Understanding the structure of liquid water
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Physicists of antiquity called it one of nature's fundamental elements; third graders know its chemical formula; and all known forms of life need it to exist. Yet what water really is--at least in its liquid form--is still, to a large extent, a mystery. A team led by scientists from SSRL and Stockholm University has now achieved a breakthrough in understanding the structure of liquid water. They found that water molecules clump together much more loosely than previously thought.
Their findings appeared in Science magazine's advance publication Web site on April 1. "The results overturn 20 years of research in the physical chemistry of water," says Anders Nilsson (ESRD), the team leader. "It's going to be a big shock in the whole field," he says.
As its H2O formula suggests, each water molecule is made of two atoms of hydrogen and one of oxygen. Water molecules tend to stick together in what chemists call hydrogen bonds. The oxygen can form two hydrogen bonds, so a molecule can link with up to four others--with two links through its oxygen atom and one through each of its hydrogens.
In ice, each molecule forms four stable bonds, while as a liquid, water bonds form and break a trillion times per second. The ephemeral patterns formed by bonding in the liquid are still far from being understood, but are thought to be responsible for the peculiar properties of water, including its relatively high boiling point, its high viscosity, and--last, but not least--its ability to sustain the chemical reactions inside a living cell.
The consensus among researchers has been that, at any given time, a molecule of water typically forms three or four hydrogen bonds--3.5 on average. "What we find," says Uwe Bergmann (ESRD), "is that there's not 3.5 hydrogen bonds, but only 2." Each molecule could still form up to four bonds, the research suggests, but two would be of different, much looser kinds.
The earlier 3.5 estimate was based on theoretical assumptions that became commonly accepted because computer simulations gave results consistent with known properties of water. "Nobody had anything to object to the prevailing model, so it became the truth," says Nilsson.
The difficulty of 'seeing' the actual molecules in action meant a dearth of real data. "There has not really been new experimental information about water in the last 20 years, except for neutrons," says Nilsson. "The amazing thing," he says, "is that hardly anything is known about the microscopic origin for the unique properties of liquid water."
The new result now reopens the hunt for the structure of liquid water. "It resurrects models that were considered inappropriate," says Bergmann. One possibility, he suggests, is that water molecules could arrange in chains or even in closed rings. Eventually, the outcome could mean a better understanding of the chemistry of the cell, which is notoriously hard to imitate using different liquids. "Nobody has a clear answer to why water is essential for life," says Nilsson.
The research was the first to apply a technique called x-ray absorption spectroscopy to the local structure of water. The technique, developed by SSRL along with other research laboratories, bombards a material with x-rays that are finely tuned to excite particular electrons in a molecule's structure. Careful measurement of the scattered radiation reveals the motions of the excited electrons which, in turn, reveal what bonds molecules are forming. While SPEAR was being upgraded, the experiments used intense x-ray sources at Argonne and LBNL.
The team is now working on several projects to extend their results. "We want to study water in a whole range of pressures and temperatures," says Bergmann. "We propose to build a new facility at SPEAR3 where the structure of water would be a large part of the scientific drive," Bergmann says.
In addition to Nilsson and Bergmann, the other scientists from SLAC included in the five year long collaboration are Philippe Wernet (first author of the paper, now at the BESSY Laboratory in Berlin), Hirohito Ogasawara (ESRD) and Lars Naslund (Stockholm University).
By Davide Castelvecchi