Encounter leads to new ice discoveries by UNL team

Sometimes in science, new research pathways are generated by unexpected suggestions.

That's what led Xiao Cheng Zeng and his research group at the University of Nebraska-Lincoln to their latest series of discoveries about the behavior of materials -- especially water -- at extremes of temperature, pressure and confinement.

Zeng gave a talk in 2008 to the Materials Research Society in San Francisco about some of his lab's earlier discoveries, including the two-dimensional, high-density "Nebraska ice" that he named for its flatness. After the talk, C. Austen Angell, professor of chemistry and biochemistry at Arizona State University, approached Zeng and suggested that it would be possible to have a two-dimensional ice clathrate with the "Nebraska ice." A clathrate is essentially a molecular cage, usually three-dimensional in nature, in which molecules of one substance are completely enclosed in the crystal structure of another.

The suggestion got Zeng's attention, especially since Angell is regarded as one of the world's foremost experts in liquid physics. When he returned to Lincoln, Zeng asked Jaeil Bai, research technologist in his lab, to create a computer simulation to test the idea. Bai's experiment led to a series of discoveries that were reported in the March 15-19 online edition of the Proceedings of the National Academy of Sciences. It's the seventh time in less than nine years that research from Zeng's lab has been published in one of the three major international multidisciplinary journals (Nature, Science and PNAS).

"That idea by Angell was realized by Jaeil in the computer simulation," said Zeng, Ameritas university professor of chemistry. "It's a very flat, single-layer ice clathrate. We call it a 'square-octagon ice clathrate' and we may nickname it 'Nebraska ice clathrate' because it's very flat."

As the two-dimensional ice formed under negative pressure, it developed into an interlocking pattern of small square-shaped openings and larger octagonal-shaped openings similar to the wallpaper pattern called Archimedean four-eight tiling. Hydrophobic argon atoms used in the experiment filled the octagonal "cages" in the ice clathrate [movie] [image].

Bai next tried the experiment without the argon atoms, again under negative pressure, in a hydrophobic slit pore about 0.6 nanometers wide (a nanometer is 1 billionth of a meter). He wanted to see if he could form the ice alone, without the argon as a "guest." The result was the discovery of a "guest-free" monolayer ice that forms spontaneously and remains stable [movie]. Zeng said his friend, professor E.G. Wang, dean of physical sciences at Beijing University, found the same pattern in 2004, but needed a silica template. "We were able to see for the first time a spontaneous formation of this ice," Zeng said.

A third discovery from the experiment was the coexistence of low- and high-density, two-dimensional ices. The low-density ice behaves like Ice I, the everyday kind of ice we find in our ice-cube trays at home. Just like regular ice, if the low-density, two-dimensional ice is subjected to increased pressure, it becomes liquid. Under even more pressure, it becomes the high-density ice, thus going from solid to liquid to solid. Putting almost any other solid under higher pressure forms only a denser solid.

"Ultimately, we want to understand the formation of three-dimensional ice clathrates on the molecular scale," Zeng said. "That could help scientists and engineers learn how to tap into the vast methane hydrates mostly trapped under the ocean floor."

Zeng said the Department of Energy estimates that the methane clathrates could provide some 20,000 terawatt-years of energy, compared to the 1,000 terawatt-years estimated to remain in conventional oil and gas. As a point of reference, global energy use in 2008 was about 15 terawatt-years. In addition, the undersea ice clathrates could be a site to store carbon dioxide, reducing the threat of global warming.

The research was conducted through the University of Nebraska's Holland Computing Center and supported by funding from the Department of Energy and the Nebraska Research Initiative.

WRITER: Tom Simons, University Communications, (402) 472-8514