan δ curve plotted as a function of temperature shows a peak, at∼ 272 K (for a vibration frequency of 1 Hz), which is likely due to a thermally activated relaxation process because its maximum shifts at higher temperature with increasing frequency (∼ 290 K for a vibration frequency 9
ACCEPTED MANUSCRIPT of 10 Hz; see Figure 2). Concomitantly, a step is observed in the modulus. On further cooling, between 245 and 220 K (for a vibration frequency of 1 Hz), the anelastic spectra show an intense stiffening of the modulus and an intense peak of tan δ. These features are the signs of the occurrence of the glass transition, as observed for other systems. Indeed previous DSC measurements on PYR14-IMT4 reported the occurrence of the glass transition around this
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temperature [15].
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1.0
0.0
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tan
1.5
0.5
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0.2
1 Hz cooling 10 Hz cooling
2.0
M /M290K
PYR14-IMT4
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0.3
230
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0.0
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0.1
240
250
260
270
280
290
300
T (K)
Figure 2. DMA spectra of the pocket containing Pyr14–IMT4 measured at two frequencies. The continuous line is a fit according to eqs. 1−4 for a thermally activated peak.
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ACCEPTED MANUSCRIPT The detection of the glass transition typical features in the spectra of the sample indicates that, at the used temperature rate the liquid does not crystallize on cooling and the observed relaxation processes occur in its supercooled liquid phase. Moreover, this thermally activated relaxation process is similar to the one found in the supercooled liquid phase of the parent compound PYR14-TFSI, which was analyzed in terms of a modified
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Debye model [10] and indeed the data measured at both frequencies can be reasonably fitted
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(continuous lines in Figures 2) using the same model (eq 4), which is appropriated for jumps in an
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asymmetrical potential well. The relaxation time (τ) was assumed to follow a VFT temperature dependence (eq 2). The values of the best-fit parameters are reported in Table 1 where, for a
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comparison, also the values previously obtained for PYR14-TFSI and Allyl-TFSI have been
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reported.
PYR14-IMT4
PYR14-IM3
Allyl-TFSI[10]
(1.7 ± 0.4) 10−13
(8±1) 10-14
(2.7±0.7) 10-14
(9.3 ± 4.5) 10−14
0.36 ± 0.01
0.51±0.02
0.43±0.01
0.37 ± 0.01
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E [eV]
PYR14-TFSI[10]
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τ0 [s]
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reference for the reported values.
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TABLE 1. Best fit parameters obtained for the relaxation processes. Superscript indicates the
T0 [K]
80± 3
61±6
76±2
67± 2
ΔE [meV]
26± 2
15± 3
2±2
30± 4
α
0.7 ± 0. 4
0.88±0.32
0.95±0.45
0.7 ± 0.5
The value obtained for pre-exponential factor of the relaxation time is typical of highly viscous liquids approaching the glass transition [27] and is in good agreement with the literature [28-29], while the width parameter α lower than 1 indicates interaction among the relaxing units. The activation energy for PYR14-IMT4 provided by our analysis is 0.51 ± 0.02 eV; to the best of our 11
ACCEPTED MANUSCRIPT knowledge, only a few data of W for this particular ionic liquid are reported in the literature. Indeed, there is only a self diffusion coefficient plot [15], which indicates that this liquid should have an activation energy slightly higher than PYR14-TFSI. This is in agreement with our results, even though the absolutes values reported in the paper for the mobility coefficients are lower than
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ours [15, 30]. Our analysis further indicates that the value for the energy separation of the non-equivalent
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configurations is 0.15 ± 0.03 eV, which is in perfect agreement with the energy separation obtained
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above for the IMT4 anion conformers, all-trans (tt) trans-gauche (tg) configurations, which is found to be 0.15 eV by means of DFT calculations. Indeed, in our previous paper by a comparison with
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the energy separation obtained for the cations conformers [10], we showed that only the anion
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conformers are likely to be involved in the different configurations among which the ions can rearrange.
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The central role of the anions conformers in the definition of the different configurations among
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which the ions can rearrange in the diffusive process is further confirmed by the data collected on sample PYR14-IM3, whose anion does not possess different conformers, according to our
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calculations. Figure 3 displays the DMA spectra (relative modulus variation and tan δ) of PYR14-IM3 measured on cooling at 4 Kmin-1. These spectra present the same features observed for
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the former two samples, i.e. on cooling, one can observe first a relaxation process (with the maximum around 246 K for a vibration frequency of 1 Hz) and at a lower temperature (around 210 K) an intense stiffening of the modulus and an intense and narrow peak of tan δ, which are likely due to the glass transition reported around 200 K by DSC measurements [15]. The relaxation peak was analyzed with the same modified Debye model using eq. 4 and the values obtained for the best fit parameters are reported in Table 1. The values obtained for the preexponential factor of the relaxation time, the width parameter α and the activation energy are close to those obtained for PYR14-IM14 and PYR14-IMT4 samples. On the contrary, the value obtained for 12
ACCEPTED MANUSCRIPT the energy separation of the non-equivalent configurations is 2 ± 2 meV, which indicates that for this sample, there is no asymmetry between the configurations among which the ions can move. This is in perfect agreement with DFT calculation, which for the IM3 anion showed that the
180
190
200
210
220
230
PYR14-IM3
260
270
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1 Hz cooling 10 Hz cooling
2 0
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0.4
6 4
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0.5
8
M /M290K
0.6
tan
250
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0.7
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0.3
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0.2
190
200
210
220
230
240
250
260
270
280
T (K)
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180
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0.1 0.0
240
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0.8
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occurrence of different conformers is not expected.
Figure 3. DMA spectra of the pocket containing Pyr14–IM3 measured at two frequencies. The continuous line is a fit according to eqs. 1−4 for the thermally activated peak.
This finding strongly support the anion central role in the diffusive motion of ions in ILs. Indeed, the importance of the anion has been highlighted also for the crystallization, in particular a thermal study of several mixtures of pyrrolidinium-based ionic liquids having a cation with different alkyl 13
ACCEPTED MANUSCRIPT chains and anions of the per(fluoroalkylsulfonyl)imide family showed that the crystallization is mainly driven by the formation of a matrix composed of the anions, no matter whether there is only one anion or two in the mixture. The crystal state or the glassy state can be achieved if the cations are able or not to fit into this resulting anions frame [3]. Furthermore, the competition between different anion conformer configurations can induce the suppression of crystalline phases in
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mixtures [8-9]. The present results contribute to enlightening the role of the anion conformers
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configuration in the definition of the dynamics and kinetics of ions in ILs. Indeed, the previous
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computational and experimental results on PYR14-TFSI and Allyl-TFSI ruled out a direct participation of the cations in the different configuration, since the energy difference between their
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lower energy conformers was different from the asymmetry between the configurations among which the ions can move. The present results further confirm that the major role in the definition of
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the non-equivalent configurations is played by the anions conformers.
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Conclusions
In the present paper DFT calculations and low-frequency mechanical experiments are performed for
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the first time on PYR14-IMT4 andPYR14-IM3. Results confirm the occurrence of a relaxation process, previously already reported for other TFSI based ILs, and attributed to the ions motion,
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which can be described by a hopping model between non-equivalent configurations. Moreover, the comparison between calculations and data analysis provide evidence of the connection between these non-equivalent configurations and the anion conformers. The present results contribute to enlightening the role of the anion conformers configuration in the definition of the dynamics and kinetics of ions in ILs.
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ACCEPTED MANUSCRIPT Rassolov, P.E. Maslen, P.P. Korambath, R.D. Adamson, B. Austin, J. Baker, E.F.C. Byrd, H. Dachsel, R.J. Doerksen, A. Dreuw, B.D. Dunietz, A.D. Dutoi, T.R. Furlani, S.R. Gwaltney, A. Heyden, S. Hirata, C-P. Hsu, G. Kedziora, R.Z. Khalliulin, P. Klunzinger, A.M. Lee, M.S. Lee, W.Z. Liang, I. Lotan, N. Nair, B. Peters, E.I. Proynov, P.A. Pieniazek, Y.M. Rhee, J. Ritchie, E. Rosta, C.D. Sherrill, A.C. Simmonett, J.E. Subotnik, H.L. Woodcock III, W. Zhang, A.T. Bell,
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A.K. Chakraborty, D.M. Chipman, F.J. Keil, A.Warshel, W.J. Hehre, H.F. Schaefer, J. Kong, A.I.
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Krylov, P.M.W. Gill and M. Head-Gordon, Advances in Methods and Algorithms in A Modern
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Quantum Chemistry Program Package, Phys. Chem. Chem. Phys. 2006, 8, 3172-3191.
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[19] W.J. Hehre, A Guide to Molecular Mechanics and Quantum Chemical Calculations,
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Wavefunction, Inc., Irvine, CA, 2003.
[20] M. Herstedt, M. Smirnov, P. Johansson, M. Chami, J. Grondin, L. Servant, J. C. Lassègues,
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Spectroscopic Characterization of the Conformational States of the
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Bis(trifluoromethanesulfonyl)imide Anion (TFSI-), J. Raman Spectrosc. 2005, 36, 762-770.
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[21] A. Martinelli, A. Matic, P. Johansson, P. Jacobsson, L. Börjesson, A. Fernicola, S. Panero, B.
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Scrosati, H. Ohno, Conformational Evolution of TFSI− In Protic and Aprotic Ionic Liquids, J. Raman Spectr. 2011, 42, 522-528. [22] J. N. Canongia Lopes, K. Shimizu, A. A. H. Pádua, Y. Umebayashi, S. Fukuda, K. Fujii, S.-I. Ishiguro, A Tale of Two Ions: The Conformational Landscapes of Bis(trifluoromethanesulfonyl)amide and N,N-Dialkylpyrrolidinium, J. Phys. Chem. B 2008, 112, 1465-1472.
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ACCEPTED MANUSCRIPT [23] T. Yamaguchi, S. Miyake, and S. Koda, Shear Relaxation of Imidazolium-Based RoomTemperature Ionic Liquids. J. Phys. Chem. B 2010, 114, 8126–8133
[24]W. Makino, R. Kishikawa, M. Mizoshiri, S. Takeda, and M. Yao,Viscoelastic properties of
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room temperature ionic liquids, J. Chem. Phys. 2008, 129, 104510-104517.
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[25] A. S. Nowick, B. S. Berry, Anelastic Relaxation in Crystalline Solids; Academic Press: New
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York, 1972.
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[26] G.Cannelli, R. Cantelli, F. Cordero, F. Trequattrini, Dynamics of Hydrogen, Oxygen, and
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Dislocations in Yttrium by Acoustic Spectroscopy, Phys. Rev. B 1997, 55, 14865-14871.
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Ionic Liquids. Phys. Rev. B. 2006, 73, 212201
[29] C. Krause, J. R. Sangoro, C. Iacob, F. Kremer, Charge Transport and Dipolar Relaxations in Imidazolium-Based Ionic Liquids. J. Phys. Chem. B 2010, 114, 382−386.
[30] F. Castiglione, E. Ragg, A. Mele, G. B. Appetecchi, M. Montanino, S. Passerini, Molecular Environment and Enhanced Diffusivity of Li+ Ions in Lithium-Salt-Doped Ionic Liquid Electrolytes. J. Phys. Chem. Lett. 2011, 2, 153−157.
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ACCEPTED MANUSCRIPT Highlights
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Conformational analysis of IMT4 and IM3 anions performed by DFT Temperature dependence of the ILs mechanical modulus both in liquid and glass phase. Observation of a relaxation which is attributed to the motion of ions. The motion occurs between non equivalent configurations involving anion conformers.
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O. Palumbo, F. Trequattrini, G.B. Appetecchi, L. Conte, A. Paolone PII: DOI: Reference:
S0167-7322(17)32750-2 doi: 10.1016/j.molliq.2017.08.017 MOLLIQ 7725
To appear in:
Journal of Molecular Liquids
Received date: Revised date: Accepted date:
26 June 2017 2 August 2017 5 August 2017
Please cite this article as: O. Palumbo, F. Trequattrini, G.B. Appetecchi, L. Conte, A. Paolone , Relaxation dynamics in pyrrolidinium based ionic liquids: The role of the anion conformers, Journal of Molecular Liquids (2017), doi: 10.1016/j.molliq.2017.08.017
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ACCEPTED MANUSCRIPT Relaxation dynamics in pyrrolidinium based ionic liquids: the role of the anion conformers. O. Palumbo1,*, F. Trequattrini1,2, G. B. Appetecchi3, L. Conte4 and A. Paolone1 CNR-ISC, U.O.S. La Sapienza, Piazzale A. Moro 5, 00185 Roma, Italy
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Physics Department, Sapienza University of Rome, Piazzale A. Moro 5, 00185 Roma, Italy Materials and Physicochemical Processes Laboratory (SSPT-PROMAS-MATPRO), Via
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3ENEA,
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1
School of Engineering, University of Padua, via Marzolo 9, 35131 Padova, Italy
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Anguillarese 301, 00123 Roma, Italy
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Abstract
We report density functional theory (DFT) calculations and low-frequency mechanical spectroscopy results on two ionic liquids having the same N-butyl-N-methyl-pyrrolidinium cation (PYR14), but anions,
(nonafluorobutanesulfonyl)(toluenesulfonyl)imide
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different
(IMT4)
and
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1,3 hexafluoropropane-disulfonylimide (IM3), belonging to the per(fluoroalkylsulfonyl)imide family. For both samples, a relaxation process is observed in their supercooled liquid phase, which can be described by a hopping model between non-equivalent configurations. In agreement with previous results, the relaxation is attributed to the ions motion. The comparison among calculations of the anion conformers population and the analysis of the experimental data points out that the
*Corresponding author: Oriele Palumbo, CNR-ISC, Piazzale A. Moro 5, I-00185 Rome, Italy e-mail: [email protected] Fax: +39-06- 49694323 Tel.: +39-06-49914400 1
ACCEPTED MANUSCRIPT non-equivalent configurations between which the ion motion occurs involve a different configuration for the anion.
Keywords
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1. Ionic liquids; 2. Conformers; 3. Ion dynamics
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ACCEPTED MANUSCRIPT Introduction In recent years, ionic liquids (ILs) have attracted great interest for both fundamental and applicative research due to their peculiar properties such us a wide liquid phase temperature range with a low melting point as they are classified as salts with melting point below 100 °C. Moreover, they
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usually show negligible vapor pressure, chemical and electrochemical stabilities up to high
reactions and synthesis but also for energy applications [1-3].
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temperatures, and high ionic conductivity, which make them attractive not only for catalytic
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These properties can be obtained by means of a proper choice of bulky and asymmetric organic
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cations, like imidazolium, pyrrolidinium, piperidinium, ammonium or alkyl phosphonium, and organic/inorganic anions, like hexafluorophosphate, tetra-fluoroborate, triflate, dicyanamide,
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tetracyanamethanide or bis(trifluoromethanesulfonyl)imide (TFSI) [1, 2]. All the properties of ILs are the consequence of competitive microscopic interaction forces, the balance of which can
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generate a wide range of results, allowing the possibility of tuning the properties of the liquids by
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assembling different combinations of cation and anion or by changing the length and the type of the side chains of the composing ions [1, 3]. A better physical understanding of the ions interactions
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and dynamics within the liquids would be highly useful also for practical applications.
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Many of the ions composing the ILs have geometric isomers, which can coexist in the liquid state according to the Boltzmann distribution [4-7] and whose configuration affects the physical and chemical properties of the ILs both in the liquid and solid phase. In particular, for ammonium and pyrrolidinium based ILs, we showed that the relative concentration of the anion conformers, and its variation as a function of temperature and pressure [6-7], is strictly related to the cation and to the length of its alkyl chain [5, 8]. Moreover, the suppression of the crystallization (and, therefore, the widening of the liquidus range) in mixed systems, composed by two ionic liquid sharing a common
3
ACCEPTED MANUSCRIPT TFSI anion and cations of the same family but differing in the length of the alkyl chain, is attributed to the competition between the two conformers of the TFSI anions [8-9]. Furthermore, by means of combined mechanical spectroscopy measurements and DFT calculation, we recently highlighted a possible role played by the anion conformers in the ion dynamics [10, 11].
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In particular, low-frequency mechanical spectroscopy experiments allowed the determination of the mechanical modulus and of its variation during the main phase transitions occurring by varying the
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temperature, in both liquid and solid state for two ionic liquids sharing the same TFSI anion but
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with different cations, 1-butyl-1-methylpyrrolidinium (PYR14) and 1-allyl-3-H-imidazolium (Allyl). A thermally activated relaxation process was reported in the liquid phase of both ILs and it was
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successfully analyzed in terms of a modified Debye model. The obtained parameters suggested the attribution of the observed relaxation peak to the ions motion, which can be described by a hopping
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model between non-equivalent configurations. DFT results on the possible conformers of the ions indicated that these configurations can be likely identified by the two anion conformers. Indeed,
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previous combined nuclear magnetic resonance and rheology experiments with ab initio calculations [12] in pyrrolidinium-based ILs provided measurements of the diffusion and selfdiffusion coefficients and suggested that any relative motion of two oppositely charged ions within
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the bulk liquid cannot just consist of simple “sliding” movements, but must involve rather complex
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intramolecular rearrangements. Moreover, the occurrence of translational motion by means of hopping processes and rearrangements of molecular network has also been suggested for various ILs [13].
In order to further ascertain the role of the anion configurations in the ion dynamics, in the present work we extended the low-frequency mechanical spectroscopy experiments to other pyrrolidinium based ILs, having the same PYR14 cation and different anions all belonging to the per(fluoroalkylsulfonyl)imide family, but having different asymmetry. In particular we studied N-butyl-N-methyl-pyrrolidinium (nonafluorobutanesulfonyl)(toluenesulfonyl)imide (PYR14-IMT4), 4
ACCEPTED MANUSCRIPT and N-butyl-N-methyl-pyrrolidinium 1,3hexafluoropropane-disulfonylimide (PYR14-IM3). Among the ILs with a per(fluoroalkylsulfonyl)imide anion, PYR14-TFSI has been firstly widely studied for its potential use as electrolyte in lithium batteries, for which it is important to enlarge the liquidus range by shifting the melting temperature to lower values. Later on, several combinations of cations belonging to the pyrrolidinium family and anions of the per(fluoroalkylsulfonyl)imide family were
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also studied to achieve this goal [3, 12, 14-16]. In particular, calorimetric measurements [3, 15-16]
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showed that an increase in the difference between the cation and the anion sizes induces a lowering of the melting transition temperature that ends in a very difficult crystallization, as observed in
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(trifluoromethanesulfonyl)(nonafluorobutanesulfonyl)imide
N-butyl-N-methyl-pyrrolidinium
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(PYR14-IM14), for which only a glass transition is detected around 190 K despite the repeated thermal cycles carried out at low rates and at low temperatures. A similar behavior has been
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reported also for PYR14-IMT4 [3], indicating that the unfavorable packing of strongly asymmetric anions such as IM14 or IMT4 remarkably lowers the cation-anion lattice energy and, by slowing
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down the kinetics, hinders the crystallization of the ionic liquid materials [15], which is detectable
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only at very low temperature rate [15].
The anions selected for the present study present different geometries (Figure 1), thus a different
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number and distribution of conformer is expected; however, to our knowledge a systematic
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conformational analysis is still lacking for IMT4 and IM3 anions. To fill this gap, in the present work density functional calculations have been extended to these two anions providing for the first time information about their possible conformers and their relative minimum energy state. In particular, with the aim of verifying the effective involvement of the anion conformers in the diffusive process, the IMT4 and IM3 anions were selected since for the former we expected a high number of possible conformer configurations, due to its long alkyl chain, while for the latter we expected a lower number of configurations.
5
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Figure 1. Geometries of the anion composing the studied samples and of their low energy
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conformers.
It is worth noting that also the PYR14 cation presents different conformers [17] but, in our previous
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work [10], the comparison of the DMA spectra measured on ILs having the same TFSI anion but
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different cations [10] suggested that the cation conformers were not involved in the relaxation process. The presently obtained results further confirm the central role of the anion conformational
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structures in the intramolecular rearrangements involved in the dynamics of the studied ILs and
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support the idea that the cation conformers are not directly involved in the process.
Computational Preliminary force field calculations were performed using the Spartan software [18, 19] to find the possible geometries of the ions composing the ILs; possible duplicates were eliminated during this procedure. Starting from these inputs, we performed DFT calculations by the same software to find the stable points on the potential energy surface (PES) and to calculate the energy of each 6
ACCEPTED MANUSCRIPT conformer. DFT calculation were performed by the B3LYP hybrid density functional theory methods, adopting the 6-31G** basis set. This particular choice of basis set and theory was extensively used in the literature in order to locate stable points on the PES of the ions composing
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the ILs [4-5, 20-22].
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Experimental
The investigated samples were synthesized by a procedure developed at ENEA and described in
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detail elsewhere [14-15]. Their geometries are shown in Figure 1.
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Dynamic mechanical analysis was carried out using a PerkinElmer DMA 8000 instrument, following a procedure already used by us in previous works [10-11]; the samples, which are all in
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the liquid phase at room temperature, were laid out into a stainless steel Material Pocket, supplied by PerkinElmer, with dimensions of 30.0 mm by 14.0 mm by 0.5 mm; the pocket, which is scored
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in the mid-point, was then folded in half and crimped closed to form a sandwich. Flexural vibration
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measurements were performed in the three-point bending configuration. The storage modulus, M, and the elastic energy dissipation, tan δ, were measured in an inert argon atmosphere, at frequencies
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of 1 Hz and 10 Hz, during cooling/heating scan at 4 Kmin-1, between 150 and 330 K.
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It should be pointed out that, with this setup, the stress applied on the sample is not a pure shear stress but, due to the spatial isotropy of liquids, the mechanical modulus presently measured is a combination of both the shear and the bulk modulus [10, 23-24]. The experimental setup provides the opportunity of measuring during the same run the mechanical response function of the sample by changing both the frequency and temperature, in a large T range. Moreover, it allows the measurement of the modulus both in the liquid and solid phase and during the phase transitions, thus allowing the application of theoretical models usually adopted for the
7
ACCEPTED MANUSCRIPT analysis of mechanical spectra of solids to the whole measured spectrum, also including contributions to the spectra coming from the non-solid phases. In case species can move between two configurations with a relaxation rate -1 by means of thermal activation in a standard anelastic solid [25], the elastic energy dissipation presents a maximum
given by:
1
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tan T
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when the Debye relaxation condition, = 1, is satisfied. For a single relaxation time, , tanis
(1)
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where is the angular vibration frequency and the relaxation strength is proportional to the
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concentration of the relaxing species, to the elastic modulus and to the change in the local distortion. α is the Fuoss–Kirkwood width parameter and is equal to 1 for a single time Debye
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relaxation; α < 1 produces broadened peaks with respect to Debye ones. For classical Arrhenius processes = 0eW/kT, where W is the activation energy, whilst assuming for
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the relaxation time a Vogel-Fulcher-Tamman type (VFT) temperature dependence: [ ⁄ (
)]
(2)
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where W is the activation energy and T0 is the VFT temperature (i.e., the temperature at which the
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configurational entropy of the system is equal to zero). The empirical VFT formula has been largely used to describe the temperature dependence of several physical properties of ionic liquids above the glass transition, like the conductivity and the inverse of the viscosity [1], as observed in many others glass-forming liquids. If relaxation occurs between two equivalent sites, the relaxation strength in eq. (1) decreases with increasing T. Instead, in the case of hopping between two non-equivalent configurations with energy separation E, the relaxation strength, which is proportional to the product of the respective populations in the two configurations, becomes [26]: 8
ACCEPTED MANUSCRIPT T
c E sech 2 T 2kT
(3)
Considering equations 1 and 3, a more general expression for tan δ is then given by: c 1 T cosh E 2kT
(4)
2
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tan
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Results and discussion
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The computational results of the present study show that the anion IM3 does not possess conformers. On the contrary, for the IMT4 ion the conformational analysis indicates that this ion
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possesses 51 conformers, with the higher energy conformers spanning the entire energy range up to ~24 kJ/mol. However, calculations pointed out the occurrence of two low energy configurations
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named as tt and tg, whose geometry is reported in Fig.1: the tt is the most stable, while at an energy 15 meV (1.31 kJ/mol) higher we found the tg conformer. Both conformers display a trans
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configuration of the S-N-S bonds. However, the lowest energy conformer shows an all-trans
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configuration of the alkyl chain, while the conformer of higher energy shows a gauche-trans configuration of the part of the chain more distant from sulfur atom.
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Figure 2 displays the DMA spectra (modulus, M, and tan δ) of PYR14-IMT4 measured on cooling at 4 Kmin-1. The storage modulus is reported as relative variation with respect to the value at 290 K
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(M/M290 K − 1) because it is not possible to separate the contribute of the ILs from that of the pocket. However, as reported elsewhere [10] the curves of both M and tan δ measured for the empty pocket are flat in the whole temperature range of the measurements and have to be considered as a background. The tan δ curve plotted as a function of temperature shows a peak, at∼ 272 K (for a vibration frequency of 1 Hz), which is likely due to a thermally activated relaxation process because its maximum shifts at higher temperature with increasing frequency (∼ 290 K for a vibration frequency 9
ACCEPTED MANUSCRIPT of 10 Hz; see Figure 2). Concomitantly, a step is observed in the modulus. On further cooling, between 245 and 220 K (for a vibration frequency of 1 Hz), the anelastic spectra show an intense stiffening of the modulus and an intense peak of tan δ. These features are the signs of the occurrence of the glass transition, as observed for other systems. Indeed previous DSC measurements on PYR14-IMT4 reported the occurrence of the glass transition around this
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temperature [15].
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1.0
0.0
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tan
1.5
0.5
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0.2
1 Hz cooling 10 Hz cooling
2.0
M /M290K
PYR14-IMT4
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0.3
230
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0.0
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0.1
240
250
260
270
280
290
300
T (K)
Figure 2. DMA spectra of the pocket containing Pyr14–IMT4 measured at two frequencies. The continuous line is a fit according to eqs. 1−4 for a thermally activated peak.
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ACCEPTED MANUSCRIPT The detection of the glass transition typical features in the spectra of the sample indicates that, at the used temperature rate the liquid does not crystallize on cooling and the observed relaxation processes occur in its supercooled liquid phase. Moreover, this thermally activated relaxation process is similar to the one found in the supercooled liquid phase of the parent compound PYR14-TFSI, which was analyzed in terms of a modified
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Debye model [10] and indeed the data measured at both frequencies can be reasonably fitted
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(continuous lines in Figures 2) using the same model (eq 4), which is appropriated for jumps in an
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asymmetrical potential well. The relaxation time (τ) was assumed to follow a VFT temperature dependence (eq 2). The values of the best-fit parameters are reported in Table 1 where, for a
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comparison, also the values previously obtained for PYR14-TFSI and Allyl-TFSI have been
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reported.
PYR14-IMT4
PYR14-IM3
Allyl-TFSI[10]
(1.7 ± 0.4) 10−13
(8±1) 10-14
(2.7±0.7) 10-14
(9.3 ± 4.5) 10−14
0.36 ± 0.01
0.51±0.02
0.43±0.01
0.37 ± 0.01
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E [eV]
PYR14-TFSI[10]
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τ0 [s]
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reference for the reported values.
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TABLE 1. Best fit parameters obtained for the relaxation processes. Superscript indicates the
T0 [K]
80± 3
61±6
76±2
67± 2
ΔE [meV]
26± 2
15± 3
2±2
30± 4
α
0.7 ± 0. 4
0.88±0.32
0.95±0.45
0.7 ± 0.5
The value obtained for pre-exponential factor of the relaxation time is typical of highly viscous liquids approaching the glass transition [27] and is in good agreement with the literature [28-29], while the width parameter α lower than 1 indicates interaction among the relaxing units. The activation energy for PYR14-IMT4 provided by our analysis is 0.51 ± 0.02 eV; to the best of our 11
ACCEPTED MANUSCRIPT knowledge, only a few data of W for this particular ionic liquid are reported in the literature. Indeed, there is only a self diffusion coefficient plot [15], which indicates that this liquid should have an activation energy slightly higher than PYR14-TFSI. This is in agreement with our results, even though the absolutes values reported in the paper for the mobility coefficients are lower than
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ours [15, 30]. Our analysis further indicates that the value for the energy separation of the non-equivalent
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configurations is 0.15 ± 0.03 eV, which is in perfect agreement with the energy separation obtained
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above for the IMT4 anion conformers, all-trans (tt) trans-gauche (tg) configurations, which is found to be 0.15 eV by means of DFT calculations. Indeed, in our previous paper by a comparison with
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the energy separation obtained for the cations conformers [10], we showed that only the anion
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conformers are likely to be involved in the different configurations among which the ions can rearrange.
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The central role of the anions conformers in the definition of the different configurations among
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which the ions can rearrange in the diffusive process is further confirmed by the data collected on sample PYR14-IM3, whose anion does not possess different conformers, according to our
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calculations. Figure 3 displays the DMA spectra (relative modulus variation and tan δ) of PYR14-IM3 measured on cooling at 4 Kmin-1. These spectra present the same features observed for
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the former two samples, i.e. on cooling, one can observe first a relaxation process (with the maximum around 246 K for a vibration frequency of 1 Hz) and at a lower temperature (around 210 K) an intense stiffening of the modulus and an intense and narrow peak of tan δ, which are likely due to the glass transition reported around 200 K by DSC measurements [15]. The relaxation peak was analyzed with the same modified Debye model using eq. 4 and the values obtained for the best fit parameters are reported in Table 1. The values obtained for the preexponential factor of the relaxation time, the width parameter α and the activation energy are close to those obtained for PYR14-IM14 and PYR14-IMT4 samples. On the contrary, the value obtained for 12
ACCEPTED MANUSCRIPT the energy separation of the non-equivalent configurations is 2 ± 2 meV, which indicates that for this sample, there is no asymmetry between the configurations among which the ions can move. This is in perfect agreement with DFT calculation, which for the IM3 anion showed that the
180
190
200
210
220
230
PYR14-IM3
260
270
280
1 Hz cooling 10 Hz cooling
2 0
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0.4
6 4
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0.5
8
M /M290K
0.6
tan
250
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0.7
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0.3
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0.2
190
200
210
220
230
240
250
260
270
280
T (K)
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180
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0.1 0.0
240
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0.8
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occurrence of different conformers is not expected.
Figure 3. DMA spectra of the pocket containing Pyr14–IM3 measured at two frequencies. The continuous line is a fit according to eqs. 1−4 for the thermally activated peak.
This finding strongly support the anion central role in the diffusive motion of ions in ILs. Indeed, the importance of the anion has been highlighted also for the crystallization, in particular a thermal study of several mixtures of pyrrolidinium-based ionic liquids having a cation with different alkyl 13
ACCEPTED MANUSCRIPT chains and anions of the per(fluoroalkylsulfonyl)imide family showed that the crystallization is mainly driven by the formation of a matrix composed of the anions, no matter whether there is only one anion or two in the mixture. The crystal state or the glassy state can be achieved if the cations are able or not to fit into this resulting anions frame [3]. Furthermore, the competition between different anion conformer configurations can induce the suppression of crystalline phases in
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mixtures [8-9]. The present results contribute to enlightening the role of the anion conformers
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configuration in the definition of the dynamics and kinetics of ions in ILs. Indeed, the previous
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computational and experimental results on PYR14-TFSI and Allyl-TFSI ruled out a direct participation of the cations in the different configuration, since the energy difference between their
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lower energy conformers was different from the asymmetry between the configurations among which the ions can move. The present results further confirm that the major role in the definition of
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the non-equivalent configurations is played by the anions conformers.
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Conclusions
In the present paper DFT calculations and low-frequency mechanical experiments are performed for
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the first time on PYR14-IMT4 andPYR14-IM3. Results confirm the occurrence of a relaxation process, previously already reported for other TFSI based ILs, and attributed to the ions motion,
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which can be described by a hopping model between non-equivalent configurations. Moreover, the comparison between calculations and data analysis provide evidence of the connection between these non-equivalent configurations and the anion conformers. The present results contribute to enlightening the role of the anion conformers configuration in the definition of the dynamics and kinetics of ions in ILs.
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Based Ionic Liquids and Their Binary Mixtures, J. Phys. Chem. C 2010, 114, 12364 12369.
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[4] F. M. Vitucci, F. Trequattrini, O. Palumbo, J.-B. Brubach, P. Roy, A. Paolone, Infrared Spectra
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of Bis(trifluoromethanesulfonyl)imide Based Ionic Liquids: Experiments and Ab-initio Simulations,
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ACCEPTED MANUSCRIPT [7] F. Capitani, F. Trequattrini, O. Palumbo, A. Paolone, P. Postorino, Phase transitions of PYR14TFSI as a function of pressure and temperature: the competition between smaller volume and lower energy conformer, J. Phys. Chem. B, 2016, 120, 2921–2928.
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[8] O. Palumbo, F. Trequattrini, G. B. Appetecchi, and A. Paolone; Influence of Alkyl Chain Length on Microscopic Configurations of the Anion in the Crystalline Phases of PYR1A-TFSI, J.
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ACCEPTED MANUSCRIPT [13] S. Schr dle, G. Annat, D. R. MacFarlane, M. Forsyth, R. Buchner, G. Hefter, High Frequency Dielectric Response of the Ionic Liquid N-Methyl-N-ethylpyrrolidinium Dicyanamide. Aust. J. Chem. 2007, 60, 6−8.
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ACCEPTED MANUSCRIPT [23] T. Yamaguchi, S. Miyake, and S. Koda, Shear Relaxation of Imidazolium-Based RoomTemperature Ionic Liquids. J. Phys. Chem. B 2010, 114, 8126–8133
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ACCEPTED MANUSCRIPT Highlights
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Conformational analysis of IMT4 and IM3 anions performed by DFT Temperature dependence of the ILs mechanical modulus both in liquid and glass phase. Observation of a relaxation which is attributed to the motion of ions. The motion occurs between non equivalent configurations involving anion conformers.
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