'Pouring' over mystery

Scientists use computer simulation to explain water's odd behaviors


Steven Schultz

 

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.


Princeton, N.J. -- Water, despite its overwhelming importance to all life, remains deeply mysterious. Unlike other liquids, 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.

Princeton scientists Pablo Debenedetti, a professor of chemical engineering, and Jeffrey Errington, a post-doctoral scholar, now have shown how these anomalies arise from water's propensity for organization and structure.

Their research, reported in the Jan. 18 issue of Nature, may yield insights into the way water participates in many biological, chemical and geological processes. Work already 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. "What they did was link ideas that no one had ever dreamed were related."

Debenedetti and Errington made the advance by developing a system for measuring structural order among water molecules and observing how these measurements changed in different situations. Their method for measuring structural order is an extension of ideas developed recently by Thomas Truskett, who received a 2001 Ph.D. in chemical engineering, Salvatore Torquato, professor of chemistry, and Debenedetti.

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, such as gasoline or antifreeze, the molecules move about randomly and tend to become slightly more structured as pressure increases -- like balloons being packed into a room. Water, however, begins with a natural propensity to form minute and fleeting patterns among its molecules. And when Debenedetti and Errington simulated a pressure increase on water, they found, paradoxically, that this natural order diminished and the molecules became more randomly spaced, even though they were packed more closely together.

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 -- as the researchers backed off on the pressure, letting the water assume more order, it began to exhibit more of the anomalous properties that have mystified scientists. That is, the more ordered it became, the more unusually it behaved.

"The idea that order brings anomalies is a very interesting concept," said Debenedetti. "We now believe we have put hard numbers into this. We have found how structured water needs to be to have strange properties."

Debenedetti and Errington 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," said Debenedetti, "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."

Errington said the next step will be to continue mapping the known anomalies onto this newly found continuum of degrees of order. Another project will be to apply the findings to a more complex liquid, such as water with sugars dissolved in it. Such liquids are of critical importance in the pharmaceutical industry, for example, in processes designed to protect proteins from damage during storage.

"Perhaps we can identify why some sugars are better than others in protecting proteins," said Errington.


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