What are ionic liquids?



Note on nomenclature:
the terms room-temperature ionic liquid (RTIL), nonaqueous ionic liquid, molten salt, liquid organic salt, and fused salt have all been used to describe salts in the liquid phase.3

In contrast to high temperature melts that are commonly referred to as molten salts, ionic liquids are, as their name implies, salts that are liquid at low temperature - many at room temperature or below - and that in a molten form are composed wholly of ions.2

Conventional molten salts exhibit a high melting point (i.e., 801 C for sodium chloride and 614 C for lithium chloride), which greatly limits their use as solvents in most applications.5 RTILs, however, remain liquids at or below room temperature.
The adopted upper temperature limit for the classification "ionic liquid" is 100 °C (though some solidify to glasses on cooling) and higher melting ion systems are typically referred to as molten salts.4 Press here to see the chemical-physical properties Table.Table 1.

There is no reliable way to predict the precise melting point of organic salts, and identification of new room-temperature ionic liquids is a somewhat hit and miss affair. After all, there is nothing special about room temperature, it just happens to be the temperature at which rooms are, and salts with melting points of 20°C and 30°C are unlikely to have great differences in their structures and interionic interactions.3

These low melting points are a result of the chemical composition of RTILs, which contain larger asymmetric organic cations compared to their inorganic counterparts of molten salts: the asymmetry lowers the lattice energy, and hence the melting point, of the resulting ionic medium. In some cases, even the anions are relatively large and play a role in lowering the melting point.5

Until recently room-temperature ionic liquids were considered to be rare, but it is now known that many salts form liquids at or close to room temperature. Invariably, these ionic liquids are either organic salts or mixtures consisting of at least one organic component.

The composition and associated properties of IL depend on the cation and anion combinations: there are litterally billions of different structures that may form an ionic liquid, the combinations cation-anion are estimated to be as high as  1018!!! 4


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