Ionic Liquids
Ionic liquids (ILs) are molten salts that originates from the association of organic cations and organic/inorganic anions.
Ionic Liquids: History, Definition and Overview
Ionic liquids, literally “liquid salts” (ILs, salts that melt at temperature below 100°C) have been known for many years, following the successful work at the Air Force Academy in use of ILs as electrolytes1 some 30 years ago. The rise of the popularity of ILs is traced back to establishment of Green Chemistry as a legitimate scientific field, and sparked a surge of interest in ILs where these fluids were used as suitable replacement for volatile organic solvents (VOCs).
The term “ionic liquid” is very broad, and multiple structures can be converted into IL form introducing complexity and diversity to IL structure and properties. Indeed, IL consist of both organic and/or inorganic ions,2 and may contain more than one cation or anion. There are electrostatic and dispersive interactions at different length scales within the liquid, which results in its highly nonisotropic character.3 There is large structural diversity of ions (Fig. 1)3 which vary from inorganic to organic, simple to complex, chiral or achiral, including fully or partially ionized acid or base, charged bridging ligands, metallate coordination polymers, organic polymeric metal ions, etc. These include the well-recognized azolium (e.g., imidazolium, triazolium), phosphonium, pyridinium, pyrrolidinium, alkylammonium, etc. cations, as well as many new cation classes. Anions include a wide variety of inorganic anions (e.g., halide, nitrate, perchlorate, sulfate, nitrite, hexafluorophosphate, tetrafluoroborate, azide), and an ever increasing number of organic anions (e.g., triflate, benzoate, sulfacetamide, alkylsulfates, alkylcarbonates, organic carboxylates).4 Such intrinsic complexity of the ILs and diversity of ions is exacerbated by the inability to attribute a single common characteristic to all ILs except melting point (mp) and there is no single model that could be comprehensive enough to describe entire family of ILs.
Yet, initial “green solvent” concept drove view of ILs as a single thing. The way the term “ionic liquid” is used leads people to group them into a single class without scientific basis, disregarding the fact that there is no similar common characteristics other than that they are liquids which describes them all. Industrial awareness is growing, and the field of ILs is moving forward exploring new modes of applications. While there is a separate section in this article on applications of the ILs, we’d like to note that if one grasps the beauty and complexity encapsulated in the IL form, transformational applications may arise from understanding of?what?are the advantages IL can provide and how we can turn what is the best thing to use into IL.
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About this series of “ionic liquids”, our laboratory can make it for you, if you are interested in it, please contact me and I will send you the catalog of ionic liquids.
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