News Services Staff
A research paper on it is published in the Feb. 2 issue of Science.
The new hybrid solvent system, called a water-in-carbon dioxide microemulsion, demonstrates the first thermodynamically stable, hydrophilic (water-loving) environment to be formed in a "green" and continuously adjustable solvent. It also can be recycled and reused.
Solvents currently in use are often harmful to the environment and present substantial disposal problems. The new solvent system has broad applications in protein and polymer chemistry, environmental and materials science, separations and reaction engineering.
"These new microemulsions in carbon dioxide open up an entirely new way of thinking about and doing chemistry," said Frank Bright, UB professor of chemistry and co-author of the paper.
Composed of water, a form of carbon dioxide and a fluorinated surfactant typically used as a blood substitute, the solvent system overcomes many of the major drawbacks associated with traditional liquid solvents.
"It is generally agreed that there are only two environmentally benign solvents in the world: water and carbon dioxide," said Bright.
Water is an ideal solvent for working with biological samples and other hydrophiles (water-loving chemical species), he explained, but once these species are dissolved, they are generally difficult to remove from water. Also, their function cannot be modulated without the addition of external stimuli like temperature changes or secondary reagents.
Proteins, other biological samples and water cannot be dissolved well in carbon dioxide alone. But carbon dioxide remains an excellent ingredient for a solvent system, since in addition to its "green" properties, it is naturally abundant, nonflammable and the least expensive solvent after water.
For these reasons, researchers have spent several decades working to find a way to marry the attractions of carbon dioxide with those of water.
The UB scientists and their colleagues achieved their breakthrough by using carbon dioxide in its supercritical state, where liquid and gas-like properties exist simultaneously.
When the supercritical carbon dioxide was combined with the fluorinated surfactant and water, it spontaneously formed stable, tiny balloon-like sacs with water in the middle, called reverse micelles.
"These reverse micelles are like microscopic chemical reactors that will trap hydrophilic species inside them," said Bright. "The surfactant serves as the micelle's walls and keeps the water within an interior pool and the carbon dioxide outside of it. So the hydrophiles are located within a tiny water microenvironment that is, in turn, surrounded by carbon dioxide."
This creates a unique environment for hydrophilic substances, such as proteins, with the added advantages associated with supercritical carbon dioxide.
"The new solvent has diffusion properties more like a gas than a liquid, so reactions that rely on diffusion go significantly faster," said Bright. "For example, a chemical reaction that might take 10 minutes in water, takes one minute in a supercritical fluid."
This is particularly attractive because several important chemical reactions cannot be harnessed in industrial or environmental applications for the reason that they take too long.
The new solvent system has the loading characteristics of a normal liquid, so it can dissolve high concentrations of relatively large hydrophiles. It also eliminates what may be the biggest drawback to liquid solvents now in use: getting the reactants or reagents out of the solvent once a reaction is finished and disposing of the solvent.
"Think of trying to remove dissolved table salt from a cup of water," said Bright. "It would be hard to remove it without adding a second solvent, another chemical reagent or energy. On the other hand, while an organic solvent may be volatile and therefore easily separated, you have to worry about how to properly dispose of the solvent."
By contrast, supercritical fluids are con- tinuously adjustable solvents, said Bright. In other words, by adjusting the amount of pressure applied to the supercritical carbon dioxide, it is possible to control the behavior of the carbon dioxide and, in turn, the reverse micelle, its water pool and chemicals within it. This property also makes extracting the chemical species from the solvent system easier. For example, if the carbon dioxide pressure is lowered, the reverse micelles become unstable, collapse and simply dump out the dissolved species.
"The beauty of this scheme is that the carbon dioxide can then be collected and recompressed so that the whole process may be repeated again and again," said Bright.
The UB portion of the research was funded by a Department of Energy grant.