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