Water, despite its overwhelming importance to all life, remains deeply mysterious. Unlike other liq uids, it expands as it cools, moves more freely as it is squeezed, and exhibits a host of other odd behaviors that have eluded quantitative explanation for centuries.

Pablo Debenedetti, professor and chairman of chemical engineering, and Jeffrey Errington, a postdoctoral scholar, have shown how these anomalies arise from water's propensity for organization and structure.

Their research, reported in the Jan. 18, 2001, issue of Nature (vol. 409, p. 300), may yield insights into the way water participates in many biological, chemical, and geological processes.

Photo by Denise Applewhite

Work is planned, for example, to apply the findings to understanding how water structures itself around different kinds of sugars used for the commercial preservation of proteins and vaccines. The technique also may offer a new approach to studying anomalous properties in other materials, including silicon, which shares some of water's quirks.

"I consider this work a major advance," said Eugene Stanley, professor of physics at Boston University and an authority on the anomalous properties of water. "They linked ideas that no one had ever dreamed were related."

Professors Debenedetti and Jeffrey made the advance by developing a system for measuring structural order among water molecules and observing how these measurements changed in different situations.

Instead of trying to measure structure in real water, they wrote computer software that simulates the interactions of many water molecules. They simulated changes in temperature and pressure, and observed whether the water became more or less structured.

In most liquids, the molecules move about randomly and tend to become slightly more structured as pressure increases This backward response occurred only in a certain range of pressures and at low temperatures. And something else odd happened in the same temperature and pressure range "The idea that order brings anomalies is a very interesting concept," Professor Debenedetti said. "We now believe we have put hard numbers into this. We have found how structured water needs to be to have strange properties."

The engineers looked in particular at two anomalies. First, water moves more freely, or diffuses faster, when the pressure around it is increased, unlike other liquids, which tend to become more stationary under pressure.

Second, at low temperatures, water expands when it cools, unlike virtually all other liquids, which shrink when cooled. In their simulation, they observed that the first anomaly happened when water reached a certain level of order, and the second became apparent only at an even higher level of order.

They concluded that seemingly unrelated anomalies are really part of a continuum that develops as water gains structural order.

"Another interesting question," Professor Debenedetti said, "is how our new measures of structural order relate to entropy (which measures water's disorder). The relationship is what we expect only in some ranges of temperature and pressure, and we don't know why."

Writing computer software that simulates the interactions of water molecules proved much more effective than working with the real thing for Jeffrey Errington, left, and Pablo Debenedetti.

This story first appeared in the Feb. 19, 2001, issue of the Princeton Weekly Bulletin and is reprinted here with permission.