An increasing interest and intensive research have been devoted towards understanding the chemistry of ionic liquids over the past decade. These liquids are not any new and have extensively been used in organic separation and synthesis. With renewed interest towards these liquids, researchers have started to realize novel uses of ionic liquids and now research efforts have been spread to other specialties within the umbrella of science and technology. While research into ionic liquids has been limited to the classification as esoteric compounds, other applications of ionic liquids continue to be realized such as performance additives, bioreactor applications and in the designing of pharmaceutical compounds has received increasing recognition.
Ionic liquids have been thought to be green although noxious ionic liquids have been synthesized with ease. An example of these potentially life threatening liquids have cyanide as their sole anion and alkaloid as the anions. Therefore, there seems to be a misunderstanding on the nature of ionic liquids which has in many times led to the poor definition of ionic liquids. A detailed understanding will generate new insights into sustainable exploitation of ionic liquids in industrial processes. Apart from ILs being praised for being excellent solvents, they are also good catalysts especially in biological systems although they take part in highly selective catalytic solvents.
Definition of Ionic Liquids
Ionic liquids, abbreviated as IL comprise of a new class of solvents that is mainly composed of ions (cations and anions). They ILs are regarded to be green solvents because of their properties such as negligibility to vapor pressure, high thermostability and non-inflammability. Due to these properties, new biotechnological applications are continuously being discovered. Among the biotechnological applications of ILs which have been developed include whole cell and enzymatic catalysis. While ILs have been labeled ‘green’, the label has recently been questioned and challenged after ILs were shown to decompose as relatively low heat intensities.
ILs have also been shown to express some toxicity levels towards microbial cells as well as higher organisms. At the same time, it is possible to modify the physicochemical properties of ILs and come up with designer solvents with unique chemical structures. Perhaps interventions required to make ILs retain their ‘green’ label could be the fine tuning so that the toxicity drawbacks are addressed. Their applications need to be broadened in order to fully maximize their benefits to man.
Important properties of ILs have been investigated in order to understand this important class of solvents. However, not every ionic liquid will show similar properties as other classes of solvents. The most common properties of ILs which can be presented by this class of solvents include the liquid range which is 3000C (-96-+2000C). ILs have also been shown to be excellent inorganic, organic and polymeric materials solvents. The acidic composition of ILs can generally be said to be superacids.
Some ILs are highly water sensitive and therefore have to be used in dry boxes. In contrast, other ILs are highly hydrophobic (water haters) and have high air stability. ILs have also shown high thermal stability up to temperatures of 2000C. vapor pressure at room temperature has not yet been estimated and is assumed negligible. While ILs are noninflammable, it has been shown recently that some can be explosive and this raises eyebrows over the tag, ‘green’ solvents.
At room temperature, ILs are basically salts with heavy phosphorous or nitrogenous organic rich organic cations with a linear skeleton or chain of alkyl groups. Extensive research has been invested mainly in the imidazolium ILs especially the cations of 1-alkyl-3-methylimidazolium. The structures of these cations have also been studied extensively. ILs are known to be of dual functionality since they have both the cations and the anions. The most common anions of ILs include, the nitrates such as chlorides (Cl-) and bromides (Br-), chloroaluminates such as [Al2Cl4]- and [Al2Cl7]-, nitrates such as [NO3]-, tetrafluorophosphates [BF6]- hexafluorophosphates, [PF6]- and alkyl sulfates [RSO4] such as ethyl sulfate [C2H5SO4]-.