UNC-CH Scientists Create World's Smallest Pieces Of Ice

Don't look for them anytime soon clinking in your tea glass, however. The pieces they made consist only of six molecules of water in flat hexagonal rings, just as ice exists in nature.

The unusual achievement could go a long way toward boosting knowledge of water, that unique and fascinating substance without which no life could exist, the scientists say. A report on the research appears in the Jan. 13 issue of Science. Chemistry graduate student Klaas Nauta and Dr. Roger E. Miller, professor of chemistry, carried out the work and wrote the report.

"Despite the fact that water is so important to us, we still don't have a really good molecular level understanding of it," Miller said. "We can do pretty well with some other systems, but water does interesting things that make it unique and also make it somewhat difficult to understand."

For example, a property of water that is bizarre -- yet taken for granted -- is that unlike almost all other substances, it becomes less dense as it freezes, Miller said. As a result, ice floats instead of sinking. The more normal behavior is for solids to sink in their own liquid phases.

"We know that when it freezes, water forms a unique hexagonal ring structure, which accounts for its low density and the fact that it floats in water," Miller said. "Understanding the hydrogen bonding forces that align the water molecules in this way is our goal."

Doing this in bulk ice and water is complicated by the fact that there are so many molecules to keep track of, he said.

"After all, a single drop of water contains about 100,000,000,000,000,000,000 water molecules," the chemist said. "When we're trying to understand water at a detailed level, having so many molecules is a real problem."

The approach he and other scientists have taken over the past decade or so has been to make small clusters of water molecules. They take three or four molecules at a time and study them in pieces, applying what they learn to water in bulk.

"The difficulty has been that every time people took six water molecules and tried to make this hexagonal ring structure characteristic of ice, they ended up with a high-density, collapsed cage structure, which is not what ice does," Miller said. "The difference is that bulk ice has a three-dimensional structure that holds the molecules in position like a scaffold. The six water molecule lacks this scaffolding, and the water molecules collapse into a non-ice-like arrangement."

The new work involved developing methods that would allow researchers to force individual ice molecules into shapes they wouldn't normally assume on their own.

"We used a liquid helium method that tricks nature into selectively making ice rings," he said. "Basically, by growing ice at very low temperatures, we starve the molecules of the energy they need to rearrange themselves from the hexagonal shape we want and into the collapsed cage shape we don't want."

The basic premise of the work is to try to understand hydrogen bonding in its most fundamental form, namely water, the chemist said.

"Pharmaceutical companies have literally spent hundreds of millions of dollars on molecular modeling simulations," he said. "They are trying to use modeling to aid in the development of new and interesting chemical systems, including new drugs. An important part of modern drug design is trying to predict the properties of new drugs on a computer. That approach is only as good as our understanding of the basic interactions between molecules, the very subject we are addressing with our research."

The work potentially could have a huge impact on the pharmaceutical industry, for example, he said.

Miller and his graduate students now are developing new ways of studying water's interacting hydrogen bonds, which control much of biochemistry, he said. The next step will be to introduce small biological molecules into the system to learn how water interacts with them.

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Note: For a computer-generated image of the ice molecule, go to http://www.unc.edu/news/newsserv/pics/smallice.jpg.