Long-Standing Quandary About Water Solved
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 precise, quantitative explanation for centuries.
Princeton scientists have now shown how these anomalies arise from water's propensity for organization and structure.
The 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 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 H. 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."
The work was done by Pablo Debenedetti, a professor of chemical engineering, and Jeffrey Errington, a post-doctoral scholar. The two developed a system for measuring structural order among water molecules and used a computer simulation to show how these measurements can be used to predict the occurrence of at least two of the most commonly observed anomalies in water.
In most liquids, 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. They developed a way of measuring this structural randomness.
This odd loss of order occurred only in a certain range of temperatures and pressures - the same low temperature range in which water's other anomalous characteristics become apparent. Debenedetti and Errington concluded that the two are linked: the level of structural order determines whether water displays it anomalous properties. Indeed, as the level of order changes, different anomalies can be observed. The researchers now believe that seemingly independent anomalies - such as the odd way water expands when it is cooled and the unusual way in which its molecules diffuse faster under pressure - are part of a continuum of characteristics that develop as water assumes progressive degrees of structural order.
"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."
Errington said the next step will be to continue mapping the known anomalies onto this newly found continuum 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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