|
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 . 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 have many applications, such as powerful solvents and electrically conducting fluids (electrolytes). Salts that are liquid at near-ambient temperature are important for electric battery applications, and have been used 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 into a liquid that consists largely of sodium cations () and chloride anions (). 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, ()()·, that melts at ;〔 〕 and 1-butyl-3,5-dimethylpyridinium bromide which becomes a glass below .〔 〕 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.〔(Mixture of nitrate salts with m.p. below 100 deg C )〕 The term "ionic liquid" in the general sense was used as early as 1943.〔 〕 ==History== 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 ()· (m.p. 12 °C), synthesized in 1914 by Paul Walden.〔 Paul Walden (1914), Bull. Acad. Sci. St. Petersburg, pages 405-422 〕 In the 1970s and 1980s ionic liquids based on alkyl-substituted imidazolium and pyridinium cations, with halide or tetrahalogenoaluminate anions, were initially developed for use as electrolytes in battery applications.〔 〕〔 〕 An important property of the imidazolium halogenoaluminate salts is that 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/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.〔 J. S. Wilkes, M. J. Zaworotko Chemical Communications 1992, 965-967 〕 Recently a new class of air– and moisture–stable, neutral ionic liquids became available. Research has also been moving away from hexafluorophosphate and tetrafluoroborate towards less toxic alternatives such as bistriflimide or away from halogenated compounds completely. Moves towards less toxic cations have also been growing, with compounds like ammonium salts (such as choline) proving to be as flexible a scaffold as imidazolium. Long thought to be solely a product of laboratory synthesis, an ionic liquid has recently been found in nature. The material, formed by tawny crazy ants (Nylanderia fulva) when they groom after being attacked by fire ants (Solenopsis invicta), is a viscous oily protic ionic liquid that may be less capable of penetrating the carapace of the would-be ant victim than would be the original venom with which it was attacked.〔See: "On the Formation of a Protic Ionic Liquid in Nature" (DOI: 10.1002/anie.201404402; Angewandte Chemie International Edition Volume 53, Issue 44, pages 11762–11765, October 27, 2014)〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Ionic liquid」の詳細全文を読む スポンサード リンク
|