The anion chemistry has a large influence on the properties of IL.
The most commonly employed IL anions are polyatomic inorganic species:

The introduction of different anions has become more popular as an increasing number of alternatives are being discovered that function as well as, or better than, the well-known anions such as AlCl4-.

While IL derived from polyatomic anions still dominate commercial offerings, this is beginning to change.

  • fluorous-anions

    • PF6-, BF4-
    • Most prominent among these is PF6-; it and the related ion BF4- are probably the most common anions used in IL research. Though little variation in properties might be expected between same-cation salts of these species, the actual differences can be dramatic: for example, [bmim]PF6- is immiscible with water, whereas [bmim]BF4- is water-soluble. This sort of variation arising from different ion pairing gave rise to IL's description as "designer solvents".

    • CF3SO3-, (CF3SO3)2N-
    • Despite their widespread use, IL featuring PF6- and BF4- have been reported to sometimes decompose when heated in the presence of water, giving off HF. This drawback has prompted to introduce alternate anions, though many of these are still fluorous materials. In these, the fluorine of the anion is bonded to carbon, the C-F bond being inert to hydrolysis. Thus, IL based upon CF3SO3-, (CF3SO3)2N- and related anions are being marketed.

    For reactions involving the use of a metal catalyst in an IL, a low capacity for coordination on the part of the anion is essential, and fluorous species like PF6- will probably remain the best in that regard. Still, fluorinated anions tend to be expensive. Some, especially (CF3SO3)2N-, seem unlikely to ever be cheap enough to use in formulating ILs for use on larger scales, save for use in highvalue applications.

  • non-fluorous anions
  • In response to safety and cost concerns new ILs with non-fluorous anions have been introduced. Among the most interesting are salts in which the anions are derived from inexpensive bulk chemicals.

    • alkylsulfate anions
    • Alkylsulfate anions are likely to be reasonably non-toxic and biodegradable.
      One specific ionic liquid, ECOENG 500 (Peg-5 cocomonium methosulfate), is the first commercially available ionic liquid for which full toxicological data is available.
      The docusate (dioctylsulfosuccinate) anion is known to exhibit low mammalian toxicity, and is widely used in food, drug and cosmetic applications. In contrast to other ionic liquids, many incorporating docusate are highly lipophilic, some being fully miscible with hydrocarbons.

      n-Octy lsulfate (top); docusate (bottom)n-Octyl sulfate (top); docusate (bottom).

      Examples of ionic liquids.

Not all ionic liquids are actually liquid at room temperature.
Left is methyltri-n-butylammonium docusate, with a mp around 40 °C. Right is an RTIL (room temperature ionic liquid) 1-butyl-3-methyl imidazolium diethyleneglycolmonomethylethersulfate.
Note that some ionic liquids are colorless, while others are pale yellow to orange in colour.


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