Switchable Solvents

Solvents that reversibly switch from being hydrophobic to hydrophilic or between extremes in ionic strength simply by the addition or removal of carbon dioxide are being reported by Philip G. Jessop and coworkers of Queen's University, in Kingston, Ontario.

The difference between the two states of the solvents is large enough that many compounds are soluble in only one form or the other. Harnessing these switchable solvents for chemical processing could make it easy in some cases to replace volatile organic solvents to improve air quality and eliminate the need for energy-intensive distillations.

Jessop and colleagues devised the first switchable solvent in 2005, opening the way for a budding list of CO₂-switchable solvents, surfactants, and catalysts. In one new study, Jessop and coworkers describe using CO₂ to reversibly switch the hydrophobic solvent N,N,N'-tributylpentanamidine into a hydrophilic solvent (Green Chem., DOI: 10.1039/b926885e).

The amidine isn't normally miscible with water and forms a biphasic system, Jessop explains. But under the influence of CO₂, the solvent converts to a bicarbonate species that is completely miscible with water. Bubbling nitrogen or air through the solution or heating it gently flushes CO₂ out of the mixture and re-forms the biphasic system.

This "switchable hydrophilicity solvent" is ideal for extracting low-polarity organic compounds, such as vegetable oils, Jessop notes. For example, his team has used it to mimic the industrial extraction of soybean oil from soybean flakes.

The current process involves extracting soybeans with hexane, followed by removing the hexane via distillation to leave the pure oil. The hydrophilicity solvent makes it possible to extract the oil with the amidine solvent and then separate the oil and solvent by adding CO₂ and water. Once the oil is removed, the solvent is separated from the water by removing CO₂ and then reused—no hexane and no distillation required. GreenCentre Canada, an institute that develops technologies from Canadian universities, is working to scale up the model process, Jessop says.

In a separate study, Jessop and graduate student Sean M. Mercer figured out a way to reversibly switch between salty and nonsalty water (ChemSusChem, DOI: 10.1002/cssc.201000001). "Salting out" is an effective method for separating water-soluble organic compounds from water, but it requires adding a large amount of sodium chloride or other salt to the solution and results in leftover salty water for disposal.

Jessop and Mercer instead added a neutral diamine to water, forming an aqueous solution with essentially zero ionic strength. After CO₂ is introduced, the diamine turns into a diammonium bicarbonate salt, significantly raising the ionic strength, Jessop says.

To illustrate how this "switchable water" system might be useful, the researchers added tetrahydrofuran, which is miscible with the nonsalty diamine solution. When exposed to CO₂, the diammonium salt forms and forces tetrahydrofuran out of solution. The tetrahydrofuran layer can be removed and the aqueous layer recycled to the nonsalty form by purging the CO₂.

"The notion of reversible salting out is pretty neat—I've never seen that before," says University of Pittsburgh chemical engineer Eric J. Beckman, an expert on using CO₂ as a solvent and chemical feedstock. It's hard to be any more environmentally benign or inexpensive than by using CO₂ and water, Beckman adds. These model solvent systems created by Jessop's group have potential if they can be optimized and put to work, Beckman says.