An overview of the field of low-melting ionic liquids is given from its inception in 1886 through to the present time. The subject is divided into an introductory section that summarizes the early history of the field, and differentiates its subsections, before addressing matters judged of some interest in "pre-surge" and "post-surge" stages of its development, focusing on physicochemical as opposed to the prolific synthetic and industrial aspects in which the author has no competence. We give a final section specifically to protic ionic liquids, which we consider to have particular scientific potential.

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A rare-earth Pr-based bulk metallic glass (BMG) is obtained in the shape of rod up to 5 mm in diameter by die cast. Unlike other rare-earth-based BMGs, it exhibits a distinct glass transition, the low glass transition temperature (Tg=409 K), a large and stable supercooled liquid region, and paramagnetic property. The glass transition as well as its kinetic nature and the fragility parameters of the BMG have been studied. The BMG offers an ideal model to investigate the nature of glass transition as well as the relaxation and nucleation with a large experimentally accessible time and temperature window at low temperatures.

Simulations and theory suggest that the thermodynamic anomalies of water may be related to a phase transition between two supercooled liquid states, but so far this phase transition has not been observed experimentally because of preemptive ice crystallization. We used calorimetry, infrared spectroscopy, and molecular dynamics simulations to investigate a water-rich hydrazinium trifluoroacetate solution in which the local hydrogen bond structure surrounding a water molecule resembles that in neat water at elevated pressure, but which does not crystallize upon cooling. Instead, this solution underwent a sharp, reversible phase transition between two homogeneous liquid states. The hydrogen-bond structures of these two states are similar to those established for high- and low-density amorphous (HDA and LDA) water. Such structural similarity supports theories that predict a similar sharp transition in pure water under pressure if ice crystallization could be suppressed.

Physicochemical properties, ionicity, and fragility for protic ionic liquids (PILs) based on the protonation of the extremely fragile molecular liquid decahydroisoquinoline (DHiQ) by various Brønsted acids have been studied. The ionicity was evaluated using the Walden plot diagnostic, while the m-fragility (slope of T(g)-scaled Arrhenius plot at T(g)) was quantitatively measured by the Moynihan-Wang-Velikov variable scan rate, differential scanning calorimetry, method. DHiQ-derived PILs prove to cover the whole range of IL ionicities from poor IL to good IL, and even superionic, assessed from the Walden plot, depending on the choice of Brønsted acid. We find that the superfragile character of the parent DHiQ becomes completely suppressed upon conversion to ionic liquid, the initial value m = 128 sinking to m = 45-91 for the ionic liquid. Such values are in the intermediate to fragile range. The DHiQ-based PIL showing superionic behavior, anion [HSO(4)(-)], proves to be the case with the lowest m value (m = 45) so far reported for either aprotic or protic ILs. Both low fragility and dry proton conductivity can be attributed to an extended hydrogen bond network that is set up by the hydrogensulfate anion. The good DHiQ PILs have m values similar to those reported for typical aprotic ILs (m = 60-80), while the poor DHiQ PILs in which proton transfer from acid to base is weak show some memory of the parent fragility. Thus, a correlation of ionicity with m-fragility is characteristic of this system. A range of noncrystallizing, and also nonglassforming, behavior is observed in this series of compounds, suggesting a possible test for ideal glassformer existence.

Salts of the small symmetrical guanidinium cation, which are important protein denaturants in biophysical chemistry, are studied in the ionic liquid state for the first time. Their conductivities prove to be among the highest measured, and their liquid fragilities prove exceptional. We link these features to the large number of exchangeable protons per cation. We present evidence that the unusual properties stem from the increasing delocalization of protons among alternative structural moieties under increasing thermal excitation, and from the associated increase in "dry" proton contribution to the mass transport properties.

The striking increases in response functions observed during supercooling of pure water have been the source of much interest and controversy. Imminent divergences of compressibility etc. unfortunately cannot be confirmed due to pre-emption by ice crystallization. Crystallization can be repressed by addition of second components, but these usually destroy the anomalies of interest. Here we study systems in which protic ionic liquid second components dissolve ideally in water, and ice formation is avoided without destroying the anomalies. We observe a major heat capacity spike during cooling, which is reversed during heating, and is apparently of first order. It occurs just before the glassy state is reached and is preceded by water-like density anomalies. We propose that it is the much-discussed liquid-liquid transition previously hidden by crystallization. Fast cooling should allow the important fluctuations/structures to be preserved in the glassy state for leisurely investigation.

Excluding aqueous solutions, the highest ambient temperature conductivities of lithium containing materials have been found, not in liquid, but in glassy, mixed glass-crystal and crystal phases. The partly recrystallized thiophosphate glass reported by Tatsumisago and co-workers [3] exhibited an ambient temperature conductivity of a remarkable 17 mS cm −1 , significantly higher than the highest reported in nonaqueous electrolyte solutions, and that has now been exceeded by a chlorinecontaining variant of the Kamaya et al. thiophosphogermanate superionic crystal [4] that exhibits σ 25 °C = 25 mS cm −1 . [5] Part of the success of these solid electrolytes is due to the fact that the alkali cation is now, not only the most mobile ion, but usually the only mobile ion. However, as rigid materials, they tend to be mechanically fragile and are prone to encounter junction problems with anode and cathode materials. Excellent cell performance has nonetheless been reported. [5] An alternative approach that avoids the liquid state is to dissolve salts in plastic crystal phases. Two types of plastic crystal solvents have been explored: (i) molecular solvent [6,7] (succinonitrile (SSN) in which a salt like LiN(Tf) 2 is dissolved) and (ii) organic cation salts in which salts like LiBF 4 and LiN(Tf) 2 are dissolved, of which many variants [8][9][10] have been employed. Although it was not mentioned in the initial publication, [6] the success of the plastic crystal state as a solvent lies primarily in the ability of the molecular solvent (or molecular ions), to reorient on short time scales (t reor ≈0.1 ns in the case of SSN [11] ) thus providing a high entropy medium within which the ions enjoy high mobility. While each case has been considered successful, the disadvantage of each is that the Li + species proves to be the least mobile species. This is because, due to its high charge radius ratio, the Li cation dominates competition for ligands and "digs itself a hole" in the same way as it does in a typical nonaqueous molecular liquid electrolyte. A consequence is low mobility of the electroactive species relative to others, and consequent polarization problems during operation at high currents.It is against this backdrop that we have sought to develop an improved type of ambient temperature plastic crystal ion conductor, one in which the only mobile species is the alkali cation so that (as in superionic glasses and crystals) the conductivity Portable electronic devices are predominantly powered by lithium ion batteries in which the electrolyte is a liquid or gel of lithium salts dissolved in molecular solvents. There have been many attempts to replace the flammable liquid component of the electrolyte by alternative alkali metal transporting media, such as superionic crystals, alkali-conducting glassy solids, ionic liquids, saltin-molecular plastic crystal solvent, and salt-in-ionic plastic crystal solvents. Except for the first two of the above, which have their own problems, all the above have the disadvantage that the alkali ...