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April 1, 2002
Volume 80, Number 13
CENEAR 80 13 p. 13
ISSN 0009-2347
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Simulations show how disordered water molecules get organized into ice


Researchers in japan have simulated the freezing of water at ambient conditions, capturing at the molecular level what occurs during the process.

8013-180ns_liquid 8013-256ns_nucleus 8013-320ns_ice
STEP BY STEP Liquid water (left) contains few long-lived hydrogen bonds (shown as tubes). A polyhedral structure formed from long-lived hydrogen bonds (middle) initiates crystalli-zation into the honeycomb-like structure of ice (right).

Although a ubiquitous phenomenon, crystal formation by water has eluded computer simulation for decades. Previous efforts have succeeded only with water constrained in defined geometries. Now, after six years of supercomputer time, chemists Iwao Ohmine, Masakazu Matsumoto, and Shinji Saito at Nagoya University have accomplished the feat for pure, unconfined water [Nature, 416, 409 (2002)].

"Their results open the way to understanding the complex kinetics of how water freezes at the microscopic level," Srikanth Sastry, a researcher at the Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore, India, writes in a Nature commentary. "Understanding the kinetics of such phase transformations is important in many areas, from designing new materials to determining protein structure by crystallography."

Ice has an ordered netted structure that looks like a honeycomb. Water molecules in bulk, however, form disordered three-dimensional hydrogen-bonded networks. "It is extremely hard to find a pathway from the disordered 3-D network to the honeycomb-like net," Ohmine says. The problem resembles that of protein folding, where a unique native structure arises from numerous denatured states, he explains.

The Japanese team calculated many so-called trajectories, which trace the evolution of molecular arrangements over microsecond timescales, to find one that leads to crystal formation. The data show that the freezing of water involves four stages.

In the quiescent period (time equals 0–256 nanoseconds for trajectories monitored over 400 ns), water molecules are in a supercooled liquid state. Hydrogen-bonded networks are broken and formed continuously, much like in ordinary liquid water. Freezing begins at the next stage (256–290 ns), as the potential energy slowly decreases. A polyhedral structure made of long-lasting hydrogen bonds then forms spontaneously, becoming the initial crystal nucleus.

In the third stage (290–320 ns), the potential energy drops rapidly. The nucleus expands quickly, transforming the hydrogen-bonded networks into six-membered rings. Finally (>320 ns), the potential energy stabilizes. The hydrogen-bonded networks stack like a honeycomb. Ice is formed.

The study suggests that large-scale density fluctuations that are intrinsic in liquid water are important in forming the initial nucleus, Ohmine says. Low-density regions facilitate not only the initial formation of the nucleus but also its subsequent rapid growth.

Water expands by about 10% when frozen to crystalline ice, damaging living cells, Ohmine notes. "It is of interest to find out whether amorphous ice may be obtained at ambient conditions instead of crystalline ice, if these large-scale density fluctuations are reduced," he says. "If so, the freezing techniques now used in science and industry could change dramatically."

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