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�The conventional wisdom is that water is unique,� said Debenedetti,
the Class of 1950 Professor in Engineering and Applied Science. �And
here we have a very simple model that displays behaviors that are very
hard to get in anything but water. It forces you to rethink what is
unique about water.�
While their water imitator is hypothetical - it was created with
computer software that is commonly used for simulating interactions
between molecules - the researchers� discovery may ultimately have
implications for industrial or pharmaceutical research. �I would be
very interested to see if experimentalists could create colloids (small
particles suspended in liquid) that exhibit the water-like properties
we observed in our simulations,� Debenedetti said. Such laboratory
creations might be useful in controlling the self-assembly of complex
biomolecules or detergents and other surfactants.
More fundamentally, the research raises questions about why oil and
water don�t mix, because the simulated molecule repels oil as water
does, but without the delicate interactions between hydrogen and
oxygen that are thought to give water much of its special behavior.
The researchers published their findings Dec. 12,
2007, in the Proceedings of the National Academy of Sciences.
The team also included lead author Sergey V. Buldyrev of Yeshiva
University, Pradeep Kumar and H. Eugene Stanley of Boston University,
and Peter J. Rossky of the University of Texas. The research was
funded by the National Science Foundation through a grant shared by
Debenedetti, Rossky and Stanley.
The discovery builds on an earlier advance by the same researchers. It
had previously been shown that simple molecules can show some
water-like features. In 2006, the collaborators published a paper
showing that they could induce water-like peculiarities by adjusting
the distance at which pairs of particles start to repel each other.
Like water, their simulated substance expanded when cooled and became
more slippery when pressurized. That finding led them to investigate
more closely. They decided to look at how their simulated molecule
acts as a solvent - that is, how it behaves when other materials are
dissolved into it - because water�s behavior as a solvent is also
unique.
In their current paper, they simulated the introduction of oily
materials into their imitator and showed that it had the same
oil-water repulsion as real water across a range of temperatures. They
also simulated dissolving oily polymers into their substance and,
again, found water-like behavior. In particular, the polymers swelled
not only when the �water� was heated, but also when it was
super-cooled, which is one defining characteristic of real water.
Proteins with oily interiors also behave in this way.
In real water, these special behaviors are thought to arise from
water�s structure -- two hydrogen atoms attached to an oxygen atom.
The arrangement of electrical charges causes water molecules to twist
and stick to each other in complex ways.
To create their simulation, the researchers ignored these complexities.
They specified just two properties: the distance at which two
converging particles start to repel each other and the distance at
which they actually collide like billiard balls. Their particles could
be made of anything -- plastic beads, for example -- and so long as
the ratio between these two distances was correct (7:4), then they
would display many of the same characteristics as water.
�This model is so simple it is almost a caricature,� Debenedetti said.
�And yet it has these very special properties. To show that you can
have oil-water repulsion without hydrogen bonds is quite interesting.�
Debenedetti noted that their particles differ from water in key
aspects. When it freezes, for example, the crystals do not look
anything like ice. For that reason, the research should not be viewed
as leading toward a �water substitute.�
As a next step, Debenedetti said he would like to see if
experimentalists could create particles that have the same simple
specifications as their model and see if their behavior matches the
computer simulation.
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