Relaxation Dynamics and Phase Transitions in Ionic Liquids

Article pubs.acs.org/JPCB

Relaxation Dynamics and Phase Transitions in Ionic Liquids: Viscoelastic Properties from the Liquid to the Solid State O. Palumbo,*,† F. Trequattrini,‡ F. M. Vitucci,† and A. Paolone† †

CNR-ISC, U.O.S. La Sapienza, Piazzale A. Moro 5, 00185 Roma, Italy Physics Department, Sapienza University of Rome, Piazzale A. Moro 5, 00185 Roma, Italy

‡

ABSTRACT: In the present work we performed low-frequency mechanical spectroscopy experiments to measure the mechanical modulus of two ionic liquids and its variation during the main phase transitions occurring by varying the temperature, in the both liquid and the solid states. The liquids share the same anion, the bis(trifluoromethanesulfonyl)imide, and present different cations, 1butyl-1-methylpyrrolidinium and 1-allyl-3-H-imidazolium. A thermally activated relaxation process is found in the liquid phase and is analyzed in terms of a modified Debye model. The obtained parameters provide indications about the nature and the mechanism giving rise to the peak, which is attributed to the ions motion by means of hopping processes. Moreover, density functional calculations were performed, and the comparison with the analysis of the experimental data suggests that the anion conformers are likely to be involved in the different configurations among which the ions can rearrange.

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INTRODUCTION

Because of their high viscosity and scarce propensity to crystallization, the dynamic behavior of ILs resembles that of supercooled liquids, and they can be typically classified as fragile glass formers.4 Indeed, they generally follow time− temperature superposition, and the temperature dependence of the relaxation time is well approximated by the empirical Vogel−Fulcher−Tamman (VFT) equation.1,5,6 Moreover, the dynamics of ionic liquids has been studied by temperaturedependent experiments using dielectric spectroscopy, nuclear magnetic resonance, and neutron scattering4,6,7 in both the supercooled liquid and glassy amorphous states. Dielectric relaxation and NMR measurements on imidazolium-based ILs observed the rotations of the cation around its main axes and an interion motion due to the variation in the distance between anion and cation in the ion-pair motion.8−11 Indeed dielectric relaxation measurements in N-methyl-N-ethylpyrrolidinium dicyanamide observed at higher frequencies a hopping process due to a translational, oscillatory motion of anions and cations relative to each other as they exchange partners in ion pairs8 and at lower frequencies the correlated rearrangement of a molecular network, similar to that of nonionic hydrogenbonded liquids.9 Besides, dielectric spectra of supercooled imidazolium based ILs showed evidence of the occurrence of a secondary Johari and Goldstein relaxation,6,10 which was attributed to a common butyl group. Moreover, the VFT temperature dependence of the conductivity is attributed to a similar dependence of the mobility, while the number density of the charge carriers follows an Arrhenius thermal law.11

Ionic liquids (ILs) are inorganic salts with melting point below 100 °C; in particular, many of them are liquid even below room temperature. To achieve such low melting temperatures, ILs are usually composed of bulky ions because increasing the size of the ions decreases ion−ion interactions and prevents efficient packing of the ions into a crystal structure.1 Similarly, these ions are usually asymmetric with a large degree of charge delocalization. Indeed, most of the cationic species that form ionic liquids contain heterocyclic rings such as imidazolium or pyridinium moieties, which have an asymmetric electric charge distribution and possess dipole moments. Ionic liquids have useful properties such as very large liquids ranges, low volatility, high conductivity, wide electrochemical windows, the ability to solubilize a broad variety of organic and inorganic materials, and viscosities exceeding those of most common solvents. Indeed, ILs have been largely studied for their possible use in a wide range of areas including catalysis and electrochemistry. In particular, they have been applied in lithium batteries,1,2 fuel cells, supercapacitors, and recently in solar cells.1 Moreover, their attractive properties can be tuned by changing the cation and the anion. To date, the progresses in ionic liquids design have been achieved through an empirical approach to property modification.3 A better physical understanding of the ions interactions and dynamics within the liquids would be highly useful also for practical applications. The viscoelastic properties of several ILs have been measured as a function of frequency and temperature in the liquid phase;4,5 however, a deep comprehension of the connection among these macroscopic features and the microscopic structure of cations and anions is still needed. © 2015 American Chemical Society

Received: June 24, 2015 Revised: September 21, 2015 Published: September 23, 2015 12905

DOI: 10.1021/acs.jpcb.5b06039 J. Phys. Chem. B 2015, 119, 12905−12911

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The Journal of Physical Chemistry B

information is available about possible conformer structures and their relative energies in AllylIm. To address this lack of knowledge in the present paper, we peformed density functional calculations to individuate possible conformers and their relative minimum energy state. Indeed our work suggests a possible role of the different conformational structures in the intramolecular rearrangements involved in the dynamics of the studied ILs.

Combined nuclear magnetic resonance and rheology experiments with ab initio calculations in pyrrolidinium-based ILs provided measurements of diffusion and self-diffusion coefficients and suggested that any relative motion of two oppositely charged ions within the bulk liquid cannot just consist of simple “sliding” movements but must involve rather complex intramolecular rearrangements.4 Moreover, they also provided evidence of the formation of aggregates in the liquids, whose organization results in a mesoscopic ordering, which is one of the most peculiar and fascinating features of ILs. The shear fluidity of several ILs was also measured at low frequency in the liquid state.12−14 To deepen the knowledge of the dynamical processes in ILs and how they are related to the microscopic structure of the cations and the anions, we studied ILs by means of low frequency mechanical spectroscopy. Indeed, mechanical spectroscopy has been widely used to study phase transitions, chemical transformations, and relaxation processes in a wide set of materials.15−19 In the present work, for the first time, we performed lowfrequency mechanical spectroscopy experiments to measure the mechanical modulus of the ionic liquids and its variation during the main phase transitions occurring by varying the temperature, in both the liquid and the solid states. This result is achieved by means of a pocket, which works as a substrate. Moreover, additional information with respect to the common calorimetric measurements is obtained. Indeed, previous works on mechanical properties of ionic liquids provided information in either their liquid or solid state. The analysis of the obtained data gives new insights into the phase transitions and the relaxation processes occurring in the ILs. In particular, in the present paper we will focus on the relaxation process, which is found in the liquid phase and is analyzed in terms of a modified Debye model. The obtained parameters provide indications about the nature and the mechanism giving rise to the peak. The ILs studied in the present work share the same anion, the bis(trifluoromethanesulfonyl)imide, and present different cations, containing both a heterocyclic ring, 1-butyl-1methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (C11H20N2F6, labeled as PYR14-TFSI), and 1-allyl-3-Himidazolium bis(trifluoromethanesulfonyl)imide (C8H9N3F6S2O4, labeled as AllylIm-TFSI). The anion and the two cations are all flexible molecules that can adopt different conformational structures, energetically inequivalent, whose concentration in the liquid or solid phase can affect the physical and chemical properties of ILs. The conformers of TFSI have been largely investigated:20−23 The transoid conformer with a C2 symmetry is more stable than the cisoid one, which has a C1 symmetry; however, the two conformers are energetically separated by only 2.2 kJ/mol (22.8 meV), so that in the liquid state both conformers are usually present. The occurrence of conformers of the pyrrolidinium ion is due to the fact that the C4N ring is not planar, and therefore it can adopt either the envelope or the twist configuration. In the first case, one of the atoms is located outside the plane defined by the other four atoms; on the contrary, in the twist configuration two atoms are located above and below the plane constructed by the other three atoms. The envelope and twist conformers can easily convert one into the other by means of pseudorotations, as the difference in energy between the different conformers can be as low as ∼2 kJ/mol;22,24 indeed, it has been suggested that these two conformers are both present in equilibrium in PYR14-TFSI. To our knowledge, very poor

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EXPERIMENTAL SECTION

The investigated PYR14-TFSI (purity >99.9%) and AllylImTFSI (purity = 99%), were purchased from Solvionic and used as received. Differential scanning calorimetry measurements were performed by a Mettler-Toledo DSC 821, under an inert nitrogen flux, cooling from room temperature to 170 K and then heating back to 290 K with a scan rate of 4 K/min. Dynamic mechanical analysis was carried out using a PerkinElmer DMA 8000 instrument. The samples, which are all in the liquid phase at room temperature, were laid out into a stainless-steel Material Pocket, supplied by PerkinElmer, with dimensions of 30.0 mm × 14.0 mm × 0.5 mm. The pocket, which is scored in the midpoint, was then folded in half and crimped closed to form a sandwich. Flexural vibration measurements were performed in three-point bending configuration. The storage modulus, M, and the elastic energy dissipation, tan δ, were measured in an inert argon atmosphere at frequencies of 1 and 10 Hz during cooling/heating at 4 K/ min between 150 and 330 K. It must be pointed out that, with this setup, the stress applied on the sample is not a pure shear stress, but because of the spatial isotropy of liquids, the mechanical moduli presently measured are a combination of both the shear and the bulk moduli.25,26 The experimental setup provides the opportunity of measuring during the same run the mechanical response functions of the sample by changing both the frequency and the temperature, in a large temperature range. It allows the measurement of the modulus in both the liquid and solid phases and during the phase transitions, thus allowing the application of theoretical models usually adopted for the analysis of mechanical spectra of solids to the whole measured spectrum, also including contributions to the spectra coming from the nonsolid phases. In case a species can move between two configurations with a relaxation rate τ−1 by means of thermal activation, in a standard anelastic solid,27 the elastic energy dissipation presents a maximum when the Debye relaxation condition, ωτ = 1, is satisfied. For a single relaxation time, τ, tan δ is given by tan δ = Δ(T )

−α

(ωτ )

1 + (ωτ )α

(1)

where ω is the angular vibration frequency and the relaxation intensity Δ is proportional to the concentration of the relaxing species, to the elastic modulus, and to the change in the local distortion and α is the Fuoss−Kirkwood width parameter; α = 1 for single Debye relaxation, while α < 1 produces broadened peaks. For classical Arrhenius processes τ = τ0eW/kT, where W is the activation energy, while assuming for the relaxation time τ a Vogel−Fulcher−Tamman type (VFT) temperature dependence 12906

DOI: 10.1021/acs.jpcb.5b06039 J. Phys. Chem. B 2015, 119, 12905−12911

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The Journal of Physical Chemistry B

τ = τ0e[E / k(T − T0)]

configuration of the four carbon atoms of the C4N ring (envelope) and the butyl group at the equatorial position against these four atoms. At an energy only 1.8 kJ/mol higher, one finds the axial-envelope configurations, with the butyl group at an axial position against the four carbon atoms of the pyrrolidinium ring. About 3 kJ/mol higher than the most stable one, we find the twist-axial conformer, in which two atoms of the C4N ring are located above and below the plane constructed by the other three atoms. These values are in good agreement with a partial study of the conformers of PYR14.24 Higher energy conformer span the entire energy range between 3 and 25 kJ/mol. In Figure 2 we report the geometry of the three lowest energy conformers.

(2)

where E is the activation energy and T0 is the VFT temperature. The empirical VFT formula has been largely used to describe the temperature dependence of several physical properties of the 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 the relaxation occurs between two equivalent sites, the relaxation intensity (Δ) in eq 1 decreases with increasing T, leading to a higher intensity for the peaks measured at lower frequencies. Instead, in the case of hopping between two nonequivalent configurations with energy separation ΔE, the relaxation intensity, which is proportional to the product of the respective populations in the two configurations, becomes28 Δ(T ) ∝

⎛ ΔE ⎞ c ⎟ sech2⎜ ⎝ 2kT ⎠ T

(3)

and leads to higher intensity of peaks detected at higher ω. Considering eqs 1 and 3, a more general expression for tan δ is then given by c 1 tan δ = −α 2 T cosh (ΔE /2kT ) (ωτ ) + (ωτ )α (4)

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COMPUTATIONAL SECTION Preliminary force-field calculations were performed using the Spartan software29,30 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 with the same software to find the stable points on the potential energy surface (PES) and to calculate the energy of each 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 previous literature to locate stable point on the PES of the ions composing the ILs.20,21,23,31−33 We found two conformers for the TFSI ion (Figure 1), namely, cis-TFSI and trans-TFSI, in agreement with the

Figure 2. Optimized structure of the three lowest energy pyrrolidinium conformers at the 6-31G**-B3LYP level.

Concerning the allyl-imidazolium ion, we performed an original investigation of its conformers, as, to the best of our knowledge, no information is available. The most stable conformer displays a planar imidazolium ring and the CH2CHCH2 chain at an axial position. (See Figure 3.) We could find that two other conformers exist, which, however, differ from the most stable one by an energy of 77.6 and 85.6 kJ/mol, respectively.

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RESULTS AND DISCUSSION Figure 4 displays the DMA spectra (modulus, M, and tan δ) of PYR14-TFSI measured on cooling and subsequent heating at 4

Figure 1. Equilibrium geometries of the trans-TFSI and cis-TFSI ions, as obtained by the DFT calculations (upper panels: top view; lower panels: side view).

previous literature.20,21,23,31−33 Calculations suggest that the transoid one is the most stable, having an energy lower than that of the cisoid conformer by ΔE ≈ 3.4 kJ/mol. This value is compatible with the ones obtained by means of spectroscopic techniques, which are in the range 3.4−7.3 kJ/mol.20,31−33 The pyrrolidinium ion possesses 33 conformers. The most stable is called equatorial-envelope, and it has a planar

Figure 3. Optimized structure of the three lowest energy allylimidazolium conformers at the 6-31G**-B3LYP level. 12907

DOI: 10.1021/acs.jpcb.5b06039 J. Phys. Chem. B 2015, 119, 12905−12911

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composite system, such as the one constituted by the same liquid swelling a polymeric membrane, which we recently studied.2 Indeed, in this case the membrane and the IL formed an interacting system, and the relaxation process presently observed on cooling the pure liquid was not observed in the composite system. The obtained results confirm that this technique can be applied to study the mechanical properties of the ionic liquids, providing information in both their liquid and solid states. Moreover, the possibility of measuring the modulus variations during phase transitions allows the study of the kinetics of the transformation, as already done for different systems.2,18,37,38 In particular, isothermal measurements of the elastic modulus performed by DMA on PYR14-TFSI swelling a PVdF membrane provided a direct measure of the Avrami index of the crystallization process observed on cooling the system; however, the study of the kinetics of the phase transformations occurring on the pure PYR14-TFSI by means of elastic modulus measurements is beyond the aim of the present paper and is reported elsewhere.39 It must be noticed, however, that the study of the kinetics of phase transformations by mechanical modulus measurements can be applied also to second-order phase transitions, which do not involve any latent heat and cannot be studied by the usual calorimetric techniques. In the following we will focus on the analysis and the possible mechanisms for the observed relaxation process. The peak measured at higher frequency displays an intensity higher than the one obtained at lower frequency, suggesting the occurrence of hopping between two non equivalent sites, 1 and 2, with an energy separation, ΔE. Indeed, as shown in Figure 5,

Figure 4. DSC traces (upper part) and DMA spectra (lower part) of PYR14-TFSI, measured on cooling (blue symbols) and subsequent heating (red symbols) at 4 K/min and at two frequencies, 1 (open symbols) and 10 Hz (full symbols). The DMA spectra of the empty pocket are shown as a background (green dots).

K/min. The storage modulus is reported as relative variation with respect to the value at 290 K (M/M290 K − 1) because it is not possible to separate the contribute of the ILs from that of the pocket. However, the curves of both M and tan δ measured for the empty pocket are flat in the whole temperature range of the measurements and are to be considered as a background (Figure 4). As previously reported,2,34−36 when cooled at this temperature rate, the PYR14-TFSI passes into the supercooled liquid state, as shown by the DSC curve,2 which presents only a small exothermic peak around 190 K, corresponding to the glass transition. At the same temperature, the anelastic spectra measured by DMA shows an intense stiffening of the modulus and an intense and narrow peak of tan δ. These features are the signs of the occurrence of the glass transition, as observed for other systems.2 Moreover, on cooling at ∼230 K (for a vibration frequency of 1 Hz) a peak is observed in the tan δ curve, with a concomitant step in the modulus (Figure 4). At the same temperature the DSC trace does not display any variation,2 indicating that the mechanism giving rise to this peak does not involve any heat exchange. It rather corresponds to a thermally activated relaxation process because its maximum shifts at higher temperature with increasing frequency (∼245 K for a vibration frequency of 10 Hz; see Figure 4). On heating back to room temperature the DSC trace2 shows an endothermic peak around 190 K (the glass transition occurring on heating back), two exothermic peaks around 210 and 240 K, and an intense endothermic peak around 265 K. In particular, the peak at 210 K corresponds to the cold crystallization of the sample and the one at 265 K to the melting.2 In correspondence with all of these peaks, the DMA spectra present clear features on both the real and the imaginary parts of the elastic modulus, which are typical of phase transitions because they do not display any temperature shift with the frequency. In particular, the occurrence of the cold crystallization is displayed by an abrupt stiffening of the modulus at 210 K. It is noticeable that in the present work the anelastic spectra are measured on the pure liquid and not on a

Figure 5. DMA spectra of the pocket containing PYR14-TFSI, measured at two frequencies. The continuous line is a fit according to eqs 1−4 for the thermally activated peak.

the data of both frequencies can be reasonably fitted (continuous line in Figure 5) using eq 4, which is appropriated for jumps between asymmetrical potential well and assuming for the relaxation time τ a Vogel−Fulcher−Tamman type (VFT) temperature dependence (eq 2). The Debye relaxation has been already used to describe the dielectric spectra of some imidazolium-based ILs;7 here we suggest that the hopping occurs between two nonequivalent configurations as a consequence of an asymmetrical potential profile. Indeed, a shear-mechanical spectroscopy study of supercooled liquids dynamics40 supported the validity of the 12908

DOI: 10.1021/acs.jpcb.5b06039 J. Phys. Chem. B 2015, 119, 12905−12911

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in PYR14-TFSI; therefore, to ascertain if one of the ions plays a major role in the observed dynamics, we performed the same kind of measurements on a different liquid having the same anion (TFSI) but a different cation (AllylIm), an imidazolium with conformers separated by a rather different energy value (80 meV, see the Computational Section). The anelastic spectra of AllylIm-TFSI measured on cooling at 4 K/min are reported in Figure 6. When cooled at this temperature rate it does not crystallize and shows a glass transition at ∼190 K(DSC results not shown).

shoving model, which follows from a classical estimation heights from curvature at non equivalent energy minima.41 The values of the best-fit parameters are τ0 = (1.7 ± 0.4) 10−13 s, E = 0.36 ± 0.01 eV, T0 = 80 ± 3 K, ΔE = 26 ± 2 meV, and α = 0.7 ± 0.4. The obtained pre-exponential factor for the relaxation time is typical of highly viscous liquids approaching the glass transition 41 and is in good agreement with previous literature.6,10 The width parameter α lower than 1 indicates interaction among the relaxing units. The value obtained for the activation energy is slightly higher but with comparable magnitude to those ones reported for several secondary relaxations despite the fact that these relaxations are different from the one presently reported. Indeed dielectric spectra in imidazolium-based supercooled ILs detected the occurrence of two secondary relaxation processes having an Arrhenius linear temperature dependence with activation energies of 0.38 and 0.22 eV.6 These processes were attributed to the common butyl group in the imidazolium cation. Moreover, a later broadband dielectric spectroscopy10 study also detected secondary relaxations with comparable energies, which were attributed to the librational motion of the imidazolium ring. The dynamic behavior of PYR14-TFSI has been studied by measuring the viscosities and the diffusion coefficients in the temperature range between 290 and 350 K.4 It has been found that their temperature dependence is very similar, leading to very close activation energies; in particular, they obtained 0.30 eV from the viscosity and 0.35 eV for the diffusion of both cation and anion.4,42 This last value is well comparable to the energy obtained by our model. Moreover, ab initio calculations lead to the identification of several local minima with an energy spread for the different ion pair configurations close to the measured activation energy, suggesting that the ions motions within the bulk liquid cannot consist of simple sliding movements but must involve rather complex molecular rearrangement. Indeed the occurrence of translational motion by means of hopping processes and rearrangements of molecular network has also been suggested for different ILs.8 In the previously cited works the activation energies are obtained by fitting the temperature dependence of the relaxation rates6,10 of the viscosities or of the diffusion coefficients.4 It is worth noting that in the present work the dynamical parameters are obtained by fitting the whole shape of the relaxation peak. This procedure allows the introduction of parameters that describe the shape and the intensity of the spectra providing more detailed information about the relaxation mechanism and particularly suggesting the existence of nonequivalent configurations having an energy separation ΔE = 26 ± 2 meV. This value is comparable to the energy separations reported in PYR14-TFSI for the two anion conformers, cis and trans configurations (22.8 meV), as well for the envelope and twist cation conformers (21.0 meV). This suggests that the different conformers are likely involved in the nonequivalent configurations, giving rise to the observed dynamic process and that the occurrence of different conformers configuration can play a role in the dynamics of ILs. The nonequivalent configurations among which the ILs rearrange can be due to different conformers state of the ions, and indeed the relaxation is observed only in the liquid phase, where both conformers should be present. The values of the energy separation between different conformers are very close for the cation and the anion

Figure 6. DMA spectra of the pocket containing AllylIm-TFSI, measured at two frequencies. The continuous line is a fit according to eqs 1−4 for the thermally activated peak.

The measured spectra (see Figure 6) are similar to the those reported for PYR14-TFSI. At ∼190 K it displays the features of the glassy transition, that is, an intense stiffening of the modulus and an intense and narrow peak of tan δ. At higher temperature (∼235 K for a vibration frequency of 10 Hz) a relaxation process is clearly detectable (symbols in Figure 6), even if the low-temperature tail of this peak is not clear due to the superposition of the high temperature side of the glasstransition peak. Similarly to the process analyzed for PYR14TFSI this peak is thermally activated, too, as its maximum shifts to higher temperature with increasing frequency (∼245 K for a vibration frequency of 10 Hz) and displays higher intensity when measured at higher frequency (Figure 6). The data can be satisfactory fitted (continuous line in Figure 6) with the same formula previously used for PYR14-TFSI (eqs 2 and 4), and we obtain the following values for the fitting parameters: τ0 = (9.3 ± 4.5) 10−14 s, E = 0.37 ± 0.01 eV, T0 = 67 ± 2 K, ΔE = 30 ± 4 meV, and α = 0.70 ± 0.5. These values are very similar to the ones obtained in PYR14-TFSI, which is consistent with the mechanisms giving rise to the relaxation being the same. In particular, the energy separation between the hopping configurations is comparable to the one previously obtained and is close to the calculated energy separation between the anion cis and trans configurations. Moreover, in the case of the 12909

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(6) Rivera, A.; Rossler, E. A. Evidence Of Secondary Relaxations In The Dielectric Spectra Of Ionic Liquids. Phys. Rev. B: Condens. Matter Mater. Phys. 2006, 73, 212201. (7) Nakamura, K.; Shikata, T. Systematic Dielectric and NMR Study of the Ionic Liquid 1-Alkyl-3-Methyl Imidazolium. ChemPhysChem 2010, 11, 285−294. (8) Schrödle, S.; Annat, G.; MacFarlane, D. R.; Forsyth, M.; Buchner, R.; Hefter, G. High Frequency Dielectric Response of the Ionic Liquid N-Methyl-N-ethylpyrrolidinium Dicyanamide. Aust. J. Chem. 2007, 60, 6−8. (9) Schrodle, S.; Annat, G.; MacFarlane, D. R.; Forsyth, M.; Buchner, R.; Hefter, G. Broadband Dielectric Response Of The Ionic Liquid Nmethyl-N-ethylpyrrolidinium Dicyanamide. Chem. Commun. 2006, 1748−1750. (10) Krause, C.; Sangoro, J. R.; Iacob, C.; Kremer, F. Charge Transport and Dipolar Relaxations in Imidazolium-Based Ionic Liquids. J. Phys. Chem. B 2010, 114, 382−386. (11) Sangoro, J. R.; Serghei, A.; Naumov, S.; Galvosas, P.; Kärger, J.; Wespe, C.; Bordusa, F.; Kremer, F. Charge Transport and Mass Transport in Imidazolium-based Ionic Liquids. Phys. Rev. E 2008, 77, 051202. (12) Santic, A.; Wrobel, W.; Mutke, M.; Banhatti, R. D.; Funke, K. Frequency-dependent Fluidity and Conductivity of an Ionic Liquid. Phys. Chem. Chem. Phys. 2009, 11, 5930−5934. (13) McHale, G.; Hardacre, C.; Ge, R.; Doy, N.; Allen, R. W. K.; MacInnes, J. M.; Bown, M. R.; Newton, M. I. Density-Viscosity Product of Small-Volume Ionic Liquid Samples Using Quartz Crystal Impedance Analysis. Anal. Chem. 2008, 80, 5806−5811. (14) Yamaguchi, T.; Nakahara, E.; Koda, S. Quantitative Analysis of Conductivity and Viscosity of Ionic Liquids in Terms of Their Relaxation Times. J. Phys. Chem. B 2014, 118, 5752−5759. (15) Cantelli, R.; Cordero, F.; Palumbo, O.; Cannelli, G.; Trequattrini, F.; Guadalupi, G. M.; Molinas, B. Mechanisms of the Semi-insulating Conversion of InP by Anelastic Spectroscopy. Phys. Rev. B: Condens. Matter Mater. Phys. 2000, 62, 1828−1834. (16) Palumbo, O.; Paolone, A.; Cantelli, R.; Jensen, C. M.; Sulic, M. Fast H-Vacancy Dynamics During Alanate Decomposition by Anelastic Spectroscopy. Proposition of a Model For Ti-enhanced Hydrogen Transport. J. Phys. Chem. B 2006, 110, 9105−9111. (17) Cantelli, R.; Palumbo, O.; Paolone, A.; Jensen, C. M.; Kuba, M. T.; Ayabe, R. Dynamics of Defects in Alanates. J. Alloys Compd. 2007, 446−447, 260−263. (18) Paolone, A.; Palumbo, O.; Teocoli, F.; Cantelli, R.; Hassoun, J. Phase Transitions in Polymers For Lithium Batteries. Solid State Phenom. 2012, 184, 351−354. (19) Paolone, A.; Vico, F.; Teocoli, F.; Sanna, S.; Palumbo, O.; Cantelli, R.; Knight, D. A.; Teprovich, J. A., Jr.; Zidan, R. Relaxation Processes and Structural Changes in Li and Na Doped Fulleranes For Hydrogen Storage. J. Phys. Chem. C 2012, 116, 16365−16370. (20) Herstedt, M.; Smirnov, M.; Johansson, P.; Chami, M.; Grondin, J.; Servant, L.; Lassègues, J. C. Spectroscopic Characterization of the Conformational States of the Bis(trifluoromethanesulfonyl)imide Anion (TFSI-). J. Raman Spectrosc. 2005, 36, 762−770. (21) Vitucci, F. M.; Trequattrini, F.; Palumbo, O.; Brubach, J.-B.; Roy, P.; Paolone, A. Infrared Spectra of Bis(trifluoromethanesulfonyl)imide Based Ionic Liquids: Experiments and Ab-initio Simulations. Vib. Spectrosc. 2014, 74, 81−87. (22) Vitucci, F. M.; Palumbo, O.; Trequattrini, F.; Brubach, J.-B.; Roy, P.; Meschini, I.; Croce, F.; Paolone, A. Interaction of 1-butyl-1methylpyrrolidinium bis(trifluoromethanesulfonyl)imide with an Electrospun PVdF Membrane: Temperature Dependence of the Anion Conformers. J. Chem. Phys. 2015, 143, 094707. (23) Vitucci, F. M.; Trequattrini, F.; Palumbo, O.; Brubach, J.-B.; Roy, P.; Navarra, M. A.; Panero, S.; Paolone, A. Stabilization of Different Conformers of Bis(trifluoromethanesulfonyl)imide Anion in Ammonium-Based Ionic Liquids at Low Temperatures. J. Phys. Chem. A 2014, 118, 8758−8764. (24) Fujimori, T.; Fujii, K.; Kanzaki, R.; Chiba, K.; Yamamoto, H.; Umebayashi, Y.; Ishiguro, S.-I. Conformational Structure of Room

AllylIm-TFSI, it is also noticeably lower than the energy separation among the cation configuration, which should be ∼80 kJ/mol (0.8 eV), as calculated in the Computational Section. These values suggest that only the anion conformers are likely to be involved in the different configurations among which the ions can rearrange. It must be noticed that in the present work the stress applied on the sample is not a pure shear stress, thus allowing the detection of relaxations thst are not necessarily observed by applying a pure shear deformation, as in the case of previous shear viscosity measurements.12−14 In fact these works did not report any shear relaxation below 1 MHz, whereas the relaxation presently observed at room temperature would occur at a frequency of ∼5 kHz, suggesting that such relaxation is related to the bulk modulus component of the measured mechanical moduli. Indeed, because bulk modulus is related to the volumetric changes,27 this would be in agreement with the involvement of the conformers in the configurations among which the relaxations occurs because ultrasonic measurements have demonstrated that the isomerization equilibrium can cause ultrasonic relaxation through the associated changes in volume and enthalpy.14,25,26

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CONCLUSIONS Mechanical spectroscopy measurements and DFT calculations were performed on two ionic liquids having the same TFSI anion but different cations, namely, PYR14-TFSI and 1-allylimidazolium-TFSI. The mechanical modulus is measured in the whole temperature range between 300 and 150 K, allowing the detection of its variation during the main phase transitions in both the liquid and the solid states. Moreover, a relaxation process is observed in the liquid phase and is possibly attributed to the ions motion, which can be described by a hopping model between nonequivalent configurations. DFT results on the possible conformers of the ions suggest that these configurations can be likely identified by the two anion conformers.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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DOI: 10.1021/acs.jpcb.5b06039 J. Phys. Chem. B 2015, 119, 12905−12911