is dominated by the study of species in solution.
any liquid may be used as a solvent, relatively few are in general use.
However, as the introduction of cleaner technologies has
become a major
concern throughout both industry and academia, the search for
alternatives to the most damaging solvents has
a high priority.
Solvents are high on the list of damaging chemicals
for two simple reasons:
- they are used in
- they are
usually volatile liquids (VOCs) that
salts are liquids containing
only ions, ionic
liquids. It is possible, by careful choice of starting materials, to prepare ionic liquids that
are liquid at and below room temperature.
RTILs are different than molecular
solvents, water and organic solvents, and chemically
analogous to molten salts. Conventional molten salts exhibit
a high melting point (i.e., 801 °C for sodium chloride and
614 °C for lithium chloride), which greatly limits their use as
solvents in most applications; RTILs, however, remain liquids at or
below room temperature.5
- Solvent properties of ILs
Some simple properties of these room-temperature
that make them interesting as potential solvents for synthesis are the
- they are good
solvents for a wide range of both
inorganic and organic materials, and unusual combinations of reagents
can be brought into the same phase;
- they are often
poorly coordinating ions, so they have the potential to be highly polar
yet noncoordinating solvents;
- they are
immiscible with a number of
organic solvents and provide a nonaqueous, polar
two-phase systems. Hydrophobic
ionic liquids can also be used as
immiscible polar phases with water;
liquids are nonvolatile,
hence they may be used in high-vacuum systems and eliminate many
containment problems. They do not evaporate!
One of the most distinct advantages of RTILs that has
been the rationale for their characterization as “Green
Solvents” is their negligible volatility. This
renters them promising replacements to volatile organic
compounds (VOCs), which are used in large
chemical and engineering industries and are a source of major
environmental problems.5 Moreover, many ILs can be recycled and reused
is the most commonly used solvent classification. Even when considering
molecular solvents it is poorly understood and often confused. Terms
such as polar, apolar, and nonpolar are used indiscriminately to apply
to values of dielectric constants, dipole moments, and
polarizabilities, even though none of these are directly correlated in
a simple way.
The simplest qualitative definition is that a polar
solvent is one that will dissolve and stabilize dipolar or charged
solutes. It is widely thought, though yet to be generally demonstrated,
that under this definition, ionic liquids will be highly polar solvents.
Liquid-crystalline properties 10
Long-chain IL salts have attracted some
interest due to their liquid crystalline (LC) properties. The origin
for these can be found in the formation of domains, "Coulombic" layers
where the ionic head-groups interact with the counterions, and "van der
Waals" layers built from (anti)parallel stacking of the alkyl chains.
Hexafluorophosphate salts with cations up to C20MIM have
been investigated by differential thermal analysis (DTA) and show one
or more LC transitions. Melting to isotropic liquids occurs at rather
high temperatures ( > 100 °C).