Why water is
weird
By Gary
Taubes
Red
Herring
March 26
This article is from the March 20, 2001, issue of
Red Herring magazine.
Water -- heavier when chilled, lighter when frozen,
absorbing enormous heat with but a slight rise in temperature,
the foundation of life, the most common of liquids, and the
strangest. If only we knew how it worked.
"We still don't quantitatively understand the physics of
liquid water," says Richard Saykally, a world-renowned chemist
at the University of California at Berkeley. As a result, our
best computer models don't simulate reactions in water, like
the folding of a protein or the docking of a hormone and its
target cell -- each a Holy Grail of biotech. Solving the
mysteries of water would do for chemicals and pharmaceuticals
what the wind tunnel did for aerospace: substitute fast, cheap
calculations for slow, costly experiments. "We're talking
about billions of dollars saved here," Mr. Saykally says.
Experimentalists like Mr. Saykally study how water actually
behaves, at temperatures ranging from just above absolute zero
to a few hundred degrees above its boiling point. Theorists,
meanwhile, attempt to hone their computer simulations to match
more closely the experimental observations. As increasingly
powerful computers bring the two approaches into somewhat
better agreement, scientists are learning that water is even
weirder than they had thought.
LIQUID MYSTERY
Take virtually any liquid --
molten iron, for instance -- and freeze part of it into a
solid; the solid will sink to the bottom. But ice floats, and
the question is why. Indeed, water reaches peak density at 4
degrees Celsius, or around 40 degrees Fahrenheit.
"In school," says H. Eugene Stanley, a physicist at Boston
University, "we learn that if water is in equilibrium with
ice, the temperature must be zero Celsius. That's not true.
The water at the bottom is not zero, but four Celsius, and the
reason is that below four Celsius, the water starts becoming
lighter, so the heavier four-Celsius water sinks to the bottom
of the glass and just stays there."
Mr. Stanley describes this as the most remarkable of the
"magical properties" of water, although there are plenty of
others. There are, for instance, 5 different kinds, or phases,
of liquid water, not to be confused with the 12 to 14
different phases of ice. Ice forms a crystal lattice, and each
phase has its own structure. As a crystal, ice is as different
from water as diamonds are from pencil lead. You can, for
example, supercool water so that rather than freezing at 0
degrees Celsius, as it prefers to do, it will stay a liquid
down to roughly -38 degrees Celsius. Water typically won't
freeze without some impurities around which its molecules can
begin to coalesce. For this reason, researchers who study
supercooled water do so with the purest water they can
get.
At -38 degrees Celsius, however, even the purest water
spontaneously turns to ice. When that happens, "it does so
with an audible bang, like a little bomb," says Austen Angell,
a University of Arizona chemist who holds the world record for
supercooling water. From -38 to -120 degrees Celsius, it's ice
all the way, a temperature regime that Mr. Stanley calls
"no-man's land," by which he means "no liquid." But below -120
degrees Celsius, it's possible to make what's known as
ultraviscous water, a liquid as thick as molasses. Below -135
degrees Celsius comes glassy water, a solid having no crystal
structure.
Most of water's strange properties stem from the peculiar
bonds formed between neighboring H2O molecules. The
bonds are formed by the two hydrogen atoms, which stick out
from the oxygen at an angle of exactly 106 degrees -- "Mickey
Mouse ears," Mr. Stanley calls them, "with the two positive
hydrogen atoms as the ears, and two little feet sticking out,
which are the negatively charged pairs."
The bond angle doesn't allow water molecules to bind
ears-to-feet. Instead, the left ear of one molecule goes to
the foot of a second, and the right ear goes to the foot of a
third. At any given moment, only a few water molecules are
likely to be bound at both ears and both feet. Others will
have only three bonds, and still others only two.
The result is hard to simulate because you can't treat
every water molecule as identical. Nor can you portray them as
spheres, with perfect symmetry that would cut back on the
number of spatial relationships, considerably easing the
calculating load. Moreover, the electromagnetic forces between
Mickey's ears and feet have a relatively long range, so you
have to take into account not merely neighboring molecules but
those farther apart as well.
TESTING THE WATERS
As computing power has grown
exponentially over the years and modeling techniques have
improved, so have simulations, which can now do a reasonable
job of modeling a few thousand water molecules at a time. The
models explain the four-degree temperature anomaly and some
other conundrums just as Mr. Stanley did in his suggestion 20
years ago -- at any one time, the water molecules are engaged
in the largest possible number of "good" hydrogen bonds. In
ice, for instance, the hydrogen bond network is fully engaged,
with each Mickey Mouse molecule locked onto its neighbors by
two ears and two feet and occupying its maximum volume.
In water, because one or more of the hydrogen bonds is
always broken, the molecules can move a little closer than
they can in ice, allowing them more ways to arrange
themselves. Lower the temperature, and you get "a little bit
of a solid phase inside the liquid phase, and, as you lower
the temperature further, you get more and more of these little
bits of ice forming," Mr. Stanley says, like "plums in the
plum pudding."
But does this transition between phases happen in reality
or just in the computer? The ultimate test of a model is
whether it predicts a phenomenon that experimentalists have
yet to discover. In the case of Mr. Stanley's water
simulation, this happened in 1992, when he and two
collaborating physicists, Peter Poole of the University of
Western Ontario and Francesco Sciortino of the University of
Roma La Sapienta, noticed a coalescing of the plums in the
plum pudding at roughly -50 degrees Celsius. The water seemed
to be separating into a less dense phase of highly bonded
water and a denser phase of less well-bonded water -- a kind
of liquid water never before seen.
The proposition was, and still is, controversial. Indirect
evidence is mounting, but direct evidence is hard to come by.
"Heat capacity, compressibility -- quite a lot of the
properties of water measured in that region show this type of
divergence," says Mr. Saykally. "That's the standard hallmark
of a phase transition near a critical point in the
neighborhood."
The only direct experimental evidence of the phenomenon
comes from Osamu Mishima of Japan's National Institute for
Research in Inorganic Materials. In 1994, Mr. Mishima
demonstrated that glassy water has high- and low-density
phases and a transition from the former to the latter that Mr.
Stanley says "pops like popcorn" as the glassy water expands.
More recently, Mr. Mishima and Mr. Stanley have plotted the
melting temperature against the pressure of superpure water
and discovered kinks in the resulting curves -- kinks that are
consistent with transition to a new form of liquid water.
Meanwhile, Mr. Saykally wants to infer Niagara Falls from a
drop of water by fully calculating the behavior first of two
water molecules, then three, four, and onward. He hopes to end
up with a water model that is demonstrably better than that of
Mr. Stanley or, for that matter, anyone else. "It should be
able to do everything," Mr. Saykally says, "to calculate any
properties whatsoever of liquid water more accurately than
they've ever been described before."
Gary Taubes is a freelance writer living in Venice,
California. Write to letters@redherring.com.
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