In a feat that would be alchemy if it weren’t clever chemistry, scientists have developed a simple technique to switch an oil-like solvent into a waterlike one. The approach provides chemists a way to use the same solvent system for a diversity of reactions that otherwise would require many different and often environmentally troublesome solvents. This advance could help the chemical industry become greener and save money at the same time, the technique’s inventors say.
In the Aug. 25 Nature, Philip G. Jessop, a chemist at Queen’s University in Kingston, Ontario, and his colleagues describe their switchable solvent—a mixture of an alcohol and an amine base. The mixture starts out as a nonpolar liquid, so it can dissolve compounds that have an affinity for oil.
But when the researchers bubble carbon dioxide through the solvent under normal temperature and pressure, it undergoes a dramatic change in personality, becoming polar. Then, it can dissolve chemicals that are water-soluble. Another bubbling, this time with nitrogen or argon gas, transforms the solvent to its former, nonpolar self.
Although the polarity of certain liquids can be altered at high pressures, the new fluid is the first practical, reversible solvent, says Jessop.
Keith P. Johnston, a chemical engineer at the University of Texas at Austin, predicts that the technique will generate interest because of its simplicity. He notes that the switch occurs “in a very straightforward manner.”
The work is also “a beautiful example for green chemistry,” says Walter Leitner, a chemist at RWTH Aachen University in Germany. “What it shows is that by understanding the chemical principles, we can lay the basis for more-sustainable production processes.”
Currently, the hundreds of millions of pounds of solvent waste generated in the chemical industry each year present disposal problems, says Jessop.
There are many details to work out before switchable solvents become routine. For example, the amine component of Jessop’s solvent is strongly basic, which limits the kinds of reactions for which it can be used. Also, while the conversion to the polar liquid occurs in about an hour, the change back to the nonpolar liquid takes a day at room temperature. The reversion does speed up significantly with a little warming.
Jessop’s group is now investigating other solvent mixtures, and Jessop says that he hopes other chemists will join the search.
“We want to have a whole suite of [quickly switchable] solvents … with different chemical characteristics,” he says. With more choices, industry could opt for switchable solvents to manufacture thousands of products.
Says Jessop: “We want to have our medicines and plastics, but we don’t want to have the pollution.”