Decompression-Induced Crystal Polymorphism in a Room-Temperature Ionic Liquid, N,N-Diethyl-N-methyl-N-(2-methoxyethyl) Ammonium Tetrafluoroborate

From stress to charge: investigating the piezoelectric response of solvate ionic liquid in structural energy storage composites

Solvate ionic liquids (SILs) are a class of ionic liquids where the liquid-state salt is chelated by a coordinating solvent, and of interest due to their advantageous properties such as low vapour pressure and superb thermal and chemical stability for energy storage applications. The electromechanical and piezoelectric effect were studied in lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) solvated by triethylene glycol dimethyl ether (triglyme, G3), forming [Li-G3]TFSI. These effects were also investigated in full solid polymer electrolyte (SPE) used in energy storage devices, consisting of [Li-G3]TFSI paired with an epoxy-based resin system. The SIL's electromechanical response was first established in isolation, as well as within the SPE. Experimental data demonstrates the effect of a major part of the SPE contributing to the electrical potential generation during application of force and subsequent pressurisation as well as depressurisation, underlined by a direct piezoelectric effect. SPE response to applied load is explored after the recent discovery of liquid-to-crystalline phase transition following pressurisation in pure ionic liquids. This finding has the potential to ameliorate the performance of energy storage composites via additional effects of charging such a device by subjecting it to stress, leading to increased efficiency. Results to date show a bulk potential difference across the SIL of up to 150 mV, while the SPE potential response is scaled down due to a significantly lower volume of SIL at the interface (∼30 mV). Nevertheless, such findings can still significantly affect the performance of carbon fibre (CF)-based structural supercapacitors and batteries that are able to store and release electrical energy whilst simultaneously contributing to load-bearing performance.

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.

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.

Isochronal superpositioning in the equilibrium regime of superpressed propylene carbonate to ∼ 1.8 GPa: A study by diffusivity measurement of the fluorescent probe Coumarin 1

We address the problem of glass-forming of liquids by superpressing. We study the pressure-induced dynamic change of the fragile van der Waals liquid propylene carbonate towards the glassy state in the equilibrium regime by measuring the diffusivity of the fluorescent probe Coumarin 1 embedded in the host liquid. The probe diffusivity is measured by the fluorescence recovery after photobleaching (FRAP) technique across a bleached volume generated by the near-field diffracted pattern of a laser beam. The recovered fluorescence intensity fits to a stretched exponential with the diffusive time [Formula: see text] and the stretched exponent [Formula: see text] as free parameters. In the pressure range [0.3-1.0]GPa the diffusivity decouples from the Stokes-Einstein relation. The decoupling correlates well to a decrease of [Formula: see text]. The variation of [Formula: see text] is non-monotonous with [Formula: see text] showing a minimum at [Formula: see text] s. We evidence an isochronal superpositioning over about 3 decades of [Formula: see text] between ∼ 10 s and [Formula: see text] s and a density scaling in the whole investigated pressure range. The pressure at which [Formula: see text] is minimum coincides to the dynamical crossover pressure measured by other authors. This crossover pressure is compatible with the critical point of MCT theory. As our studied pressure range encompasses the critical pressure, the non-monotonous variation of [Formula: see text] opens new insight in the approach to the critical point.

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

We conducted high-pressure Raman spectroscopy measurements on a series of 1-alkyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide ([Pyr_1n][TFSI], n = 3, 4, 6 and 8) homologues that have different alkyl chain lengths, n, at room temperature. The results showed that all [Pyr_1n][TFSI] samples formed a glassy state in which the glass transition pressure (p_g) slightly increased with an increase in n. This tendency is similar to prior results of high-pressure glass formation of [C_nmim][TFSI], although the p_gs for [Pyr_1n][TFSI] are larger than those for [C_nmim][TFSI] with corresponding n by ∼0.5 GPa. We discuss the local structural changes occurring in [Pyr_1n][TFSI] in view of the conformational changes of the Pyr⁺_1n cation and TFSI^- anion.

Low-temperature and high-pressure phases of a room-temperature ionic liquid and polyiodides: 1-methyl-3-propylimidazolium iodide

Experimental results are summarized on the P–T–m diagram. In pure [C₃mim][I], amorphous phase appeared both at low-temperature and high-pressure. Stoichiometric [C₃mim][I₃] promotes crystallization, while non-stoichiometric [C₃mim][I_3.66] indicates anomalies.

High-pressure glass formation of a series of 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide homologues

[C_nmim][TFSI] resists external pressure and retains the local liquid structure, as if a sponge absorbs a stimulus.

Synthesis, characterization and cellulose dissolution capabilities of ammonium-based room temperature ionic liquids (RTILs)

Green synthesis of room temperature ionic liquids (RTILs), are presented as friendly and challenging solvents for the effective dissolution of oil palm-lignocellulosic biomass. A series of Bronsted acidic-ionic liquids were prepared by the direct neutralization of diethyl dimethyl ammonium hydroxide with several (economical and environmental friendly) Bronsted acids as RTILs. The structural and physicochemical characterization was performed by applying various techniques as Fourier transform infrared (FT-IR), nuclear magnetic resonance (NMR), thermo gravimetric analysis (TGA), differential scanning calorimetry (DSC), zeta-nanosizer and dynamic light scattering (DLS) respectively, to state the effect of anion on the extended cellulose dissolution capabilities of the synthesized RTILs under mild conditions. As a polysaccharide solvent, diethyl dimethyl ammonium phosphate (A1P) showed the extreme capability to extract 65 % of cellulose from biomass without any pretreatment for 30 min. The present study could be a significant step toward the synthesis of efficient RTILs and generating upgraded cellulose for Hi-tech engineered composites and energy concerns.

Vibrational Spectroscopy of Ionic Liquids

Vibrational spectroscopy has continued use as a powerful tool to characterize ionic liquids since the literature on room temperature molten salts experienced the rapid increase in number of publications in the 1990's. In the past years, infrared (IR) and Raman spectroscopies have provided insights on ionic interactions and the resulting liquid structure in ionic liquids. A large body of information is now available concerning vibrational spectra of ionic liquids made of many different combinations of anions and cations, but reviews on this literature are scarce. This review is an attempt at filling this gap. Some basic care needed while recording IR or Raman spectra of ionic liquids is explained. We have reviewed the conceptual basis of theoretical frameworks which have been used to interpret vibrational spectra of ionic liquids, helping the reader to distinguish the scope of application of different methods of calculation. Vibrational frequencies observed in IR and Raman spectra of ionic liquids based on different anions and cations are discussed and eventual disagreements between different sources are critically reviewed. The aim is that the reader can use this information while assigning vibrational spectra of an ionic liquid containing another particular combination of anions and cations. Different applications of IR and Raman spectroscopies are given for both pure ionic liquids and solutions. Further issues addressed in this review are the intermolecular vibrations that are more directly probed by the low-frequency range of IR and Raman spectra and the applications of vibrational spectroscopy in studying phase transitions of ionic liquids.

Conformational adjustment for high-pressure glass formation of 1-alkyl-3-methylimidazolium tetrafluoroborate

The role of the alkyl-chain length (the conformational adjustment effect) in high pressure glass formation of 1-alkyl-3-methylimidazolum tetrafluoroborate.