Ionic liquids (IL) are compounds made up exclusively of ions and their combinations, but unlike the salts, they appear in liquid form at room temperature (or at temperatures close to the latter) even without the presence of a molecular solvent.
A generic definition of ionic liquids is that which describes them as salts that have melting points lower than the boiling point of water. This is a definition based solely on temperature, which does not provide information on the composition of the material except for the fact that it is made up of only ions.
Among the chemical-physical properties that most characterize ionic liquids, differentiating them from common organic solvents, the main ones are low volatility, low electrical conductivity, low flammability, relatively high thermal and chemical stability, the amplitude of the temperature range in which they occur in the liquid state, and favorable solvating properties for a range of polar and non-polar compounds.
The first room-temperature ionic liquid discovered appears to have been ethylammonium nitrate EtNH3NO3, first described in 1914. The first patent for a room temperature ionic liquid based on chloroaluminate anions dates back to 1948. In 1963 a system based on chlorocuprate anions and tetraalkylammonium cations was described, while in 1967 the first application of a room temperature ionic liquid based on tetra-n-hexylammonium as solvent was published.
In the 1970s, based on Osteryoung’s work, an important series of studies were conducted on ionic liquids based on tetraalkylammonium cations and chloroaluminate anions.
In the 1980s, systematic studies of ionic liquids of chloroaluminates were conducted. Early applications as electrolytes in solvent batteries and catalysts in organic chemistry are studied.
In the 1990s a new series of ionic liquids based on the 1-ethyl-3-methylimidazole cation and the tetrafluroborate anion were studied and described, which showed for the first time the possibility of creating ionic liquids based not exclusively on chloroaluminate salts.
Since then a large number of new ionic liquids have been described, studied and characterized and an increasing number of publications have been devoted to this category of compounds, including several reviews and books. This growing interest has also led to the commercialization of a large number of ionic liquids.
Properties of ionic liquids
Ionic liquids identify a very large class of compounds, estimated in the order of 1018 units, but in which some specific properties can be identified.
The values of the melting points of ionic liquids can vary greatly and those that have a low melting point resulting liquid at room temperature, are distinguished and defined as room temperature ionic liquids (or RTIL, acronym of room temperature ionic liquids). The chemical and physical properties of these compounds are the same as those of high-temperature ionic liquids, but the practical aspects of their maintenance and treatment are different enough to warrant a distinction.
Ionic liquids have almost zero vapor pressure (with a few exceptions), high electrical conductivity, and a large electrochemical window that allows for anions and cations that resist oxidation-reduction processes. In particular, the electrochemical properties can be modulated by modifying the anion and cation in various ways. These specificities favor their use in the electrochemical field (batteries, accumulators, sensors, photovoltaic panels).