Molecules with minds of their own

17 Jun 2002

Laser technology probes the mysteries of single-molecule motion in super-cooled liquids.

Understanding the nature of glass transition has plagued scientists for more than half a century. However, researchers from the University of Texas, Austin, US believe they have solved the long-standing controversy of whether or not molecules in super-cooled systems behave like those in normal liquids. By using single-molecule microscopy, Laura Deschenes and David Vanden Bout followed the motion of individual molecules within a dye that was held just above its glass transition temperature, see Science 292, 255-258, (2001).

The researchers separated molecules of Rhodamine 6G by diluting it into a nanometer-thick film of poly(methyacrylate). They then scanned the film's surface with a focused laser to see how each molecule rotated. "If you look at all of the molecules at the same time, you can't tell the difference between their movements," explained Vanden Bout. "But [this technique] allows you to study individual molecular environments one at a time to avoid the group averaging that masks these dynamics in bulk studies."

The study showed that the single molecules in the dye exhibited unusual behavior. "Unlike molecules in a regular liquid, the rotations of the individual molecules were different from each other," said Vanden Bout. "Some of the molecules moved quickly while other moved slowly."

The researchers noticed that the super-cooled liquid molecules were grouped into a number of small domains. "[Molecular motion] within each domain looked like that in a normal liquid," said Vanden Bout. "But the domains moved at different rates." The rate of molecular rotation within the domains also changed: domains with a fast molecular rotation would suddenly rotate more slowly. "The exchange times [between rotation rates] showed a distinct temperature dependence," the researchers reported. "As the temperature decreases, the exchange times grew longer."

Although the researchers have unravelled many of the secrets clouding molecular motion in super-cooled liquids, they acknowledge that all is not yet clear. "Many questions remain," they said, "such as the relationship between exchange lifetimes and the onset of glass transition".