Buckyball additives improve lubrication.

Molecules shaped like spheres conjure up images of microscopic ball bearings that can reduce friction between tiny moving components.

When researchers discovered the buckminsterfullerene molecule in 1985, they thought they had found the ideal candidate for such a role. Consisting of 60 carbon atoms, this molecule is a nearly perfect sphere. In the crystalline solid state, it spins at a rate dependent on temperature, while remaining in its position in the crystal lattice.

Those hopes were dashed when subsequent experiments demonstrated that buckyballs are not particularly good lubricants. Now, a team of chemical engineers and chemists has discovered that C60 molecules dissolved in the organic solvent toluene greatly reduce the friction between the liquid and the surface across which it flows.

This finding suggests that the addition of buckyballs to conventional lubricating fluids may enhance their performance. Jacob N. Israelachvili, Fred Wudl, and their coworkers at the University of California, Santa Barbara report their results in the Aug. 8 Nature.

The researchers measured oscillating, attractive forces between a pair of smooth, transparent sheets of mica immersed in toluene as the distance between the surfaces was varied. When the liquid contained small quantities of C60 molecules, the forces between the mica plates changed in a manner suggesting that thin layers of buckminsterfullerene molecules had settled on the mica surfaces. The molecular spheres allowed the liquid to pass easily between the mica sheets, "giving rise to flow behavior that is totally different from conventional fluid flow through narrow pores," the researchers note.

A possible explanation of this result is that the adsorbed layer of molecules is only weakly bound to the surface and, as in the crystalline solid, the buckyballs may be rotating rapidly, allowing toluene molecules to slide by easily.

In contrast, measurements of the forces between a pair of solid layers of C60 in the absence of a liquid confirm that the molecules by themselves offer no significant reduction in friction. "The results . . . indicate that C60, although not a good lubricant, shows great promise as an additive," Israelachvili and his colleagues conclude.