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June 11, 2013
The Overselling of Ionic Liquids
Ionic liquids (molten salts at relatively low temperatures) have been a big feature of the chemical literature for the last ten or fifteen years - enough of a feature to have attracted a few disparaging comments here, from me and from readers. There's a good article out now that talks about the early days of the field and how it grew, and it has some food for thought in it.
The initial reports in the field didn't get much attention (as is often the case). What seems to have made things take off was the possibility of replacing organic solvents with reusable, non-volatile, and (relatively) non-toxic alternatives. "Green chemistry" was (and to an extent still is) a magnet for funding, and it was the combination of this with ionic liquid (IL) work that made the field. But not all of this was helpful:
The link with green chemistry during the development of the IL field, propelled both fields forward, but at times the link was detrimental to both fields when overgeneralizations eroded confidence. ILs were originally considered as green since many of these liquid salts possess a negligible vapor pressure and might replace the use of volatile organic solvents known to result in airborne chemical contamination. The reported water stability and non-volatility led to the misconception that these salts were inherently safe and environmentally friendly. This was exacerbated by the many unsubstantiated claims that ILs were ‘green’ in introductions meant to provide the motivation for the study, even if the study itself had nothing to do with green chemistry. While it is true that the replacement of a volatile organic compound (VOC) might be preferred, proper knowledge of the chemistry of the ions must also be taken into account before classifying anything as green. Nonetheless, the statement “Ionic Liquids are green” was widely published (and can still be found in papers published today). Given the number and nature of the possible ions comprising ILs, these statements are similar to “Water is green, therefore all solvents are green.”
There were many misunderstandings at the chemical level as well:
However, just as the myriad of molecular solvents (or any compounds) can have dramatic differences in chemical, physical, and biological properties based on their chemical identity, so too can ILs. With the potential for 10^18 ion combinations, a single crystal structure of one compound is not a good representation of the chemistry of the entire class of salts which melt below 100 °C and would be analogous to considering carbon tetrachloride as a model system for all known molecular solvents.
The realization that hexafluorophosphate counterions can indeed generate HF under the right conditions helped bring a dose of reality back to the field, although (as the authors point out), not without a clueless backlash that decided, for a while, that all ionic liquids were therefore intrinsically toxic and corrosive. The impression one gets is that the field has settled down, and that its practitioners are more closely limited to people who know what they are talking about, rather than having quite so many who are doing it because it's hot and publishable. And that's a good thing.
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