September 20, 1999
Volume 52, No. 5
Menger's emulsion system waters down mustard gas
The first recorded act of chemical warfare occurred in 423 B.C. during the Peloponnesian War. Through a hollow beam, Spartan allies directed a noxious gas from lighted coals, sulfur and pitch into an Athenian-held fort, taking the stronghold from their enemies. Fire and flame continued to be employed throughout the ages until the birth of modern chemistry during the late 18th and early 19th centuries.
On July 12, 1917, chemical warfare reached a new level. German artillery shells containing a new chemical agent, sulfur mustard, resulted in more than 20,000 casualties and terrorized Allied troops huddled in their trenches. Unlike previous chemicals employed by both sides in World War I, mustard was a persistent agent-not only the air the soldiers breathed but also anything they touched became toxic, as mustard does not quickly dissipate.
Since that time more-and deadlier-chemical agents have been developed even as international agreement has banned them. While research has effectively produced whole new classes of chemical agents and specialized delivery systems, not much time or money has been spent to defend against these horrible agents.
However Fred Menger, Candler Professor of Chemistry, has developed a viable decontamination system. "It was a challenging problem, a kinetic problem," he said. "How do you destroy an undesirable material, and how do you do it with an aqueous (water) system? We didn't want to do it with gasoline or organic solvents; we wanted to use water."
Certain chemical aspects of water determine how it interacts with the environment, including other chemicals. The two hydrogen atoms of a water molecule have a positive charge, while the oxygen atom has a negative charge. Since opposite charges attract, water molecules line up like little magnets. This interesting property also makes some substances unlikely to dissolve in water. "Normally, toxic materials are water insoluble," Menger said. "So if you spray water on them, nothing happens."
He overcame this problem by using an emulsion, a solution with tiny suspended droplets. "The decon system is based on a microemulsion system," Menger explained. "Basically it's an aqueous system, a water system, that dissolves large amounts of organic substances, including toxic substances."
A common emulsion that one might see around the house is milk. Milk consists of a water solution with large droplets of fat. Inside the fat droplet, the fat molecules line up with their hydrophilic (water-loving) heads facing the water and their hydrophobic (water-fearing) tails tucked inside.
The droplets in Menger's system consist of hydrocarbons that are stabilized with a surfactant (soap). Added to this, another chemical must destroy the toxin. Menger uses a common household substance: bleach. Sprayed onto a toxin, the emulsion quickly traps the poison inside the tiny droplets where the bleach destroys them. "Mustard has a sulfur atom in it, and we bond an oxygen to that sulfur," Menger said. "By reacting the sulfur, we make it inactive."
Menger's experiments never involved the actual chemicals used in chemical warfare: "We work with safe simulants. These are compounds that are not toxic but have similar chemistry to mustards and nerve agents." However he has been told that his system performed well during tests on the real agents, and an emulsion system similar to his was used to decontaminate a Japanese subway system after a Sarin gas attack by a Japanese cult. He also further refined the emulsion formula so it could be used in the Arctic without freezing.
Military applications notwithstanding, Menger has not involved himself in finding uses for his discoveries. "We're not applied chemists; we do basic science," he said. "Right now we're defining the structure and behavior of these surfactants. We don't look for specific applications."
And not all his work has military connections, Menger stressed, adding that much of is funded by the National Institutes of Health and the National Science Foundation. Some properties he studies are change in surface tension, thermal characteristics and flow behavior (rheology). Rheology is perhaps the most intriguing of these properties since the effects are apparent in everyday life. "There are properties that change with how you handle a solution physically," Menger said. "The most common example is ketchup [that won't come out of the bottle]. You change the viscosity by shaking it, changing the alignment of the molecules so they flow more easily."