This overview deals with the current and future scopes of ionic liquid studies. The hot topics (1) fundamental understanding of ionic liquid properties, (2) utilization of ionic liquids as new materials, and (3) realization of green chemistry by using ionic liquids as new solvents, are individually presented in relation to our own research. The characteristics of ionic liquids deeply owe to the facts that they keep liquid states at ambient temperatures, in spite of the large ion-ion interaction, and dissociate into ions in the absence of solvents. The role and position of ionic liquids as “designers solvent” should be established based on the fundamental understanding between the ionic structure and the properties.
The air/liquid interface of room temperature ionic liquids with 1-alkyl-3-methylimidazolium cations was studied using infrared-visible sum frequency generation (IV-SFG) vibrational spectroscopy and metastable atom electron spectroscopy (MAES). The air/liquid interface is covered with equally anions and cations if the alkyl chain is rather short, but longer alkyl chains tend to shield the anions. The probability of the gauche defect per CH2-CH2 bond in the alkyl chain decreases as the number of carbon atoms in the alkyl chain increases. This finding suggests that the interaction between the alkyl chains is enhanced as the alkyl chain length becomes longer. A local structure, which originates from the intermolecular interaction between the ionic liquid molecules, is proposed to explain these observations.
The tribological properties of ionic liquids were outlined from the viewpoint of tribo-chemistry. The tribological properties of two hydrophobic anions, trifluorotris (pentafluoroethyl) phosphate (FAP) and bis (trifluoromethylsulfonyl) imide (TFSI), were compared by a pendulum type friction test. FAP salts of imidazolium possess much better lubricity than TFSI salts of imidazolium. Tribo-chemical reactions of the ionic liquids were studied by time-of-flight secondary ion mass spectroscopy. It was found that content of phosphorus on the inside wear track was higher than that on the outside wear track. The results indicate that reaction of FAP took place under the tribo-chemical conditions. Straight-chain carboxylic acids were found to improve tribological properties of the ionic liquids. Direct interaction of the acid with the rubbed surfaces was confirmed by using stable isotopic tracers.
Self-assembly of liquid-crystalline molecules that combine order and fluidity is one of promising approaches to the preparation of nanostructured functional materials. Here we present a development of liquid-crystalline materials exhibiting anisotropic ionic conductivities by self-organization of ionic liquids. A new class of liquid-crystalline materials with layered (smectic) and cylindrical (columnar) structures was prepared by supramolecular association of conventional ionic liquids and hydrogen-bonded mesogenic molecules and by the chemical modification of ionic liquids. These materials formed nano-segregated structures consisting of ion-conducting and insulating parts in the liquid-crystalline states. Anisotropic ionic conductivities were induced by the formation of a single domain in the liquid-crystalline phases and the macroscopic alignment of the ion-conductive paths. Free-standing ion-conductive polymer films were also prepared by fixation of aligned structures of polymerizable liquid-crystalline ionic liquids through photoirradiation.
Noticing of the extremely low volatility of ionic liquids, the authors attempted to make metal vapor deposition to the liquids and observation of it with an electron microscope. We discover that the ionic liquids can be observed by electron microscopy because there is no problem of charging and that the metal vapor deposition to the liquids generates stable metal nanoparticles in the liquid. The former discovery allows us to observe insulating materials by electron microscopy by wetting the samples with ionic liquid. The size of metal nanoparticles obtained by vapor deposition was found to depend on the kinds of ionic liquids. These findings suggest various applications in nanotechnology field.
Ionic liquids (ILs) are comprised entirely of ions, and they possess unique physico-chemical properties such as non-volatility, non-flammability, high thermal stability, and high ionic conductivity, which make so distinct from conventional molecular liquids. Since ILs are essentially ionic conductors, their utilization as novel electrolytes for electrochemical devices, such as rechargeable lithium batteries, double-layer capacitors, dye-sensitized solar cells, and fuel cells, has also been the subject of intense study. The authors present our early trial to apply an IL as an electrolyte for a rechargeable lithium cell. We also review scientific progresses and problems for practical application in this field.
New approaches in the measurements and analyses with LEED and RHEED lead us to another view of these techniques as a scan method for a wide-range of crystal reciprocal space. Recent progress of the constant momentum-transfer averaging (CMTA) method for LEED structure analysis and development of Weissenberg RHEED method, in which a principle of a Weissenberg Camera for X-ray crystallography was imported to RHEED measurements, are briefly introduced in this review. Both methods capture a huge number of diffraction patterns, and can survey a crystal reciprocal space in three dimension. It is possible to determine the surface structure directly from a simple analysis based on a Fourier transformation of the obtained reciprocal data.