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Slowing chemical reactions in tight spaces.

No action comes more naturally to a chemist than swirling the contents of a test tube to hasten a chemical reaction. But the chemistry that takes place when reacting chemicals must thread long, narrow tubes or wander through a maze of tiny pores doesn't necessarily follow the same familiar rules as the chemistry in spacious, well-stirred vessels.

Raoul Kopelman and his co-workers at the University of Michigan in Ann Arbor have now demonstrated experimentally that under certain conditions, two reacting chemicals confined to a thin tube spontaneously tend to segregate themselves rather than mix. Because molecules of the two substances rarely get close enough for a reaction to occur, the rate at which the reactants combine to form a product decreases as the reaction proceeds. In contrast, conventional theory, which assumes mixing, predicts that this reaction rate should actually increase.

Kopelman and his colleagues first came across the phenomenon of reactant segregation in their computer simulations of reactions on confined surfaces, which severely restrict molecular movement. Because a collision between two different molecules leads to the immediate formation of a product, any mixing naturally gets rid of both reactants. But because molecules diffuse extremely

slowly, replacement molecules take a long time to arrive in the depleted areas. The combination of these two factors produces a curious patchwork of depleted zones and concentrations of one or the other reactant.

Any molecules straying to a boundary get "killed" before they can penetrate each other's territories, Kopelman says. It's the survival of the most isolated.

"This doesn't come out of any of our [conventional] physical or chemical formulations because they always assume that everything is nicely stirred up," he adds. "But when you treat it as a Darwinian-like principle, it's obvious."

To demonstrate the same effect in the laboratory, the Michigan researchers devised an experiment in which two substances of contrasting colors diffuse from opposite ends toward the middle of a narrow, horizontal, gel-filled tube. A vertical boundary forms between the two reactants where they meet and combine to create a new chemical substance.

Classical theory predicts that the reaction rate in this situation should increase steadily as the two substances gradually interpenetrate and mix. Instead, the Michigan group found that the reaction front actually develops into a distinct region where the concentrations of both reactants sink very low. In effect, this visible gap keeps the reactants segregated, and in the absence of mixing, the reaction rate eventually falls nearly to zero.

"Here you can see it with your own eyes," Kopelman says. "This is the first experimental evidence that different molecular reactants segregate themselves into like groups when confined to small spaces."

These findings may prove important in the study of a variety of chemical processes. "Once you establish a certain principle, you can use it to explain and interpret a lot of other situations," Kopelman says. "If the conditions are right, it should also happen in less controlled situations -- inside a biological cell or on a catalytic surface."

Kopelman described this research at an American Physical Society meeting held last week in Indianapolis.
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Author:Peterson, Ivars
Publication:Science News
Date:Mar 28, 1992
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