Abstract

1. Introduction

Ionic liquids (ILs) have been largely studied because their properties are attractive for a wide range of applications. ILs consist of large organic cations and inorganic/organic anions, which can be tuned for particular applications by changing ions or even the functional groups on the ions. Information regarding the molecular dynamic behavior of each ion forming an IL is especially important because several of the basic properties of ILs are determined by their behavior at a molecular level. Concerning their mechanical properties, ILs generally show high viscosity and scarce propensity to crystallization¹1 Matic A, Scrosati B. Ionic liquids for energy applications. MRS Bulletin. 2013;38(7):533-537.; therefore, their dynamic behavior resembles that of supercooled liquids²2 Palumbo O, Trequattrini F, Vitucci FM, Paolone A. Relaxation Dynamics and Phase Transitions in Ionic Liquids: Viscoelastic Properties from the Liquid to the Solid State. Journal of Physical Chemistry B. 2015;119(40):12905-12911. and they can be typically classified as fragile glass formers³3 Castiglione F, Moreno M, Raos G, Famulari A, Mele A, Appetecchi GB, et al. Structural Organization and Transport Properties of Novel Pyrrolidinium-Based Ionic Liquids with Perfluoroalkyl Sulfonylimide Anions. Journal of Physical Chemistry B. 2009;113(31):10750-10759.. Indeed, they generally follow time − temperature superposition laws, and the temperature dependence of the relaxation time is well approximated by the empirical Vogel−Fulcher−Tamman (VFT) equation above the glass temperature, and by the Arrhenius equation in the glass phase¹1 Matic A, Scrosati B. Ionic liquids for energy applications. MRS Bulletin. 2013;38(7):533-537.^,44 Shamim N, McKenna GB. Glass Dynamics and Anomalous Aging in a Family of Ionic Liquids above the Glass Transition Temperature. Journal of Physical Chemistry B. 2010;114(48):15742-15752.^,55 Rivera A, Rössler EA. Evidence of secondary relaxations in the dielectric spectra of ionic liquids. Physical Review B. 2006;73(21):212201.. The dynamics of ionic liquids has been studied by temperature dependent experiments using dielectric spectroscopy, nuclear magnetic resonance, and neutron scattering³3 Castiglione F, Moreno M, Raos G, Famulari A, Mele A, Appetecchi GB, et al. Structural Organization and Transport Properties of Novel Pyrrolidinium-Based Ionic Liquids with Perfluoroalkyl Sulfonylimide Anions. Journal of Physical Chemistry B. 2009;113(31):10750-10759.^,55 Rivera A, Rössler EA. Evidence of secondary relaxations in the dielectric spectra of ionic liquids. Physical Review B. 2006;73(21):212201.^,66 Nakamura K, Shikata T. Systematic Dielectric and NMR Study of the Ionic Liquid 1-Alkyl-3-Methyl Imidazolium. ChemPhysChem. 2010;11(1):285-294. in both the supercooled liquid and the glassy amorphous states. Recently we measured the mechanical modulus of some ionic liquids²2 Palumbo O, Trequattrini F, Vitucci FM, Paolone A. Relaxation Dynamics and Phase Transitions in Ionic Liquids: Viscoelastic Properties from the Liquid to the Solid State. Journal of Physical Chemistry B. 2015;119(40):12905-12911.^,77 Trequattrini F, Paolone A, Palumbo O, Vitucci FM, Navarra MA, Panero S. Low Frequency Mechanical Spectroscopy Study of Three Pyrrolidinium Based Ionic Liquids. Archives of Metallurgy and Materials. 2015;60(1):385-390.^,88 Palumbo O, Trequattrini F, Appetecchi GB, Conte L, Paolone A. Relaxation dynamics in pyrrolidinium based ionic liquids: Role of the anion conformers. Journal of Molecular Liquids. 2017;243:9-13. and its variation during the main phase transitions occurring by varying the temperature, in both the liquid and the solid states. The low frequency mechanical spectroscopy experiments were performed by means of a pocket, which works as a substrate. It is worth noting that this setup allows the use of low frequency mechanical spectroscopy to follow the main phase transitions occurring in ionic liquids by cooling down from room temperature, even in their liquid state, while, other previous works⁹9 Yamaguchi T, Nakahara E, Koda S. Quantitative Analysis of Conductivity and Viscosity of Ionic Liquids in Terms of Their Relaxation Times. Journal of Physical Chemistry B. 2014;118(21):5752-5759.

10 Yamaguchi T, Miyake S, Koda S. Shear Relaxation of Imidazolium-Based Room-Temperature Ionic Liquids. Journal of Physical Chemistry B. 2010;114(24):8126-8133.^-1111 Makino W, Kishikawa R, Mizoshiri M, Takeda S, Yao M. Viscoelastic properties of room temperature ionic liquids. Journal of Chemical Physics. 2008;129(10):104510. on mechanical properties of ionic liquids provided information either in their liquid or solid state. These works showed the effect of cation and anion on the shear viscosity¹⁰10 Yamaguchi T, Miyake S, Koda S. Shear Relaxation of Imidazolium-Based Room-Temperature Ionic Liquids. Journal of Physical Chemistry B. 2010;114(24):8126-8133. and the viscoelastic properties¹¹11 Makino W, Kishikawa R, Mizoshiri M, Takeda S, Yao M. Viscoelastic properties of room temperature ionic liquids. Journal of Chemical Physics. 2008;129(10):104510. of some imidazolioum based ILs; mainly reporting that when the alkyl chain of the cation is lengthened, the shear viscosity increases because the increase in the relaxation time overwhelms the decrease in the shear modulus.

In particular, in pyrrolidinium based ILs, depending both on the ions composition of the material and the different thermal history, crystallization from the melt or glass transitions, cold-crystallization and solid-solid phase transitions were observed and the kinetics of the crystallization processes was studied in the framework of the Johnson-Mehl-Avrami-Kolmogorov theory⁷7 Trequattrini F, Paolone A, Palumbo O, Vitucci FM, Navarra MA, Panero S. Low Frequency Mechanical Spectroscopy Study of Three Pyrrolidinium Based Ionic Liquids. Archives of Metallurgy and Materials. 2015;60(1):385-390.; the Avrami parameters, namely the activation energy and the Avrami index, were derived by the measure of the modulus variation under isothermal and non-isothermal conditions for both the crystallization from the melt and the cold crystallization observed on heating, thus providing additional information with respect to the common calorimetric measurements⁷7 Trequattrini F, Paolone A, Palumbo O, Vitucci FM, Navarra MA, Panero S. Low Frequency Mechanical Spectroscopy Study of Three Pyrrolidinium Based Ionic Liquids. Archives of Metallurgy and Materials. 2015;60(1):385-390..

Moreover, a thermally activated relaxation process has been detected in the liquid phase of several ILs having different cations and anions belonging to the per(fluoroalkylsulfonyl)imide family, but with different asymmetry²2 Palumbo O, Trequattrini F, Vitucci FM, Paolone A. Relaxation Dynamics and Phase Transitions in Ionic Liquids: Viscoelastic Properties from the Liquid to the Solid State. Journal of Physical Chemistry B. 2015;119(40):12905-12911.^,88 Palumbo O, Trequattrini F, Appetecchi GB, Conte L, Paolone A. Relaxation dynamics in pyrrolidinium based ionic liquids: Role of the anion conformers. Journal of Molecular Liquids. 2017;243:9-13.. This relaxation was analyzed in terms of a modified Debye model and the obtained parameters suggested the attribution of the observed peak to the ions motion, which can be described by a hopping model between nonequivalent configurations. DFT results on the possible conformers of the ions indicated that these nonequivalent configurations can be likely identified by the two lowest energy anion conformers. Indeed, previous combined nuclear magnetic resonance and rheology experiments with ab initio calculations³3 Castiglione F, Moreno M, Raos G, Famulari A, Mele A, Appetecchi GB, et al. Structural Organization and Transport Properties of Novel Pyrrolidinium-Based Ionic Liquids with Perfluoroalkyl Sulfonylimide Anions. Journal of Physical Chemistry B. 2009;113(31):10750-10759. in pyrrolidinium-based ILs provided measurements of the 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 rearrangements³3 Castiglione F, Moreno M, Raos G, Famulari A, Mele A, Appetecchi GB, et al. Structural Organization and Transport Properties of Novel Pyrrolidinium-Based Ionic Liquids with Perfluoroalkyl Sulfonylimide Anions. Journal of Physical Chemistry B. 2009;113(31):10750-10759.. Moreover, the occurrence of translational motion by means of hopping processes and rearrangements of molecular network has also been suggested for various ILs¹²12 Schrödle S, Annat G, MacFarlane DR, Forsyth M, Buchner R, Hefter G. High frequency dielectric response of the ionic liquid N-methyl-N-ethylpyrrolidinium dicyanamide. Australian Journal of Chemistry. 2007;60(1):6-8.. Also the reorientation of ions or ions pair can contribute to the relaxation dynamics. Indeed the main result of our previous work was the identification of a role of the possible anion conformers in the complex ions dynamics of ILs, which is further supported by the present work.

It must be noticed that with the previously described setup the stress applied on the sample is not a pure shear stress, thus allowing the detection of relaxations that are not necessarily observed by applying a pure shear deformation, as in the case of previous shear viscosity measurements⁹9 Yamaguchi T, Nakahara E, Koda S. Quantitative Analysis of Conductivity and Viscosity of Ionic Liquids in Terms of Their Relaxation Times. Journal of Physical Chemistry B. 2014;118(21):5752-5759.^,1313 Šantić A, Wrobel W, Mutke M, Banhatti RD, Funke K. Frequency-dependent fluidity and conductivity of an ionic liquid. Physical Chemistry Chemical Physics. 2009;11(28):5930-5934.^,1414 McHale G, Hardacre C, Ge R, Doy N, Allen RWK, MacInnes WK, et al. Density-Viscosity Product of Small-Volume Ionic Liquid Samples Using Quartz Crystal Impedance Analysis. Analytical Chemistry. 2008;80(15):5806-5811.. 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, since bulk modulus is related to the volumetric changes, 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 enthalpy⁹9 Yamaguchi T, Nakahara E, Koda S. Quantitative Analysis of Conductivity and Viscosity of Ionic Liquids in Terms of Their Relaxation Times. Journal of Physical Chemistry B. 2014;118(21):5752-5759.

10 Yamaguchi T, Miyake S, Koda S. Shear Relaxation of Imidazolium-Based Room-Temperature Ionic Liquids. Journal of Physical Chemistry B. 2010;114(24):8126-8133.^-1111 Makino W, Kishikawa R, Mizoshiri M, Takeda S, Yao M. Viscoelastic properties of room temperature ionic liquids. Journal of Chemical Physics. 2008;129(10):104510..

In the present work we extend the low frequency mechanical spectroscopy experiments to N-butyl-N-methyl-pyrrolidinium (trifluoromethanesulfonyl)(nonafluorobutanesulfonyl)imide (PYR₁₄-IM₁₄). Indeed, pyrrolidinium based ILs are largely studied as safe electrolytes for lithium cells¹⁵15 Navarra MA. Ionic liquids as safe electrolyte components for Li-metal and Li-ion batteries. MRS Bulletin. 2013;38(7):548-553. and the IM₁₄ anion is suggested³3 Castiglione F, Moreno M, Raos G, Famulari A, Mele A, Appetecchi GB, et al. Structural Organization and Transport Properties of Novel Pyrrolidinium-Based Ionic Liquids with Perfluoroalkyl Sulfonylimide Anions. Journal of Physical Chemistry B. 2009;113(31):10750-10759.^,1616 Appetecchi GB, Montanino M, Carewska M, Moreno M, Alessandrini F, Passerini S. Chemical-physical properties of bis(perfuoroalkylsulfonyl)imide-based ionic liquids. Electrochimica Acta. 2011;56(3):1300-1307. as a possible alternative to the usually adopted PF₆ and TFSI anions. Similar to TFSI, the IM₁₄ ion possesses different conformers, whose configuration and distribution we recently studied by means of combined DFT calculation and infrared spectroscopy¹⁷17 Palumbo O, Trequattrini F, Appetecchi GB, Paolone A. A study of the Conformers of the (Nonafluorobutanesulfonyl)imide Ion by Means of Infrared Spectroscopy and Density Functional Theory (DFT) calculations. Challenges. 2017;8(1):1-10.. In particular, DFT calculation¹⁷17 Palumbo O, Trequattrini F, Appetecchi GB, Paolone A. A study of the Conformers of the (Nonafluorobutanesulfonyl)imide Ion by Means of Infrared Spectroscopy and Density Functional Theory (DFT) calculations. Challenges. 2017;8(1):1-10. showed that the IM₁₄ anion possesses 74 conformers. The most stable configuration is the one with the alkyl chain in the all-trans configuration (tt), while at an energy 1.8 kJ/mol higher we can find the trans-gauche (tg) conformer (Figure 1), in agreement with a previous study¹⁸18 Russina O, Lo Celso F, Di Michiele M, Passerini S, Appetecchi GB, Castiglione F, et al. Mesoscopic structural organization in triphilic room temperature ionic liquids. Faraday Discussions. 2013;167:499-513.. Moreover analysis of the infrared spectra of PYR₁₄-IM₁₄ allowed a direct measure of the difference of the enthalpy between these two configurations in the liquid phase, which is found to be positive and equal to 1.0 ± 0.3 kJ/mol, further confirming that the tt IM₁₄ conformer is the most energetically favored.

2. Experimental

The investigated samples were synthesized by a procedure developed at ENEA and described in detail elsewhere¹⁹19 Montanino M, Alessandrini F, Passerini S, Appetecchi GB. Water-based synthesis of hydrophobic ionic liquids for high-energy electrochemical devices. Electrochimica Acta. 2013;96:124-133.^,2020 Jeremias S, Carewska M, Conte L, Passerini S, Appetecchi GB. Asymmetry effect of novel per(fuoroalkylsulfonyl)imide anions in pyrrolidinium ionic liquids. RSC Advances. 2013;3(39):17755-17761..

Dynamic mechanical analysis was carried out using a PerkinElmer DMA 8000 instrument, following a procedure already used by us in previous works²2 Palumbo O, Trequattrini F, Vitucci FM, Paolone A. Relaxation Dynamics and Phase Transitions in Ionic Liquids: Viscoelastic Properties from the Liquid to the Solid State. Journal of Physical Chemistry B. 2015;119(40):12905-12911.^,77 Trequattrini F, Paolone A, Palumbo O, Vitucci FM, Navarra MA, Panero S. Low Frequency Mechanical Spectroscopy Study of Three Pyrrolidinium Based Ionic Liquids. Archives of Metallurgy and Materials. 2015;60(1):385-390.^,88 Palumbo O, Trequattrini F, Appetecchi GB, Conte L, Paolone A. Relaxation dynamics in pyrrolidinium based ionic liquids: Role of the anion conformers. Journal of Molecular Liquids. 2017;243:9-13.: the sample, which is liquid at room temperature, was 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 in the mid-point, was then folded in half and crimped closed to form a sandwich. Flexural vibration 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 of 1 Hz and 10 Hz, during cooling/heating at 4 Kmin^-1, between 150 K 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, due to the spatial isotropy of liquids, the mechanical modulus presently measured is a combination of both the shear and the bulk modulus²2 Palumbo O, Trequattrini F, Vitucci FM, Paolone A. Relaxation Dynamics and Phase Transitions in Ionic Liquids: Viscoelastic Properties from the Liquid to the Solid State. Journal of Physical Chemistry B. 2015;119(40):12905-12911.^,1010 Yamaguchi T, Miyake S, Koda S. Shear Relaxation of Imidazolium-Based Room-Temperature Ionic Liquids. Journal of Physical Chemistry B. 2010;114(24):8126-8133.^,1111 Makino W, Kishikawa R, Mizoshiri M, Takeda S, Yao M. Viscoelastic properties of room temperature ionic liquids. Journal of Chemical Physics. 2008;129(10):104510..

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 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 analysis of the mechanical spectra of solids to the whole measured spectrum, also including contributions to the spectra coming from the non-solid 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 solid²¹21 Nowick AS, Berry BS. Anelastic Relaxation in Crystalline Solids. New York: Academic Press; 1972., 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:

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. α is the Fuoss-Kirkwood width parameter and is equal to 1 for a single time Debye relaxation; α < 1 produces broadened peaks with respect to Debye ones.

For classical Arrhenius processes τ = τ₀e^W/kT, where W is the activation energy, whilst assuming for the relaxation time τ a Vogel-Fulcher-Tamman type (VFT) temperature dependence:

where W is the activation energy and T₀ is the temperature at which the configurational entropy is equal to zero (e.g., the theoretical glass transition temperature). For instance, 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 viscosity¹1 Matic A, Scrosati B. Ionic liquids for energy applications. MRS Bulletin. 2013;38(7):533-537., 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, becomes²²22 Cannelli G, Cantelli R, Cordero F, Trequattrini F. Dynamics of hydrogen, oxygen, and dislocations in yttrium by acoustic spectroscopy. Physical Review B. 1997;55(22):14865.:

Considering equations 1 and 3, a more general expression for tanδ is then given by:

3. Results and Discussion

The DMA spectrum (modulus, M, and tan δ) of PYR₁₄-IM₁₄ measured on cooling at 4 K/min is displayed in Figure 2. The storage modulus is reported as relative variation with respect to the value at 290 K (M/M_{290 K} − 1) because it is not possible to separate the contribute of the ILs from that of the pocket. However, as already pointed out elsewhere²2 Palumbo O, Trequattrini F, Vitucci FM, Paolone A. Relaxation Dynamics and Phase Transitions in Ionic Liquids: Viscoelastic Properties from the Liquid to the Solid State. Journal of Physical Chemistry B. 2015;119(40):12905-12911.^,77 Trequattrini F, Paolone A, Palumbo O, Vitucci FM, Navarra MA, Panero S. Low Frequency Mechanical Spectroscopy Study of Three Pyrrolidinium Based Ionic Liquids. Archives of Metallurgy and Materials. 2015;60(1):385-390., 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.

The tan δ curve plotted as a function of temperature shows, at ∼ 250 K (for a vibration frequency of 1 Hz) a peak, which is likely due to a thermally activated relaxation process because its maximum shifts at higher temperature with increasing frequency (∼ 272 K for a vibration frequency of 10 Hz; see Figure 2). Concomitantly, a step is observed in the modulus (Figure 2). On further cooling, around 190 K, the anelastic spectra shows an intense stiffening of the modulus and an intense peak of tan δ. These features indicate the occurrence of the glass transition, as observed for other systems. Indeed previous DSC measurements on PYR₁₄-IM₁₄ reported the occurrence of the glass transition around this temperature¹⁶16 Appetecchi GB, Montanino M, Carewska M, Moreno M, Alessandrini F, Passerini S. Chemical-physical properties of bis(perfuoroalkylsulfonyl)imide-based ionic liquids. Electrochimica Acta. 2011;56(3):1300-1307.^,1919 Montanino M, Alessandrini F, Passerini S, Appetecchi GB. Water-based synthesis of hydrophobic ionic liquids for high-energy electrochemical devices. Electrochimica Acta. 2013;96:124-133.^,2020 Jeremias S, Carewska M, Conte L, Passerini S, Appetecchi GB. Asymmetry effect of novel per(fuoroalkylsulfonyl)imide anions in pyrrolidinium ionic liquids. RSC Advances. 2013;3(39):17755-17761..

Moreover, these thermally activated relaxation processes are similar to those ones found in the supercooled liquid phase of the parent compounds PYR₁₄-TFSI and PYR₁₄-IM_T4 which were analyzed through a modified Debye model²2 Palumbo O, Trequattrini F, Vitucci FM, Paolone A. Relaxation Dynamics and Phase Transitions in Ionic Liquids: Viscoelastic Properties from the Liquid to the Solid State. Journal of Physical Chemistry B. 2015;119(40):12905-12911.^,88 Palumbo O, Trequattrini F, Appetecchi GB, Conte L, Paolone A. Relaxation dynamics in pyrrolidinium based ionic liquids: Role of the anion conformers. Journal of Molecular Liquids. 2017;243:9-13. and indeed the data of both frequencies can be reasonably fitted (continuous lines in Figures 2 and 3) using eq. 4, which is appropriated for jumps in an asymmetrical potential well, and assuming for the relaxation time (τ) a Vogel−Fulcher−Tamman type (VFT) temperature dependence (eq. 2). The values of the best-fit parameters are reported in Table 1 where, for a comparison, also the values previously obtained for the parent compounds PYR₁₄-TFSI and PYR₁₄-IM_T4 have been reported. The values obtained for pre-exponential factor of the relaxation time are in good agreement with previous literature⁵5 Rivera A, Rössler EA. Evidence of secondary relaxations in the dielectric spectra of ionic liquids. Physical Review B. 2006;73(21):212201.^,99 Yamaguchi T, Nakahara E, Koda S. Quantitative Analysis of Conductivity and Viscosity of Ionic Liquids in Terms of Their Relaxation Times. Journal of Physical Chemistry B. 2014;118(21):5752-5759.^,2323 Krause C, Sangoro JR, Iacob C, Kremer F. Charge Transport and Dipolar Relaxations in Imidazolium-Based Ionic Liquids. Journal of Physical Chemistry B. 2010;114(1):382-386., while the width parameter α lower than 1 indicates interaction among the relaxing units. The activation energies provided by the analysis is 0.45 ± 0.04 eV, which is in perfect agreement with the activation energy obtained by viscosity and diffusion coefficients measurements (0.43 eV)³3 Castiglione F, Moreno M, Raos G, Famulari A, Mele A, Appetecchi GB, et al. Structural Organization and Transport Properties of Novel Pyrrolidinium-Based Ionic Liquids with Perfluoroalkyl Sulfonylimide Anions. Journal of Physical Chemistry B. 2009;113(31):10750-10759.^,2424 Castiglione F, Raos G, Appetecchi GB, Montanino M, Passerini S, Moreno M, et al. Blending ionic liquids: how physico-chemical properties change. Physical Chemistry Chemical Physics. 2010;12(8):1784-1792..The value for the energy separation of the non-equivalent configurations is 29 ± 1 meV, which is close to the energy separations obtained for similar compounds and is comparable to the energy separations obtained for the anion conformers, IM₁₄ all-trans (tt) and trans-gauche (tg) conformer (19 meV)¹⁷17 Palumbo O, Trequattrini F, Appetecchi GB, Paolone A. A study of the Conformers of the (Nonafluorobutanesulfonyl)imide Ion by Means of Infrared Spectroscopy and Density Functional Theory (DFT) calculations. Challenges. 2017;8(1):1-10..

These results further confirm that the intramolecular rearrangements involved in the dynamics of the studied ILs are strongly connected to the anion conformational structures. Indeed, also the PYR₁₄ cation presents different conformers²⁵25 Fujimori T, Fujii K, Kanzaki R, Chiba K, Yamamoto H, Umebayashi Y, et al. Conformational structure of room temperature ionic liquid N-butyl-N-methyl-pyrrolidinium bis(trifluoromethanesulfonyl) imide - Raman spectroscopic study and DFT calculations. Journal of Molecular Liquids. 2007;131-132:216-224.. However, in our previous work²2 Palumbo O, Trequattrini F, Vitucci FM, Paolone A. Relaxation Dynamics and Phase Transitions in Ionic Liquids: Viscoelastic Properties from the Liquid to the Solid State. Journal of Physical Chemistry B. 2015;119(40):12905-12911., the comparison of the DMA spectra measured on ILs having the same TFSI anion but different cations, also including PYR₁₄²2 Palumbo O, Trequattrini F, Vitucci FM, Paolone A. Relaxation Dynamics and Phase Transitions in Ionic Liquids: Viscoelastic Properties from the Liquid to the Solid State. Journal of Physical Chemistry B. 2015;119(40):12905-12911., suggested that the cation conformers were not involved in the relaxation process since the calculated energy difference between their lower energy conformers was different from the asymmetry between the configurations among which the ions can move.

The present results strongly support for the central role of the anion in the diffusive motion of ions in ILs. The importance of the anion and of its conformers distribution has been suggested also to explain the different results of a cooling, i.e. a phase transition to a crystal state (crystallization) or to a glassy state²⁶26 Kunze M, Jeong S, Paillard E, Winter M, Passerini S. Melting Behavior of Pyrrolidinium-Based Ionic Liquids and Their Binary Mixtures. Journal of Physical Chemistry C. 2010;114(28):12364-12369.

27 Palumbo O, Trequattrini F, Appetecchi GB, Paolone A. Infuence of Alkyl Chain Length on Microscopic Confgurations of the Anion in the Crystalline Phases of PYR1A-TFSI. Journal of Physical Chemistry C. 2017;121(21):11129-11135.^-2828 Palumbo O, Trequattrini F, Navarra MA, Brubach JB, Roy P, Paolone A. Tailoring the physical properties of the mixtures of ionic liquids: a microscopic point of view. Physical Chemistry Chemical Physics. 2017;19(12):8322-8329.. Moreover, the competition between different anion conformer configurations can induce the suppression of crystalline phases in mixtures²⁸28 Palumbo O, Trequattrini F, Navarra MA, Brubach JB, Roy P, Paolone A. Tailoring the physical properties of the mixtures of ionic liquids: a microscopic point of view. Physical Chemistry Chemical Physics. 2017;19(12):8322-8329.^,2929 Vitucci FM, Trequattrini F, Palumbo O, Brubach JB, Roy P, Navarra MA, et al. Stabilization of Different Conformers of Bis(trifluoromethanesulfonyl)imide Anion in Ammonium-Based Ionic Liquids at Low Temperatures. Journal of Physical Chemistry A. 2014;118(38):8758-8764.. 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.

4. Conclusions

This work extends previous low frequency mechanical spectroscopy measurements to PYR₁₄-IM₁₄. The results show that this liquid undergoes a glass transition around 190 K. The relaxation process detected in the liquid phase is similar to the one observed in other ILs having different anion of the same per(fluoroalkylsulfonyl)imide family. The analysis of this peak confirms its attribution to the ion dynamics among different configurations which are strongly affected by the different anion conformers. In this framework, the central role of the anion in the ions dynamics of ILs is further confirmed.

5. References

Publication Dates

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