Phase transitions of choline dihydrogen phosphate: A vibrational spectroscopy and periodic DFT study

Choline dihydrogen phosphate, [Chol][H2PO4], is a proton-conducting ionic plastic crystal exhibiting a complicated sequence of phase transitions. Here, we address the argument in the literature around the thermal properties of [Chol][H2PO4] using Raman and infrared microspectroscopy. The known structure of the low-temperature crystal, which contains the anti-conformer of [Chol]+ and hydrogen-bonded dimers of anions, was used to do periodic density functional theory calculations of the vibrational frequencies. Raman spectra indicate that the solid-solid transition at 20 °C is linked to a conformational change to the gauche [Chol] conformer with a concurrent local rearrangement of the anions. The distinct bands of lattice modes in the low-frequency range of the Raman spectra vanish at the 20 °C transition. Given the ease with which metastable crystals can be produced, Raman mappings demonstrate that a sample of [Chol][H2PO4] at ambient temperature can contain a combination of anti- and gauche conformers. Heating to 120 °C causes continuous changes in the local environment of anions rather than melting as suggested by a recent calorimetric investigation of [Chol][H2PO4]. The monotonic change in vibrational spectra is consistent with earlier observations of a very small entropy of fusion and no abrupt jump in the temperature dependence of ionic conductivity along the phase transitions of [Chol][H2PO4].

Towards safer non-volatile tissue fixatives: Evaluation of choline-based ionic liquids for fixing ocular tissues

Volatile organic chemicals (VOCs) are routinely used for processing biological tissue samples in clinical laboratories. Recognizing their serious health and environmental impacts, a few non-volatile green solvents (choline based ionic liquids, ILs) were evaluated as tissue fixatives here. Microscopic evaluation of histo-morphology, fixation and staining quality, and macromolecular integrity (DNA and proteins) were assessed in human eye tissues (sclera, choroid, retinal layers and retinal pigmented epithelium, eyelid and orbit) after IL-fixation. Formalin-fixed tissues were used as standard reference. Microscopic examination revealed favorable histomorphology, tissue fixation and staining characteristics in most tissues immersed in ILs. Time taken to fix, and stability over a period of time (24 h, 48 h, 1 week, 1 month) was also recorded. Electrophoretic analysis revealed stability of cellular proteins and nucleic acids in IL-fixed scleral tissues. Heterogeneity in tissue fixation property relative to the type of ocular tissue, duration of fixation and storage, warrant further design and optimization of ILs to fix biological tissues. The simple cholinium salts based ILs tested here show favorable potential for tissue fixation application, and as an alternative approach to the use of VOCs, towards sustainable biomedical practice.

Organic salts utilising the hexamethylguanidinium cation: the influence of the anion on the structural, physical and thermal properties

The synthesis and characterisation of new solid-state electrolytes is a key step in advancing the development of safer and more reliable electrochemical energy storage technologies. Organic ionic plastic crystals (OIPCs) are an increasingly promising class of material for application in devices such as lithium or sodium metal batteries as they can support high ionic conductivity, with good electrochemical and thermal stability. However, the choice of OIPC-forming ions is still relatively limited. Furthermore, understanding of the influence of different cations and anions on the thermal, structural and transport properties of these materials is still in its infancy. Here we report the synthesis and in-depth characterisation of a range of new OIPCs utilising the hexamethylguanidinium cation ([HMG]) with five different anions. The thermal, structural, transport properties and free volume in the different salts have been investigated. The free volume within the salts has been investigated by positron annihilation lifetime spectroscopy, and the single crystal and powder X-ray diffraction analysis of [HMG] bis(trifluoromethanesulfonyl)imide ([TFSI]) in phase I and II, [HMG] hexafluorophosphate ([PF₆]) and [HMG] tetrafluoroborate ([HMG][BF₄]) are reported. The HMG cation can exhibit significant disorder, which is advantageous for plasticity and future use of these materials as high ionic conductivity matrices. The bis(fluorosulfonyl)imide salt, [HMG][FSI], is identified as particularly promising for use as an electrolyte, with good electrochemical stability and soft mechanical properties. The findings introduce a range of new materials to the solid-state electrolyte arena, while the insights into the physico-chemical relationships in these materials will be of importance for the future development and understanding of other ionic electrolytes.

Ionic Liquids as Environmentally Benign Electrolytes for High-Performance Supercapacitors

Electrochemical capacitors (ECs) are a vital class of electrical energy storage (EES) devices that display the capacity of rapid charging and provide high power density. In the current era, interest in using ionic liquids (ILs) in high-performance EES devices has grown exponentially, as this novel versatile electrolyte media is associated with high thermal stability, excellent ionic conductivity, and the capability to withstand high voltages without undergoing decomposition. ILs are therefore potentially useful materials for improving the energy/power performances of ECs without compromising on safety, cyclic stability, and power density. The current review article underscores the importance of ILs as sustainable and high-performance electrolytes for electrochemical capacitors.

Protic organic ionic plastic crystals based on a difunctional cation and the triflate anion: a new solid-state proton conductor

A new family of ammonium based organic ionic plastic crystals exhibits exciting solid-state proton conductivity.

A novel class of gas separation membrane based on organic ionic plastic crystals

The first demonstration of organic ionic plastic crystal-based membranes for CO₂ separation.

Ionic liquids for energy, materials, and medicine

As highlighted by the recent ChemComm web themed issue on ionic liquids, this field continues to develop beyond the concept of interesting new solvents for application in the greening of the chemical industry. Here some current research trends in the field will be discussed which show that ionic liquids research is still aimed squarely at solving major societal issues by taking advantage of new fundamental understanding of the nature of these salts in their low temperature liquid state. This article discusses current research trends in applications of ionic liquids to energy, materials, and medicines to provide some insight into the directions, motivations, challenges, and successes being achieved with ionic liquids today.