Chemistry Reference
In-Depth Information
When discussing ionic liquid properties, it is important to realize that
there is no such thing as a single ionic liquid because an extremely wide range
of compounds can be categorized as ionic liquids. As such, one class may pos-
sess a particular property, while another class can have very different proper-
ties. Many people have been tripped up over the years by making broad
general statements about ionic liquids and their properties, only to have coun-
terexamples crop up. Given this caveat, however, we can list some general
properties that many ionic liquids possess. First, ionic liquids have an exceed-
ingly low volatility, to the point of being effectively nonvolatile. Of course, no
substance is truly nonvolatile, and a recent article
2
has shown that some ionic
liquids have a detectable vapor pressure and can actually be distilled! How-
ever, even these have
very low
vapor pressures, such that for all practical
purposes they can be considered about as volatile as a solid. This is one of their
major beneficial properties. Ionic liquids have a very broad liquidus range, on
the order of several hundred degrees Celsius. Many tend to have thermal
decomposition temperatures of around 300-400
C, making them quite stable.
Second, ionic liquids can be made to dissolve a wide range of compounds,
from polar molecules such as water and alcohols, to aromatic substances
such as benzene, to polymeric substances such as cellulose, to gases such
as carbon dioxide and sulfur dioxide.
3,4
This, combined with their low vola-
tility, stimulated a great deal of research directed at their use as ''green''
solvents. The idea was that, unlike traditional organic solvents, ionic liquid
solvents do not evaporate and thus cannot contribute to air pollution like con-
ventional volatile organic compounds. The jury is still out, however, as to how
truly green these substances are. Some have been shown to be highly toxic to
aquatic organisms, while others have very low toxicity.
5
Third, ionic liquids
are conductive, which opens up the possibility of using them in fuel cells, bat-
teries, and other electrochemical devices. Ionic liquids can be made energetic,
which suggests they could be used as low-volatility propellants.
6
Ionic liquids
have also been shown to have highly desirable lubrication properties,
7
which
when coupled with their thermal stability and low volatilty suggests great
potential in high-performance applications. Ionic liquids have been used in
sensing and other analytical devices, as catalysts, magnetic fluids, heat transfer
fluids, and as heavy-metal extraction agents. The list could go on and on,
which is really why these compounds are under such scrutiny today.
The other important factor to realize when talking about ionic liquids is
that they have tremendous chemical diversity. An almost limitless number of
compounds exist that can be made into an ionic liquid. Figure 3 shows a very
small example of the types of cations and anions that can be combined to give
ionic liquids. Estimates suggest that more than 10
8
different compounds can
be made into an ionic liquid. This is due to the fact that different functional
groups can be added to base cation or anion structures, which can have a
profound effect on properties. For example, by pairing the same cation with
a different anion, the melting point can be changed by over 100
C.
8
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