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Computing liquid water as a transient gel.

Computing liquid water as a transient gel

Water, the most common liquid on Earth, is also among the least typical and most puzzling of liquids. Many of its key properties, such as the way its density depends on temperature, differ sharply from those of other liquids. These peculiarities become even more pronounced when water is carefully cooled to temperatures below its normal freezing point.

Scientists still don't know exactly how the motion of water molecules and the formation of hydrogen bonds -- each bond representing the attractive force between a hydrogen atom in one water molecule and the oxygen atom of another -- add up to such anomalous liquid behavior. But a team of researchers, using computer simulations of water molecules in motion, has now found a remarkable pattern of hydrogen-bond formation and dissolution that may eventually lead to an explanation for water's many strange characteristics.

"Our long-term goal is to elucidate the dynamics of water," says H. Eugene Stanley of Boston University. "This [study] is an important first step because it focuses on the bond network itself." Stanley, Francesco Sciortino, Peter H. Poole and Shlomo Havlin report their findings in the April 2 PHYSICAL REVIEW LETTERS.

The researchers picture liquid water as a transient gel consisting of a random network of hydrogen bonds linking water molecules over large distances. Because the hydrogen bonds break and reform on a picosecond time scale, the network restructures itself rapidly. But at any given time, enough bonds remain intact to keep it from falling apart.

To find out how the hydrogen-bond network in water evolves, the team computed the motion of 216 water molecules in a cubic box at five different temperatures, keeping track of how long individual hydrogen bonds last. And to get a better grip on the effects of molecular movements on the hydrogen-bond network, they adopted an unconventional definition of what constitutes a hydrogen bond.

"We take into account all the bonds," Sciortino says. "If there is a little bit of an interaction, this is already a bond for us." Normally, scientists count as hydrogen bonds only those representing an attractive force higher than a certain cutoff value.

An analysis of the simulation results shows that for this particular model, the distribution of bond lifetimes fits a mathematical relationship known as a power law. That startling finding suggests that hydrogen bonds in water don't have a characteristic lifetime; instead, some last a short time and others a very long time.

This seems to contradict what many researchers have long believed. If the energy required to make or break a hydrogen bond has a particular value, there should be a characteristic time for a bond to break, they say.

The group also found that the average bond lifetimes increase rapidly as the temperature declines, especially at temperatures below the normal freezing point. Other researchers have seen similar power-law behavior in experimental studies of temperature-dependent properties such as the rate of diffusion in water.

Trends in both the experimental results and the computer simulations suggest that liquid water may undergo some kind of transformation, perhaps to a glassy state, at roughly -46[degrees]C. However, no one has yet succeeded in cooling liquid water to such a low temperature without ending up with solid ice.

"Our ... results may provide a microscopic interpretation for the experimental evidence that suggests a region of power-law behavior in the dynamic properties of liquid water," the Boston scientists conclude. Computations based on alternative molecular models of liquid water produce similar results.
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Author:Peterson, I.
Publication:Science News
Date:Apr 14, 1990
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