How does the flexibility of pyrrolidinium cations affect the phase behaviour of 1-alkyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide homologues under stressful conditions?

Structure-Dependence and Mechanistic Insights into the Piezoelectric Effect in Ionic Liquids

We reported recently that two imidazolium room-temperature ionic liquids (RTILs) exhibit the direct piezoelectric effect (J. Phys. Chem. Lett., 2023, 14, 2731-2735). We have subsequently investigated several other RTILs with pyrrolidinium and imidazolium cations and tetrafluoroborate and bis(trifluoromethylsulfonyl)imide anions in an effort to gain insight into the generality and mechanism of the effect. All the RTILs studied exhibit the direct piezoelectric effect, with a magnitude (d33) and threshold force that depend on the structures of both the cation and anion. The structure-dependence and existence of a threshold force for the piezoelectric effect are consistent with a pressure-induced liquid-to-crystalline solid phase transition in the RTILs, and this is consistent with experimental X-ray diffraction data.

Spectroscopic Investigation of the Structure of a Pyrrolidinium-Based Ionic Liquid at Electrified Interfaces

The molecular structure of electric double layers (EDLs) at electrode-electrolyte interfaces is crucial for all types of electrochemical processes. Here we probe the EDL structure of an ionic liquid, 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (BMPy-TFSI), using electrochemical shell-isolated nanoparticle-enhanced Raman spectroscopy (EC-SHINERS). We extract the position and intensity of individual peaks corresponding to either intra- or inter-molecular vibrational modes, and examine their dependence on the electrode potential. The observed trends suggest that the molecular reconfiguration mechanism is distinct between cations and anions. BMPy+ is found to always adsorb on the Au electrode surface via the pyrrolidinium ring while the alkyl chains strongly change their orientation at different potentials. In contrast, TFSI- is observed to have pronounced position shifts but negligible orientation changes as we sweep the electrode potential. Despite their distinct reconfiguration mechanisms, BMPy+ and TFSI- in the EDL are likely paired together through strong intermolecular interaction.

Comparison between pyrrolidinium-based and imidazolium-based dicationic ionic liquids: intermolecular interaction, structural organization, and solute dynamics

With an aim to understand the difference in the behaviour of imidazolium and pyrrolidinium-based dicationic ionic liquids (DILs) in terms of the intermolecular interactions, microscopic-structure and dynamics, two DILs, the imidazolium-based 1,9-bis(3-methylimidazolium-1-yl)nonane bis(trifluoromethanesulfonyl)imide and the pyrrolidinium-based 1,9-bis(1-methylpyrrolidinium-1-yl)nonane bis(trifluoromethanesulfonyl)imide, have been synthesized and subsequently investigated by exploiting combined steady sate and time resolved fluorescence, electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR) spectroscopic techniques. Data obtained for DILs have also been compared with their corresponding mono-cationic counterpart (MILs) to evaluate and understand the distinctive characteristics of the DILs in contrast with the corresponding MILs. Steady state emission and EPR data have revealed that the pyrrolidinium-based DIL is slightly less polar than the imidazolium-based DIL. Temperature-dependent fluorescence anisotropy decay of two probes, perylene and MPTS (8-methoxypyrene-1,3,6-trisulfonate), has been measured in DILs as well as in MILs. Solute-solvent coupling constants obtained from the experimentally measured rotational correlation times with the aid of Stokes-Einstein-Debye hydrodynamic theory have indicated appreciable differences in the dynamics of both the solutes on going from MILs to DILs. More interestingly, the outcome of the NMR study has suggested that the alkyl spacer chain in the imidazolium-based DIL exists in the folded form, but the pyrrolidinium-based DIL remains in the straight chain conformation. Inherently, the outcomes of all of these studies have depicted that the microscopic structural organisations in imidazolium and pyrrolidinium-based DILs are different from each other as well as from their respective mono-cationic counterparts.

High-Pressure Raman Study of n-Octane up to 15 GPa

The high-pressure (HP) phase transition and conformational change of n-octane (hereafter abbreviation as n-C₈) up to 15.3 GPa were studied using Raman spectroscopy to investigate the relationship between the HP phase state and the alkyl chain length of n-alkanes. The Raman spectral analysis of n-C₈ indicated that the liquid-solid transition (solidification) occurs at ∼0.9 GPa and that the corresponding transition pressure of n-alkanes depends on their density. Further pressurization at ∼4 GPa increased the population of the gauche conformer, while the solid (order)-amorphous transition occurred at ∼6 GPa along with a change in the full width at half maximum of the ruby R₁ fluorescence line. The comparison of our findings with previously reported results suggested that the even-odd effect in the HP phase transition after solidification of n-alkanes appears between n-C₇ and n-C₈ as their HP phase transition up to ∼15 GPa was different.

Peculiar High-Pressure Phase Behavior of 1-Butyl-3-methylimidazolium Iodide

We investigated the stability of the liquid phase of 1-butyl-3-methylimidazolium iodide (hereafter abbreviated as [C₄mim][I]) up to 16.7 GPa at room temperature. We observed a peculiar phase transition behavior in the [C₄mim][I] sample. In particular, a glassy state was formed at ∼1.3 GPa; however, the reddish-brown precipitate was formed probably due to concentrated I₃^- or I₂^- species that were formed above 12 GPa; [C₄mim][I] showed a pressure-induced partial crystallization from the glassy state. We concluded that the conformation of [C₄mim]⁺ is essential in iterative modulation to control the environmental formation of iodide precipitate.