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Rutgers chemistry researchers Hideaki Shirota and
Edward Castner, writing in the American Chemical Society's Journal of
Physical Chemistry B, describe chemicals that can perform many of the
same functions as organic, petroleum-based solvents but will not burn
or evaporate into the atmosphere. Thus, the chemicals wouldn't
contribute to air pollution and would likely cut the risk of workplace
accidents.
The chemicals, known as room temperature ionic
liquids (RTILs), can be used in industries such as chemical and
pharmaceutical manufacturing, electroplating, pulp and paper
production, and radioactive waste handling.
A major barrier to widespread adoption of RTILs is
that they are significantly thicker – or more viscous – than common
organic solvents, such as acetone, alcohol or benzene. The Rutgers
scientists invented a variant of these chemicals that could help them
overcome this problem.
"RTIL viscosity compares to traditional solvents
the way honey compares to water," Castner said. "It impedes their flow,
making lab procedures more difficult and manufacturing steps more
energy intensive and costly. We have discovered that by substituting
silicon for carbon at a key location in some RTIL molecules, we can
cut the liquid's viscosity almost tenfold relative to the same ionic
liquid without the silicon substitution."
In spite of RTIL's safety and environmental
advantages, higher costs could slow their adoption. Still, the Rutgers
advance could make these chemicals suitable for some near-term
specialty applications even though it may be too early to predict a
widespread industrial market for RTILs.
"By pairing the molecules we've studied with
molecules containing boron, we have a natural choice for handling
radioactive wastes, such as plutonium, in spent reactor fuel rods," he
said. "The boron would absorb neutrons generated by radioactive decay
while the solvent would safely withstand the elevated temperatures
that this decay causes."
Other likely applications include protein
production and analysis, where specifically tailored RTILs could
promote more complete reactions than water-based solvents and protect
proteins from breaking down under analytic procedures.
The molecules invented and studied by the Rutgers
chemists are organic salts that are liquid at room temperature. By
comparison, normal table salt melts only at the extreme temperature of
801 degrees Celsius. The Rutgers ionic liquids are comprised of
positively charged molecules, or cations, based on a carbon-nitrogen
structure known as imidazolium. The scientists paired these cations
with negatively charged molecules, or anions, including
tetrafluoroborate, a molecular structure of boron and four fluorine
atoms, and a more complex structure called
bis(trifluoromethylsulfonyl)imide. On the imidazolium cation, they
replaced an alkyl group (a common carbon and hydrogen grouping) with a
similar structure that substitutes a silicon atom for the central
carbon atom. This weakened the interaction between the ions and
resulted in liquid viscosities between two and eight times lower than
those for liquids with the alkyl cations when measured near room
temperature.
In their article, the scientists describe a
spectroscopy technique that allowed them to measure ultrafast
movements of the atoms and molecules in the RTILs. The technique
enabled the scientists to detect molecular vibrations and rotations as
fast as 20 quadrillionths of a second. They have applied these
research methods to several other classes of the ionic liquids, in
work that has also been published recently in the Journal of Chemical
Physics and in the Journal of Physical Chemistry A. |
