Researchers Model Four Kinds Of One-Dimensional Ice

Working with large-scale computer simulations, a team of scientists that included Xiao Cheng Zeng, professor of chemistry at the University of Nebraska-Lincoln, has modeled four new kinds of crystalline ice, all by adjusting the diameter of a carbon nanotube by less than one-quarter of a nanometer (a nanometer is one-billionth of a meter).

A carbon nanotube can be viewed as a graphite sheet that has been rolled up to create a tube that can be as small as one-half a nanometer in diameter, Zeng said.

Zeng and his team, which included three other scientists, Kenichiro Koga of the Fukuoka University of Education in Japan, G.T. Gao of the U.S. Naval Academy (both of whom had been postdoctoral scholars for three years in his Nebraska laboratory) and Hideki Tanaka of Okayama University in Japan, studied the formation of quasi-one-dimensional ice crystals in carbon nanotubes in the range of 1.0 to 1.4 nanometers in diameter.

In results published in the Aug. 23 edition of Nature, the international weekly journal of science, they found that ice crystals formed a square structure in a 1.108-nanometer tube. As they increased the size of the tube's diameter, they found structural changes in the ice crystals at roughly .07-nanometer intervals to pentagonal, hexagonal and heptagonal crystals.

Zeng said the ice crystals in the tubes are "quasi-one-dimensional" because they are almost but not quite mathematically one-dimensional ( that is, a line with no width). The four new ice crystals that his team modeled follow the two-dimensional "Nebraska" ice that he, Koga and Tanaka modeled in 1997.

"If our study is confirmed experimentally, it would extend the crystalline ice family," Zeng said. "Right now, there are 13 types of (three-dimensional) ice that have been discovered in nature. We earlier reported the two-dimensional 'Nebraska' ice and this time we found four members in one-D. They all satisfy the 'ice rule' that every water molecule (except on the surface) forms four hydrogen bonds with its nearest neighbor water molecules."

If confirmed, Zeng said the discovery announced in this week's Nature could contribute to the study of molecular biological science. "It is known that water provides the 'glue' for the binding of protein molecules via the hydrophobic (water-repellant) attraction force," he said. "Our study of water in hydrophobic micropores is of fundamental importance to this because it will help us to gain deeper insights into the interactions between proteins."

Zeng said his team's investigation also indicated some new knowledge in the area of physics. In phase transitions from ice to liquid to vapor or vice versa in the 13 known types of three-dimensional water, scientists have never found a point beyond which there is no difference in structure between the liquid and solid states. He said there might be such a point at the one-dimensional level.

"We found some evidence from our simulation that maybe in quasi-one-D that beyond the critical point, there is no difference between liquid and solid," he said.

The research was funded by $800,000 in grants over the last six years from the National Science Foundation's Division of Electrical Communications Systems and the Office of Naval Research Physical Science and Technology Division, with matching funds from the Center for Materials and Analysis at NU. Zeng also credited one of his former Ph.D. students, Ruben D. Parra, an assistant professor of chemistry at De Paul University in Chicago, for important contributions through his quantum-mechanical calculations of the finite-size ice tubes.

This week's Nature paper was the second in less than a year for Zeng and his team, following the Nov. 30 publication of a paper on the formation of ice glass.

"We feel very fortunate and very rewarded," Zeng said. "My professional career started in 1980 and it took 20 years to see one Nature article, and this is the second one in one year. It's almost like I can compare this to the Husker players when they win a championship game. The joy is as good as that for us."