Image: UNIVERSITY OF NORTH CAROLINA, CHAPEL HILL

ICE consists of water molecules that have joined to form unique, hexagonal rings that, like little life rings, float on water.


Images: NEW YORK UNIVERSITY

WATER MOLECULES form hydrogen bonds when an oxygen atom (red) in one attracts a hydrogen atom (grey) in another (top). In liquid water, the molecules pack closely together (middle). In ice, however, they take on a less dense configuration (bottom).


That ice floats is a fact we all take for granted. Why ice floats is still a bit of a mystery. Indeed, water is one of very few substances that actually become less dense as they freeze. Scientists have long known that in nature, six frozen H2O molecules will join in a flat hexagonal ring that, like a little life ring, bobs on top of water. But when they have tried to re-create these rings in the lab, their efforts have failed: the molecules have instead assembled themselves into collapsed cages that readily sink.

Now, however, chemists have copied nature's curious trick--by resorting to a few of their own. Professor Roger Miller and graduate student Klaas Nauta of the University of North Carolina at Chapel Hill reported in the January 13 issue of Science that by using liquid helium to cool water molecules, they conned them into forming flat, floating rings.

"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," Miller says. Ultimately, he hopes to understand the hydrogen bonding that in nature similarly forces the molecules away from sinking configurations.

Unraveling such fundamentals as hydrogen bonding in ice is, in fact, a hot research topic. "Pharmaceutical companies have literally spent hundreds of millions of dollars on molecular modeling simulations," Miller notes. "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."