Ionic liquids are already known as „green“ solvents since they don’t evaporate and neither burn nor explode, which is what every chemist really is looking for in a solvent. Additionally, they are powerful solvents with easily tunable properties and of course electrically conducting. Very useful stuff, ionic liquids. Unfortunately they are generally at least as toxic as normal solvents, and since they look rather promising for everyday applications like batteries and even cosmetics, that is a serious drawback.
There is, however, one huge advantage to ionic liquids: Since they all consist of cations and anions which can be combined at will, every new compound really means that there are dozens of new, unique solvents available. Although many current compounds are based on quaternary ammonium salts, some of which are potent biocides, the structural diversity of known compounds suggest that truly green ionic liquids are well within reach.
There have been recent advances regarding non-toxic and especially biodegradable ionic liquids, the latter can easily be obtained by an approach similar to the one that yields biodegradable detergents (pdf), i.e. incorporating esters in the hydrophobic side chain. Toxicity, on the other hand, is difficult to assess. It appears that it’s not only the toxicity of the component ions, but cooperative effects seem to play a role as well. Hydrophobic cations are usually more toxic, but changing the anion may make a huge difference, as newer results show. One example of such a compound is 1-n-butyl-3-methylimidazolium saccharinate, but extensive testing will be required to find more examples.
The problem is not just that there are millions of possible compounds, but also several different options what to test. Cell lines? Aquatic organisms? Whole ecosystems? Different systems will react differently, and there is very little experience yet with the toxicity of ionic liquids. Recent advances include the adaptation of the common agar diffusion test, which is usually employed to assess antibiotics. It won’t cut the need for extensive testing, but it can be used easily in the lab to determine on a preliminary basis, if a compound is worth pursuing or should be dropped.