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Simulated fullerene tubules act as straws.

Few children fail to marvel when they first discover that water, unassisted, can sneak part way up a thin straw. Now, computer simulations show that carbon tubules -- cylindrical versions of the all-carbon molecules called fullerenes -- act as molecular straws.

These nanotubes can suck up all sorts of small molecules, says Jeremy Q. Broughton, a physicist at the Naval Research Laboratory in Washington, D.C.

Broughton and Navy lab colleague Mark R. Pederson created on their computer a tubule with 120 carbon atoms and then calculated how that tubule's electrons shift when a hydrogen fluoride molecule is brought close to each of its ends.

For the simulation, they arranged the tubule's hexagons of carbon to make the open-ended cylinder metallic. They then put hydrogen atoms on the ends of the tubule so the dangling bonds of the carbons there would not latch onto any molecule that came near them.

As the hydrogen fluoride molecules close in on the tubule, the two researchers found, the distribution of positive and negative charges in these dipolar molecules causes the tubule's mobile electrons to bunch up near the hydrogen fluoride. The attraction of the electrons, in turn, sucks the hydrogen fluoride molecules up and holds them, Pederson and Broughton report in the Nov. 2 PHYSICAL REVIEW LETTERS.

In more recent work, the Navy lab theorists have discovered that tubules will also take up molecules that are not dipoles. In these new simulations, the scientists tracked the movements of atoms, not electrons. They brought a 960-carbon tubule close to a 20,000-atom liquid reservoir of neon warmed to about its melting point. As it neared the neon surface, the tubule drew neon atoms from the surface into itself, Broughton told SCIENCE NEWS.

With nonpolar atoms or molecules, weak attractions called van der Waal's forces lead to the capillary action, Broughton says.

These results demonstrate that tubules will suck up and retain any molecule small enough to fit into them, he adds.

Broughton and Pederson think scientists can tailor the tubule to be selective about the molecules it picks up, in part by changing the way the hexagonal sheets of carbon wind around to form it. They have also determined that they can increase the sucking strength by narrowing the tubule's radius.

"With [these tubules], you can start making nanoscale devices that are mechanical, not electronic," Broughton notes. He envisions molecular-sized solenoids, pistons, and pumps, some of which may become components of nanoscale engines or devices used to restore function to ailing body parts.

Even though scientists know how to mass-produce layered tubules (SN: 7/18/92, p.36) and fullerenes in the shape of layered spheres (SN: 10/24/92, p.277), no one has made single tubes. But, says Broughton, now that he and Pederson have come up with uses for these fullerene cylinders, chemists are more likely to synthesize single tubules.

Meanwhile, Broughton offers a new name for them. "Rather than call them 'bucky tubes,' we can say they are 'sucky tubes,'" he says.
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Title Annotation:nanotubes can suck up any molecules small enough to fit
Author:Pennisi, Elizabeth
Publication:Science News
Date:Nov 14, 1992
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