Ionic liquids are expected to be used as advanced lubricants because of their low volatility, high thermal stability, and high oxidation stability. It is well known that halogen-containing ionic liquids exhibit excellent lubricity for metals. On the other hand, there is concern about the corrosiveness of ionic liquids caused by tribo-chemical reactions. Formation of the metal-fluoride boundary layer reduced friction; however, corrosion occurred on the worn surfaces after the sliding. In order to apply the ionic liquids as lubricants, it is necessary to prevent the corrosiveness. It is an effective method to control the sliding atmosphere such as in vacuum, because the hydrofluoric acid, which is produced by the reaction between metal fluoride and contaminating water from the atmosphere, causes the corrosion. The phosphorus element in the halogen-containing ionic liquids composed of a phosphate anion or phosphonium cation also inhibits the corrosion. The halogen-free ionic liquids do not cause any remarkable corrosion, though they show inferior lubricity compared with the halogen-containing ionic liquids. It is one of the important solutions to find the best combination of the halogen-free ionic liquid and the sliding material. The lubricity of ionic liquids shows domination in vacuum. Especially, the superior performance is possessed as a space lubricant compared with the conventional lubricants. The view in the future as the lubricant fluids is introduced with the basic tribological properties of the ionic liquids.

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  Ionic liquid possesses negligible vapor pressure that allows us to put the liquids in a vacuum chamber without any vaporization of the liquid. This fact led us to attempt to observe ionic liquid by a scanning electron microscope (SEM). Surprisingly it has been found that ionic liquid can be observed without any charging of the liquid. This result means that ionic liquid behaves as electrically conducting material in SEM observation. Based on this finding, it was initiated to develop new electron microscopy techniques; putting ionic liquid on the surfaces of insulating materials for their observation by SEM, SEM observation of bio-materials wetted by ionic liquid, and SEM observation of chemical reactions occurring in ionic liquid.

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  In the present paper, gas solubilities in ionic liquids (ILs) have extensively been reviewed in terms of physical and chemical absorption capabilities. The phase behaviors and volume expansivities were compared between IL-CO₂ and molecular liquid-CO₂ systems. The effects of anion and cation structures on the physical solubilities of CO₂ were described, and the dissolution mechanism was discussed on the basis of the thermodynamic parameters with the related spectroscopic and simulation results. The chemical absorption properties of ILs were also summarized in several types, i.e., the cation- and anion-functionalized ILs, hybrid ILs, and the others. Using the above-mentioned ILs, three different types of gas separation processes, the high-pressure physical absorption, temperature-swing chemical absorption, and vacuum-swing absorption techniques, have been surveyed with their strong advantages over conventional processes.

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  Development of ionic liquids for biomass processing, especially our recent development of functional ionic liquids has been described. For efficient biomass processing, we analyzed the physical chemistry of ionic liquids, focusing on the possibility to dissolve naturally-derived materials for energy conversion. We herein reviewed design of a wide variety of functional ionic liquids with special ion structure to enable dissolution of cellulose, extraction of valuable materials from biomass, and hydrolysis of cellulose. Moreover, we described a new class of an analytical method to analyze biomass processing with ionic liquids.

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