Salts are mixtures of charged species, or ions. They have an avid tendency to remain solid due to the strong interactions between their charged components. About a hundred years ago, people started noticing certain compounds that were ionic would nonetheless melt below or around room temperature. This is a unique class of liquid - extremely polar, and capable of dissolving lots of odd stuff. They also tend not to evaporate very easily, the ionic interactions between components of the salt being so strong. For these reasons, a lot of people have suggested they could be used in "green chemistry," where the ILs are reused over and over (and with much less toxic fumes than our usual solvents).
It turns out ILs aren't quite so innocuous - just because they don't have much of a vapor pressure doesn't mean they aren't toxic. A lot of people have done a lot of stuff with them, just because they're neat and they're going through a vogue lately. Not everything that people are trying is necessarily worthwhile - lots of chemists joke that putting "ionic liquids" in your article's title is second only to putting "nano" in there in terms of improving its chances of getting published - but chemists have fads just like everybody else.
Transient vogues aside, ILs can do some pretty neat stuff. You can, for example, dissolve wood in IL. The salt below, 1-butyl-3-methylimidazolium tetrafluoroborate, or "bmim," is a well-studied IL (which stays liquid well below the freezing point of water!).
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In the course of doing some regulatory compliance, we received notice from the EPA that an intermediate in our process, BMIM chloride, was surprisingly toxic to lab animals (rats) when applied dermally. The EPA insisted that our LVE had to include this tox data in it. Fortunately the project died before we scaled up. BMIMCl is fairly ubiquitous as a starting material for many RTIL's.
N,N'-Imidazole-based ionic liquids can be remarkably toxic, do not biodegrade, and may bioaccumulate.
Aldrich Chemfiles 6(9) 2006 is devoted to ionic liquids, especially cellulose to rayon applications (be afraid, Lyocell, be very afraid!). See also Chemfiles 5(6) 2005 for a huge listing. See also 01 January 2007 C&EN, p. 23.
The prices are obscene. You can diddle on the cheap with feed additive plus lawn fertilizer or antifreeze, 12 September 2005 C&EN. Molar 1:2 choline chloride/urea dissolves all the transition metal oxides - including magnetite. I find that to be quit amazing. Will it dissolve ferrites in general?
Then, Choline chloride/Urea would dissolve chromium oxides, right?