Ionic liquid

An ionic liquid (IL) is a salt in the liquid state. In some contexts, the term has been restricted to salts whose melting point is below some arbitrary temperature, such as 100 °C (212 °F). While ordinary liquids such as water and gasoline are predominantly made of electrically neutral molecules, ionic liquids are largely made of ions and short-lived ion pairs. These substances are variously called liquid electrolytes, ionic melts, ionic fluids, fused salts, liquid salts, or ionic glasses. When Tawny crazy ants (Nylanderia fulva) combat Fire ants (Solenopsis invicta), the latter spray them with a toxic, lipophilic, alkaloid-based venom. The Tawny crazy ant then exudes its own venom, formic acid, and self-grooms with it, an action which de-toxifies the Fire ant venom. The mixed venoms chemically react with one another to form an ionic liquid, the first naturally occurring IL to be described. An ionic liquid (IL) is a salt in the liquid state. In some contexts, the term has been restricted to salts whose melting point is below some arbitrary temperature, such as 100 °C (212 °F). While ordinary liquids such as water and gasoline are predominantly made of electrically neutral molecules, ionic liquids are largely made of ions and short-lived ion pairs. These substances are variously called liquid electrolytes, ionic melts, ionic fluids, fused salts, liquid salts, or ionic glasses. Ionic liquids are described as having many potential applications. They are powerful solvents and electrically conducting fluids (electrolytes). Salts that are liquid at near-ambient temperature are important for electric battery applications, and have been considered as sealants due to their very low vapor pressure. Any salt that melts without decomposing or vaporizing usually yields an ionic liquid. Sodium chloride (NaCl), for example, melts at 801 °C (1,474 °F) into a liquid that consists largely of sodium cations (Na+) and chloride anions (Cl−). Conversely, when an ionic liquid is cooled, it often forms an ionic solid—which may be either crystalline or glassy. The ionic bond is usually stronger than the Van der Waals forces between the molecules of ordinary liquids. For that reason, common salts tend to melt at higher temperatures than other solid molecules. Some salts are liquid at or below room temperature. Examples include compounds based on the 1-Ethyl-3-methylimidazolium (EMIM) cation and include: EMIM:Cl, EMIM dicyanamide, (C2H5)(CH3)C3H3N+2·N(CN)−2, that melts at −21 °C (−6 °F); and 1-butyl-3,5-dimethylpyridinium bromide which becomes a glass below −24 °C (−11 °F). Low-temperature ionic liquids can be compared to ionic solutions, liquids that contain both ions and neutral molecules, and in particular to the so-called deep eutectic solvents, mixtures of ionic and non-ionic solid substances which have much lower melting points than the pure compounds. Certain mixtures of nitrate salts can have melting points below 100 °C. The term 'ionic liquid' in the general sense was used as early as 1943. The discovery date of the 'first' ionic liquid is disputed, along with the identity of its discoverer. Ethanolammonium nitrate (m.p. 52–55 °C) was reported in 1888 by S. Gabriel and J. Weiner. One of the earliest truly room temperature ionic liquids was ethylammonium nitrate (C2H5)NH+3·NO−3 (m.p. 12 °C), reported in 1914 by Paul Walden. In the 1970s and 1980s, ionic liquids based on alkyl-substituted imidazolium and pyridinium cations, with halide or tetrahalogenoaluminate anions, were developed as potential electrolytes 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 (PF−6) and tetrafluoroborate (BF−4), allowing a much wider range of applications. Although many classical ILs are hexafluorophosphate and tetrafluoroborate salts, bistriflimide − are also popular.