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.
The Rutgers paper will appear in the Nov. 10, 2005 print and Web editions of the Journal of Physical Chemistry B, and is now posted to the journal's ASAP Web site at http://pubs.acs.org/cgi-bin/asap.cgi/jpcbfk/asap/pdf/jp053930j.pdf.
The work was supported by a grant from the National Science Foundation.
Journal
The Journal of Physical Chemistry B