What is an Ionic Liquid?
It is not a question with an easy or a straightforward answer. In the words of Prof. Tom Welton from Imperial College London, “…an ionic liquid is a salt that does not make a very good solid”. In general, an Ionic Liquid -literally “liquid salt”- can be defined as any salt that melts (becomes liquid) before decomposing.
Ionic liquids are often composed of both organic and inorganic cations and anions. There is an extensively but controversial accepted division of these ionic materials into two groups: the so-called “room temperature ionic liquids” that are those salts becoming liquid around or below 100 °C, and the so-called “molten salts” that melt at much higher temperatures (e.g., NaCl at 803 °C). This controversial division of ionic liquids is arbitrary based on melting temperatures, but it can also be justified on evident practical applicability differences between them.
In this post, we will focus on the so-called “room temperature ionic liquids”, their unique properties and their application scope.
A brief history of Ionic Liquids
The discovery of the first compound considered as an ionic liquid can be attributed to Paul Walden in 1914 when he was searching for molten salts that could be liquid at his equipment working temperature. This first described ionic liquid was ethylammonium nitrate ([EtNH3][NO3]) with a melting temperature of 12 °C, and it was the first ionic liquid reported in the scientific literature.
Afterward, the research and the potential of ionic liquids breakthrough went unnoticed for a long time. It was not until the 60s of the last century when the research on ionic liquids was promoted thanks to an initial research project of the U.S. Force Academy led by Major (Dr.) Lowell A. King. The main objective then was to find out a replacement for the LiCl/KCl molten salts electrolytes used in thermal batteries. Since then, the U.S. Force Academy bets on a continuous molten salts/ionic liquids research program where they developed many ionic liquids combining different cations and anions for almost four decades.
Some of the relevant ionic liquids discovered during the decades from the 60s to the 90s are shown in the following table.
Ionic Liquid |
Year (ref.) |
|
[Et3NH][CuCl2] |
1963 (Inorg. Chem.,1963, 2, 1210) |
|
[N2222][GeCl3] and [N2222][SnCl2] |
1972 (J. Am. Chem. Soc., 1972, 94, 8716) |
|
[RPy][AlCl4] and [R2Im][AlCl4], R=alkyl] |
1982 (Inorg. Chem. 1982, 21, 1263) |
|
[R2Im][BF4] and [R2Im][PF6] |
1992 (Chem. Commun., 1992, 965) |
|
[R2Im][NTf2] |
1996 (Inorg. Chem. 1996,35,1168) |
|
Im = imidazolium, Py = pyridinium, NTF2 = bis(trifluoromethylsulfonyl)amide, N2222 = tetraethylammonium, EtPyBr = n-ethyl pyridinium bromide and Et3NH = triethyl ammonium. |
Afterward, in the early 21st century, the interest in ionic liquids and their potential application experimented a renaissance and started to attract huge interest within the scientific community. This interest was clearly reflected in the number of scientific publications where “ionic liquid” appears as a keyword. Having a look at the following chart, one can deduce that during the last 10 years (2012-2021), the interest of the scientific community in ionic liquids (> 54,000 scientific publications) became enormous, and the tendency is to continue growing in the next years (already ≈ 3,000 scientific publications in 2022 in the period January to June).