Phase Transitions of PYR₁₄-TFSI as a Function of Pressure and Temperature: the Competition between Smaller Volume and Lower Energy Conformer

Abstract

A detailed Raman study has been carried out on the ionic liquid 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR14-TFSI) over a wide pressure (0-8 GPa) and temperature (100-300 K) range. The explored thermodynamic region allowed us to study the evolution of the system across different solid and liquid phases. Calculated Raman spectra remarkably helped in the spectral data analysis. In particular, the pressure behavior of the most intense Raman peak and the shape analysis of the ruby fluorescence (used as a local pressure gauge) allowed us to identify a liquid-solid transition around 2.2 GPa at T = 300 K. The low-frequency Raman signal as well as the absence of remarkable spectral shape modifications on crossing the above threshold and the comparison with the spectra of the crystalline phase suggest a glassy nature of the high-pressure phase. A detailed analysis of the pressure dependence of the relative concentration of two conformers of TFSI allowed us to obtain an estimate of the volume variation between trans-TFSI and the smaller cis-TFSI, which is the favored configuration on applying the pressure. Finally, the combined use of both visual inspection and Raman spectroscopy confirmed the peculiar sequence of phase transitions observed as a function of temperature at ambient pressure and the different spectral/morphological characteristics of the two crystalline phases.

Pressurizing the mixtures of two ionic liquids: Crystallization versus vetrification

“…All bands shift to higher wavenumbers with increasing pressure, as expected. After pressurization to 1 GPa, one can note the absence of the spectral line that, at room pressure, is centered around 327 cm −1 , due to cis ‐TFSI . This finding is consistent with infrared spectroscopy measurements performed on TMPA‐TFSI, which indicated that between 0.1 and 0.2 GPa, the system crystallizes into a structure containing trans ‐TFSI only …”

“…The Raman spectrum of MIX98‐2 measured at 0 and 1 GPa, after overnight equilibration of the pressure, is shown in Figure . Among the bands centered in the wavenumber range between 260 and 350 cm −1 , four of them shift to higher wavenumbers as the pressure is increased (285, 301, 319, and 345 cm −1 ) whereas the band centered at 326 cm −1 at ambient pressure ascribed to cis ‐TFSI completely disappears. This behavior suggests that MIX98‐2, similarly to the pure TMPA‐TFSI compound, crystallizes upon compression into a structure containing only trans ‐TFSI.…”

“…Indeed, the experimental spectrum measured between 800 and 1,150 cm −1 , a wavenumber range where only the bands of TMPA are visible, displays only the five lines attributable to Conf1; on the contrary, the lines of Conf2 expected around 690, 849, and 911 cm −1 are not detectable. Concerning the anion, information about the occurrence of conformers can be obtained from the spectral response over the 250–380 cm 1 wavenumber range: Indeed, the bands that at ambient pressure (hereafter mentioned as 0 GPa) are centered around 279, 298, and 313 cm −1 , together with the shoulder at 288 cm −1 , contain contributions from both conformers of TFSI, whereas those at 327 and 340 cm −1 were attributed to cis and trans ‐TFSI, respectively …”

“…These three mixtures display all the bands over the 270–350 cm −1 up to the highest pressure investigated (4–5 GPa), and therefore, the band ascribed to cis ‐TFSI (326 cm −1 at p = 0 GPa) does not disappear in these cases. This finding suggests that these three mixtures do not form a crystalline phase at high pressure, because, usually in crystals, only one TFSI conformer is retained . We want to stress that the maximum care was used to allow the pressure equilibration at ~1 and ~2 GPa overnight for excluding that a slow kinetics could prevent the onset of a crystalline phase.…”

“…While the onset of a crystalline phase can be excluded by a close inspection of the pressure evolution of the Raman spectra, a careful analysis of the spectral bands is needed to ascertain whether the mixtures remain liquid or they transform in a glass state at high pressure. It has been previously reported that the study of the pressure dependence of the relative concentration of the TFSI conformers can be a useful tool to monitor the transition towards a glassy state . It should be considered that the ratio, r , of the conformer concentrations, C cis and C trans , is proportional to the ratio of the integrated intensities I cis and I trans of the spectral lines ascribed to cis and trans conformers: …”