Ionic Liquid - Characteristics

Characteristics

Ionic liquids are often moderate to poor conductors of electricity, non-ionizing (e.g. non-polar), highly viscous and frequently exhibit low vapor pressure. Their other properties are diverse: many have low combustibility, excellent thermal stability, wide liquid regions, and favorable solvating properties for a range of polar and non-polar compounds. Many classes of chemical reactions, such as Diels-Alder reactions and Friedel-Crafts reactions, can be performed using ionic liquids as solvents. Recent work has shown that ionic liquids can serve as solvents for biocatalysis. The miscibility of ionic liquids with water or organic solvents varies with side chain lengths on the cation and with choice of anion. They can be functionalized to act as acids, bases or ligands, and have been used as precursor salts in the preparation of stable carbenes. Because of their distinctive properties, ionic liquids are attracting increasing attention in many fields, including organic chemistry, electrochemistry, catalysis, physical chemistry, and engineering; see for instance magnetic ionic liquid.

Despite their extremely low vapor pressures (≈10^-10 Pa at 25˚C), some ionic liquids can be distilled under vacuum conditions at temperatures near 300 °C. In the original work by Martyn Earle, et al., the authors wrongly concluded that the vapor was made up of individual, separated ions, but was later proven that the vapors formed consisted of ion-pairs. Some ionic liquids (such as 1-butyl-3-methylimidazolium nitrate) generate flammable gases on thermal decomposition. Thermal stability and melting point depend on the liquid's components. Thermal stability of various RTILs (Room Temperature Ionic Liquid) are available. The thermal stability of a task-specific ionic liquid, protonated betaine bis(trifluoromethanesulfonyl)imide is of about 534 K (502 °F) and N-Butyl-N-Methyl pyrrolidinium bis(trifluoromethanesulfonyl)imide was thermally stable up to 640 K. The upper limits of thermal stability of ionic liquids reported in the literature are usually based upon fast (about 10 K/min) TGA scans, and they do not imply long-term (several hours) thermal stability of ionic liquids, which is limited to less than 500 K for most ionic liquids.

The solubility of different species in imidazolium ionic liquids depends mainly on polarity and hydrogen bonding ability. Saturated aliphatic compounds are generally only sparingly soluble in ionic liquids, whereas olefins show somewhat greater solubility, and aldehydes can be completely miscible. This can be exploited in biphasic catalysis, such as hydrogenation and hydrocarbonylation processes, allowing for relatively easy separation of products and/or unreacted substrate(s). Gas solubility follows the same trend, with carbon dioxide gas showing exceptional solubility in many ionic liquids. Carbon monoxide is less soluble in ionic liquids than in many popular organic solvents, and hydrogen is only slightly soluble (similar to the solubility in water) and may vary relatively little between the more commonly used ionic liquids. Different analytical techniques have yielded somewhat different absolute solubility values.