liquid: two solids make
RTILs possess a
unique array of physico-chemical properties that make them
suitable in numerous task-specific applications
conventional solvents are non-applicable or insufficiently
Such properties include:
- high thermal stability,
- high electrical conductivity,
- large electrochemical
- low nucleophilicity and capability of providing
weekly coordinating or non-coordinating environment,
good solvents properties for a wide variety of organic, inorganic and
organometallic compounds: in some cases,
of certain solutes in RTILs can be several orders of
magnitude higher than that in traditional solvents.
Moreover, by fine-tuning
the structure, these properties can
be tailor-designed to satisfy the specific application
As a result, ionic
liquids are very popular materials and they enjoy a plethora of applications in various
domains of physical sciences.Table 1.
In most of these applications, the stability
of ionic liquids, at
least at a certain extent, is crucial
for optimum process performance. Several studies have indicated
that, although not 100% inert, certain ionic liquids
incorporating 1,3-dialkyl imidazolium
cations are generally
more resistant than traditional solvents under
process conditions, such as those occurring in oxidation, photolysis and radiation processes.
High quality ionic
liquids incorporating [bmim]+ cation
and a variety of anions, such as [PF6]−,
[CF3CO2]− and [(CF3SO2)2N]−
have been reported to
be colorless, even though they are not 100% pure.
The color of less pure ionic liquids generally ranges from
yellowish to orange.
The formation of the color has been
attributed to the use of raw materials with color or excessive
heating during the synthesis of imidazolium salt.
number of precautions for synthesis of colorless ionic liquids
have been described, and a procedure
of color from impure ionic liquids using acidic alumina and
activated charcoal has also been proposed.
The water content
has an influence on the viscosity of the ionic liquids.
Viscosity measurement indicate that ionic liquids became
less viscous with increasing water content. Hydrolysis
problems can also occur.
The degree of polarity
can be varied by adapting the length of the 1-alkyl chain (in
1,3-substituted imidazolium cations), and the counterion. Long-chain IL
salts have attracted some interest due to their liquid-crystalline
chemistry has a
large influence on the properties
of IL. 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.2