floats on top of a conventional ionic liquid (left).
However, adding a small amount of a task-specific ionic liquid that
acts like a detergent enables the fluorohexane and the conventional
ionic liquid to mix into a yellow emulsion (right).
are not, as is
sometimes asserted, simply a dispersive
matrix; rather, they can and do interact
with dissolved molecules - as was recently reemphasized by recent
findings by Rogers and others that hard, coordinating anions like
chloride can be critical in facilitating the dissolution by IL of
biomolecules like cellulose and other oligosaccharides.
capacity of ILs to interact
with dissolved molecules in specific ways
is the focus of several research programs.
New IL are being introduced in which a
functional group is incorporated as a part
of the cation or anion structure. These
functional groups can impart a particular
reactivity pattern to the IL, enhancing its
capacity for interaction with specific solute
Brönsted acidic IL with appended sulfonic
acid groups were recently reported, as was
their use as solvent-catalysts for
esterification and other organic reactions;
- IL bearing appended
separate CO2 from gas streams;
with large aromatic head groups show
enhanced activity for the extraction of
aromatics in aqueous biphasic systems;
- IL with
tethered carboxylate groups have
been used as supports for “IL-phase
synthesis,” a versatile extension of the
solid-phase synthesis concept.
the development of such “task-specific” IL has been largely a
design, there is a huge role for discovery
to play as well.2
example of TSIL:
Ionic Liquids incorporating
novel cations for the coordination and extraction of Hg2+
The advent of moisture stable
IL and their unique chemical and
has brought about the
widespread application of IL with
from their use as alternatives to traditional
organic solvents and
unique reaction and as synthesis media.
In most cases, the IL characteristic that has been exploited
is their ability to dissolve a variety of solutes. For example,
IL can be used in place of traditional organic solvents in
liquid/liquid extractions where hydrophobic molecules such
as simple benzene derivatives will partition to the IL phase.
Conversely, for metal ions in [Cnmim][PF6]/aqueous
systems, the distribution ratios are below 1, indicating their
hydrated nature and preference for the aqueous phase (e.g.,
the distribution ratios for Hg2+ and Cd2+ in [C4mim][PF6]/
water systems are 0.84 and 0.03, respectively).
solvent extraction, increasing the metal ion partitioning to
the more hydrophobic phase is accomplished by adding
extractants that reside quantitatively in the extracting phase.
The extractant molecules serve to dehydrate the metal ions
and to offer a more hydrophobic environment that enables
their transport to the extracting phase. To that end have been
- crown ethers to aid in the extraction of Sr2+
from NO3- media,
- azo molecules for the extraction of
- halides and pseudohalides
for Hg2+ extraction in liquid/liquid systems with
ethers for Sr2+ extraction in liquid/liquid systems using IL
composed of the bis(trifluoromethylsulfonyl)imide anion.
associated with this approach lie in
extractant molecules that remain exclusively in the IL and
also understanding the increased complexity of the system
upon the addition of solutes, prompting the investigation of
the task-specific ionic liquid (TSIL) concept for metal ion
extraction in IL. Attaching a metal ion coordinating group
directly to the imidazolium cation makes the extractant an
integral part of the hydrophobic phase and greatly diminishes
the chance for loss to the aqueous phase.
Despite the added cost of TSIL, they
used in a mixture with less expensive
IL to achieve the same result.
Ligands employed in metal ion extraction have appended
functional groups (e.g., carboxylic acids) or contain donor
atoms (e.g., crown ethers) that interact to form complexes
with metal ions. Ligands containing soft donor atoms such as N or S
have been explored in other systems for the
extraction of heavy metals through the use of thioether,
thiourea, and urea functional groups. Other reports
confirm that acidic conditions can be sufficient to induce
Hg2+ and Cd2+ stripping from thiourea and
extractants. Hence, IL with
thioether, thiourea, and urea functional groups incorporated
in the alkyl chains of the imidazolium cations have been prepared
(figure on the right).
The results indicate that appending thioether, urea, and
thiourea functional groups to imidazolium cations does
produce TSIL cations that can be considered either a new
class of IL or novel class of IL extractants. The TSIL cations,
in combination with PF6- and used alone or in a
with [C4mim][PF6], result in significant
for Hg2+ and Cd2+ in liquid/liquid separations
the reliance on traditional organic solvents
The concept of TSIL illustrates howfunctional groups
can be introduced into the scheme of IL synthesis with the
inherent potential for achieving desirable properties tuned
to specific applications.