We
drink it, swim in it, and our bodies are largely made of it. But as
ubiquitous as water is, there is much that science still doesn’t
understand about this life-sustaining substance.
For
example, unlike almost all other compounds, which typically shrink as
they get colder, water expands when it freezes — which is why ice floats
on water. Yet even the reasons for this unusual fundamental property
remain elusive.
Now
an MIT doctoral student and a team of researchers have carried out new
experiments supporting a controversial theory about water’s behavior
that could help explain some of its mysteries.
Their findings, recently published in the Proceedings of the National Academy of Sciences,
could have important implications for fields ranging from biology to
construction, the researchers say, because the behavior of water affects
so many important processes.
Water is “probably the most weird substance on Earth,” says Yang Zhang PhD ’10, lead author of the PNAS
paper, which was based on his doctoral thesis research. “It behaves
very differently from other materials,” he says, with scores of
anomalous characteristics. The work was done in collaboration with
Zhang’s doctoral supervisor, Sow-Hsin Chen, professor in MIT’s
Department of Nuclear Science and Engineering, and six other co-authors.
All
materials undergo phase transitions between the basic states of matter —
solid, liquid and gas. At these transitions, a material’s properties
can change significantly and suddenly. A theory proposed about two
decades ago explained some of water’s odd behavior by suggesting that a
similar transition may take place between two different liquid states,
in which the arrangement of the water molecules changes so that the two
states have very different densities.
The
new research, which probed water’s molecular structure under a wide
range of pressures and temperatures, provided some evidence for the
existence of this liquid-liquid transition, though the evidence falls
short of proof.
Evidence
for this posited transition has been very difficult to obtain because
it occurs only at temperatures and pressures at which water normally
could not exist in liquid form: For instance, the temperature at which
the liquid-liquid transition may occur lies far below the normal
freezing point, at about minus 60 degrees Celsius. So the researchers
had to find a clever way to get around that limitation.
One
key trick: the use of tiny tubes of silica, in which the molecules of
water were tightly confined so that they were unable to crystallize into
ice. This tight confinement made it possible to maintain water in
liquid form far below its normal freezing point.
With
the water molecules in this state, Zhang, now the Clifford G. Shull
Fellow at Oak Ridge National Laboratory, was able to probe their density
using a neutron beam from a reactor at the National Institute of
Standards and Technology. In the experiments, he gradually varied the
pressure from normal sea-level atmospheric pressure (or 1 bar) up to
about 3,000 times that amount, and varied the temperature over a range
of 170 degrees Celsius. He found a difference in water’s density by
approaching the expected transition temperature from opposite
directions, as predicted by the theory.
Pablo
Debenedetti, a professor of engineering and applied science at
Princeton University who was not involved in this research, says “these
are beautiful experiments” that address “one of the most interesting
open questions on the liquid state of matter, and in particular on
water: the possible existence of a phase transition between two distinct
phases of liquid water.”
While
the experiments support the theory, he says, interpretation is
complicated because confined water might behave differently from water
in bulk. “The theoretical tools needed to unambiguously relate
observations in nanoscale confinement to the behavior of bulk water are
not available at present,” he says.
“Supercooled”
water that remains liquid below the normal freezing point is relatively
easy to produce; Zhang even filmed a short demonstration using an
ordinary water bottle cooled in the refrigerator. Water can also be
“superheated” in a microwave oven to well above the boiling point,
flashing to a boil all at once only when it is disturbed in some way.
(In both freezing and boiling, water usually needs a nucleation point,
such as a bubble or a ripple, to trigger the change of phase.)
Because
water is key to so many aspects of people’s lives, these phenomena
could have important consequences. For example, Chen delivered a keynote
speech this July, at a conference on low-temperature agriculture, on
the possible impact of these supercooled states on plant life. He
believes the fact that living organisms apparently cannot be revived
after being subjected to temperatures below about minus 45 C is
explained by water’s transition to a lower-density state that prevents
proteins, the molecules on which living organisms are built, from
functioning.
This
density difference could also affect construction, because concrete
contains tiny amounts of water that can cause buildings and roads in
polar regions to suffer serious cracking when temperatures plunge below
minus 45 C. If the theory is correct, this critical temperature could
set a fundamental limitation for both organisms and concrete buildings.
“The
building blocks of our bodies and the building blocks of our society,”
Zhang says, “both have a lower limit of temperature that is based on the
properties of water.” But by understanding those limits, he says, it
might be possible to alter the water — for example, by dissolving
certain chemicals in it — to change the transition points and lower that
limit.
Density hysteresis of heavy water confined in a nanoporous silica matrix
