Gubbins and his colleagues are studying idealized models of either real, existing materials or potentially new and better materials. Post-doctoral fellow Michael Maddox, for instance, is simulating adsorption of gas mixtures in buckytubes -- hollow, tube-shaped molecules closely related to buckyballs, the recently discovered spherical carbon structures that have many potential industrial applications.
"The buckytubes consist of graphite sheets," Gubbins says, "rolled into the shape of a tube, whereas buckyballs are graphite sheets rolled into a ball. We became interested in buckytubes because it seems they can be made with pores of very uniform sizes, and so it might be possible to tailor them to particular applications." Carbon atoms are highly selective and can absorb one gas from a gaseous mixture. For now, Maddox and Gubbins are studying how buckytubes adsorb one gas -- argon -- commonly used in adsorption experiments.
Maddox used the C90 at the Pittsburgh Supercomputing Center to simulate how 1,300 argon atoms interact with a buckytube 12 to 15 atoms in diameter, a simulation that required 60 hours of C90 computing. "At any given step," Maddox says, "you need to know how each atom is interacting with 1,000 to 2,000 other atoms. On first exposure, the argon atoms go in very easily, forming a first layer on the inside of the tube. Then, as you increase the pressure to force the gas into the tube, you get a sudden jump in the number of adsorbed atoms as a second layer starts to form. This happens again for several more layers in the larger buckytubes, until a saturation point is reached, when the tube is filled with condensed fluid."
The next step, says Gubbins, is to examine how gaseous mixtures interact with buckytubes: "We're starting to look at mixtures with the idea of finding optimum buckytube diameters for particular separations, but this will take some time. Also, they cannot be immediately verified by laboratory experiments; experimentalists still are trying to find ways to purify the buckytubes so they have only tubes of a particular size. Our work is to point out possible uses of these materials and to indicate what's possible once pure materials are available."
Because buckytubes were discovered just a few years ago, laboratory researchers have not yet examined their adsorption properties. "We hope to provoke some experimental work in this area," Maddox says, "and if this looks interesting on the computer, experimental groups will have some reason to look at it."
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