The anion chemistry has a
large influence on the properties
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
IL derived from polyatomic
anions still dominate commercial
offerings, this is beginning to change.
prominent among these is PF6-; it and the
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
their widespread use, IL
featuring PF6- and BF4-
reported to sometimes decompose
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-
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,
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
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
Alkylsulfate anions are
likely to be reasonably non-toxic and
One specific ionic liquid, ECOENG 500 (Peg-5 cocomonium
methosulfate), is the first commercially
available ionic liquid for which full toxicological
(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.
sulfate (top); docusate (bottom).
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
Note that some ionic liquids are colorless, while others are pale
yellow to orange in colour.