in batteries. For the imidazolium halogenoaluminate salts, their physical properties—such as viscosity, melting point, and acidity—could be adjusted by changing the alkyl substituents and the imidazolium/pyridinium and halide/halogenoaluminate ratios. Two major drawbacks for some applications were moisture sensitivity and acidity or basicity. In 1992, Wilkes and Zawarotko obtained ionic liquids with 'neutral' weakly coordinating anions such as hexafluorophosphate () and tetrafluoroborate (), allowing a much wider range of applications. Characteristics IL's are typically colorless viscous liquids. They are often moderate to poor conductors of electricity, non-ionizing. They exhibit low vapor pressure. Many have low combustibility and are thermally stable. The solubility properties of ILs are diverse. Saturated aliphatic compounds are generally only sparingly soluble in ionic liquids, whereas alkenes show somewhat greater solubility, and aldehydes often completely miscible. Solubility differences 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 good 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 common ionic liquids. Many classes of chemical reactions, 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 are precursors salts in the preparation of stable carbenes. Because of their distinctive properties, ionic liquids have been investigated for many applications. Some ionic liquids can be distilled under vacuum conditions at temperatures near 300 °C. The vapor is not made up of separated ions, but consists of ion pairs. ILs have a wide liquid range. Some ILs do not freeze down to very low temperatures (even −150 °C), The glass transition temperature was detected below −100 °C in the case of N-methyl-N-alkylpyrrolidinium cations fluorosulfonyl-trifluoromethanesulfonylimide (FTFSI). Low-temperature ionic liquids (below 130 K) have been proposed as the fluid base for an extremely large diameter spinning liquid-mirror telescope to be based on the Moon. Water is a common impurity in ionic liquids, as it can be absorbed from the atmosphere and influences the transport properties of RTILs, even at relatively low concentrations. Varieties Classically, IL's consist of salts of unsymmetrical, flexible organic cations with symmetrical