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Water, Water...

I always fancied the idea of polywater and what it might be able to do. But, then I also quite like the idea of using chemistry to convert lead into gold, a money tree and the magic porridge pot. While, polywater may have turned out to be a lost cause, chemists have for many years unearthed some quite bizarre properties from the liquid of life, writes David Bradley.


The discoveries about this elemental material continue to this day with a collaborative team from Japan and the US publishing results in Nature recently (30 November 2000) that show that water becomes a two-dimensional glass and shrinks under extreme pressure when cooled and confined.

waterwater.jpg - 21988 Bytes A simple yet marvellous material
To the ancients, water must have seemed such a simple yet marvellous material - primordial, straightforward, life-giving, ubiquitous and, to them, elemental. Indeed, until we began looking more closely at its physical properties and the underlying physical chemistry, the hydrogen bonds, polarity and such it remained that way.

Water is indeed a simple-seeming substance - a couple of hydrogen atoms stuck on an oxygen making a boomerang shape. Couldn't really have been any more uncomplicated, really, straight perhaps? But, water is not, as any high school science student would hopefully be able to tell you. Up to a point it expands when it is cooled below 4 Celsius. It expands just enough to make the perfect Scotch on the rocks and to have left the Titanic in the same predicament.

"Water is also rather unusual in that unlike most other materials it exists in all three standard states of matter - solid, liquid and gas."
Water is also rather unusual in that unlike most other materials it exists in all three standard states of matter - solid, liquid and gas - at temperatures that are not at all outside our everyday experience. The likes of carbon dioxide, common salt and egg white, just don't have that ability to flip between states within a 100 degree range. Add to that the fact that it is far more viscous than other similarly sized molecules, it can readily be converted into that increasingly familiar supercritical fluid state for which chemists are finding green applications at every turn.

The list goes on?unexpectedly high heat capacity, solubilising capacity, hydrating ability?

Microstructure of water
Much of water's anomalous behaviour boils down to the formation of hydrogen bonds between those dangling hydrogens on the boomerang tips and the oxygens on neighbouring molecules and the tiny clusters of water molecules that exist fleetingly in the liquid state but lost in the gas and frozen in the solid.

In 1992, I reported on work from Sydney Benson and Eleanor Siebert of the University of Southern California at Los Angeles for New Scientist (see New Scientist archives). They used experimental data for ice and for pairs of water molecules in the gas phase to construct a theoretical model of liquid water. They claimed that the microstructure of water could help explain many of
"Water still holds plenty of surprises for those scientists who keenly take to it."
water's unusual properties. Their model help them envisage transient cubes of water molecules held together briefly in groups of three or more - with their hydrogen bonds breaking and reforming some 500 billion times a second.

Where are the clues?
Later work provided further solid theoretical clues about water's hidden properties. David Clary and John Gregory of the chemistry department at University College London used quantum Monte Carlo methods to simulate millions of possible random configurations of water molecules and came up with a hexamer that would be plausible under Schrödinger's equation. While such theorising may ultimately lead to a way to predict the properties of water from first principles, since it is this molecular behaviour that gives rise to the bulk effects, water still holds plenty of surprises for those scientists who keenly take to it.

Xiao Cheng Zeng, Associate Professor of Chemistry at the University of Nebraska, Lincoln, working with Kenichiro Koga of Fukuoka University of Education and Hideki Tanaka of Okayama University in Japan have found that they can make water form a glass rather than ice crystals at -10 Celsius by confining it in a tiny slit just 1 nm across.

Amorphous structure of the water. Yellow dotted lines represent hydrogen-bonds, white circles hydrogen, and red circles oxygen. Adapted from Nature courtesy of Xiao Cheng Zeng.
Three years ago, Zeng and Koga who was at the time a postdoctoral fellow at UNL, and `ice expert Tanaka' were using computer modelling to look at the way water changes when it is put under extremes of pressure. The model showed that rather than expanding on freezing water it can contract if it is squeezed at 493 atmospheres at -40 Celsius between two hydrophobic plates held a nanometre apart. The model showed that water was freezing into ice crystals with a hexagonal structure where every water molecule is hydrogen bonded to its four nearest neighbours but rather than being in a three-dimensional lattice the crystals were planar. Zeng confesses that he figured Koga's model was simply incorrect, they were looking for water glass, or ice glass, and had stumbled across a new two-dimensional crystalline form instead. "We ran many, many trials for about six months," Zeng says, "but we found the water froze into crystals and shrank every time."

Koga, Zeng and Tanaka were actually hoping to find a mixture of pentagons, hexagons and heptagons in the molecular structures of the water and thought it would be fairly easy to reproduce in the laboratory. But, it has taken three more years to come up with the real thing.

Frustration was the answer
The trick that finally did it was to introduce `frustration' into the process. This simply involved holding the two hydrophobic plates immobile while the water was compressed and frozen. The effect was to totally inhibit the formation of a true crystal and force the water to form a glass instead. It worked.

Zeng says he has nicknamed the new form of ice `Nebraska' ice from the Otoe word for `flat water'. But, aside from an interesting addition to the list of water's bizarre behaviour is there likely to be any immediate applications? Zeng does not think so, his reward, he says, is the simple joy of discovery. "Water is such a fundamental substance that it deserves a lot of attention and we want to
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understand it from every aspect, from its nanoscale behaviour, from its molecular properties, and all the way up," he explains.

Maybe what we have learned so far about water is just the tip of the iceberg. Now, pass me that Scotch, with a touch of water?of course.

For subscribers to Nature the url to the full article is http://www.nature.com/cgi- taf/DynaPage.taf?file=/nature/journal/v408/n6812/full/408564a0_fs. html


David Bradley is a freelance science writer and editorial consultant
based in Cambridge, England.

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