Using molecular modeling, University of Nebraska chemistry professor Xiao Cheng Zeng recently found that double helixes of ice molecules self-assemble under high pressure inside carbon nanotubes, and that they resemble the structure of DNA. This discovery could have wide-ranging utility. It may be useful to scientists looking for ways to direct self-assembly in nanomaterials. It may also help predict the kind of ice future astronauts will find on Mars and moons in the solar system. And it could have major implications for scientists who study the protein structures that cause diseases such as Alzheimer's and bovine spongiform encephalitis (mad cow disease).

Zeng and his colleagues use powerful computers to model how materials behave at the nanoscale under extremes of temperature, pressure, and confinement. The team found the self-assembling double helix of nano-ice following a months-long experiment on UNL's PrairieFire supercomputer.

The experiment followed up on a 2001 discovery, also made through computer modeling, of four new kinds of one-dimensional ice inside carbon nanotubes. Scientists elsewhere later confirmed, through laboratory experiments, the existence of three of the new nano-ices.

One result in particular intrigued Zeng. Scientists at Argonne National Laboratory confirmed the existence of a chain of octagon-shaped ice crystals inside a 1.4-nanometer carbon tube, just as Zeng and company expected. But the Argonne group also found an additional, unexpected chain of water molecules inside the octagon.

"Maybe the helix is a way for molecules to arrange themselves in a very compact, efficient way under high pressure."

Zeng said that report inspired his team to take another look at one-dimensional ice, this time with a PrairieFire supercomputer that was 20 times more powerful that it had been five years earlier. The 2001 results were achieved at atmospheric pressures, but PrairieFire's added processing power enabled Zeng to design simulations that greatly increased the pressure on the water molecules.

"We were shocked to see these molecules arrange themselves in this way," said Zeng. "We thought it would be like two tubes, one inside the other, but it didn't do that. It was helical, like DNA. I'm just speculating, but maybe the helix is a way for molecules to arrange themselves in a very compact, efficient way under high pressure.

"This ice formation can be viewed as a self-assembling process, and self-assembly is a way for molecules to bond together through weak hydrogen bonds. One example of a self-assembling material is protein. Proteins can self-assemble into unwanted structures like amyloid fibrils that can build up in the brain to cause Alzheimer's disease or prions that cause mad cow disease."

"This ice formation can be viewed as a self-assembling process, and self-assembly is a way for molecules to bond together through weak hydrogen bonds."

Another implication, Zeng said, is that these self-assembling helical ice structures may give scientists and engineers a different way to think about weak molecular bonds and the self-assembly process as they try to develop ways to direct self-assembly in making new materials. He said that while scientists have a good understanding of covalent bonds (the strong type of bonding where atoms share electrons), knowledge is not as complete about the weak bonds, such as hydrogen bonds, that are essential to the self-assembly process. In weak bonding, atoms don't share electrons.

"We're happy to see potential applications that can maybe advance some fundamental science," Zeng said. "We're not engineers in direct contact with technology, but if our research can make some contribution, we're happy."

Zeng and colleagues obtained their results by running their simulations at Earth-like temperatures ranging from -9 to 117 degrees Fahrenheit, but at un-Earth-like pressures ranging from 10 to 40,000 atmospheres.

Most of the simulations produced the expected tubular structures. But in a simulation at -9 degrees F. and 40,000 atmospheres, the ice transformed into a braid of double helix. In another simulation at the same temperature, the pressure was instantly raised from 10 atmospheres to 8,000; and the confined liquid froze spontaneously into a high-density, triple-walled helical structure.

This research was funded by the Department of Energy, the National Science Foundation, the Nebraska Research Initiative and the John Simon Guggenheim Foundation. The findings were reported in the Dec. 11 edition of the Proceedings of the National Academy of Sciences.