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"Water-surface interactions are ubiquitous in
nature and play an important role in many technological applications
such as catalysis and corrosion," said Greg Kimmel, staff scientist at
the Department of Energy lab and lead author of a paper in the current
issue (Oct. 15 advance online edition) of Physical Review Letters. "It
was assumed that one end of the water molecule would bind to metal,
and at the other end would be these nice hydrogen attachment points
for the atoms in next layer of water."
A theory out of Cambridge University last year
suggested that these attachment points, or "dangling OH's," did not
exist, that instead of dangling, the OH's were drawn by the geometry
of hexagonal noble-metal surfaces and clung to that.
Kimmel and his co-authors, working at the
PNNL-based W.R. Wiley Environmental Molecular Sciences Laboratory,
tested the theory with a technique called rare gas physisorption that
enlists krypton to probe metal surfaces and water layers on those
surfaces. They found that the first single layer of water, or
monolayer, wetted the platinum surface as they had expected but "that
subsequent layers did not wet the first layer," Kimmel said. "In other
words, the first layer of water is hydrophobic."
The results jibe with an earlier Stanford
University study that used X-ray adsorption to show that rather than
being fixed pointing outward in the dangling position, wet and ready
to receive the next water layer, the arms of a water monolayer on a
metal surface are double-jointed. They swivel back toward the surface
of the metal to find a place to bind. To the water molecules
approaching this bent-over-backward surface, the layer has all the
attractiveness of a freshly waxed car's hood.
The second layer beads up, but that's not all:
Additional water's attraction to that first hydrophobic water
monolayer is so weak that 50 or more ice-crystal layers can be piled
atop the first until all the so-called non-wetting portions are
covered--akin to "the coalescence of water drops on a waxed car in a
torrential downpour," said Bruce Kay, PNNL laboratory fellow and
co-author with Kimmel and PNNL colleagues Nick Petrik and Zdenek
Dohnálek.
Kimmel said that self-loathing water on metal is
more than a curiosity and will come as a surprise to many in the field
who assumed that water films uniformly cover surfaces. Hundreds of
experiments have been done on thin water films grown on metal surfaces
to learn such things as how these films affect molecules in which they
come into contact and what role heat, light and high-energy radiation
play in such interactions. |
