Successful "Green" Solvent Found For Problematic Chemicals

It is the vapor pressure of traditional solvents that generally make them hazardous, explains Joan F. Brennecke, professor of chemical engineering at the University of Notre Dame and principal investigator of the study. Because the solvents evaporate easily into the air, factory workers may inhale them. Additionally, the solvents add to damage of the earth's atmosphere, because organic solvents eventually will oxidize and create carbon dioxide, a green house gas with potential impact on global warming.

In recent years, ionic liquids have emerged as possible "green" solvents -- that is, environmentally benign -- largely because they have no measurable vapor pressure. They do not evaporate

Ionic liquids are liquid salts -- not salt dissolved in a liquid, but a salt that exists in liquid form at room temperature. As salts, they have a plus charge and a negative charge. They don't occur naturally, but are manufactured, and there are dozens of various ionic liquids.

And ionic liquids are potentially good solvents, Brennecke says. "People are now showing that you can do all kinds of reactions in them, and many reactions must be done in a solvent. But once the reaction is complete, we need to get the chemical product out of the ionic liquid in pure form."

This has posed a problem in certain cases. Water soluable compounds can be extracted with water, and distillation can be used to remove chemicals with high vapor pressures. "But you would need very high temperatures to distill larger compounds that have low vapor pressures," Brennecke says. "This isn't practical."

In addition there are many chemicals that would break down, or decompose, when heated. Many pharmaceuticals fall into this category. For these, some other way is needed to remove the chemicals from the ionic liquids.

In experiments described in the Nature article, Brennecke and her colleague, Eric J. Beckman of the University of Pittsburgh, forced supercritical carbon dioxide through a solution of naphthelene (moth balls) dissolved in an ionic liquid, 1-butyl-3-methylimidazolium hexaflourophosphate. Carbon dioxide is considered to be an environmentally benign solvent because it is nontoxic and nonflammable and isn't being created in the process; what already exists is simply being used.

The carbon dioxide, in its supercritical state, is near room temperature (40 C) but is highly pressurized so that it has a liquid-like consistency yet, like gas, expands to fill a container. When droplets of supercritical carbon dioxide were forced through the ionic liquid/naphthalene solution, the carbon dioxide completely pulled the naphthalene out with it, yet left all of the ionic liquid behind.

When the resulting supercritical carbon dioxide/ naphthalene mixture was depressurized, the carbon dioxide returned to its gaseous form, leaving pure solid naphthalene. While lots of carbon dioxide remained dissolved in the ionic liquid, no measurable ionic liquid dissolved into the carbon dioxide, Brennecke says. "Carbon dioxide is not polar enough to support [dissolve] ions."

Brennecke and Beckman believe that supercritical carbon dioxide will likely work similarly with other such problematic chemicals.