Wetness-defying Water? Pacific Northwest National Laboratory Group Discovers A Paradox: Hydrophobic H₂O

The textbooks say that water readily comes together with otherwater, open arms of hydrogen clasping oxygen attached to other OHmolecules. This is the very definition of "wetness." But scientists atPacific Northwest National Laboratory have observed a first: a singlelayer of water--ice grown on a platinum wafer--that gives the coldshoulder to subsequent layers of ice that come into contact with it.

"Water-surface interactions are ubiquitous in nature and play animportant role in many technological applications such as catalysis andcorrosion," said Greg Kimmel, staff scientist at the Department ofEnergy lab and lead author of a paper in the current issue (Oct. 15advance online edition) of Physical Review Letters. "It was assumedthat one end of the water molecule would bind to metal, and at theother end would be these nice hydrogen attachment points for the atomsin next layer of water."

A theory out of Cambridge University last year suggested that theseattachment points, or "dangling OH's," did not exist, that instead ofdangling, the OH's were drawn by the geometry of hexagonal noble-metalsurfaces and clung to that.

Kimmel and his co-authors, working at the PNNL-based W.R. WileyEnvironmental Molecular Sciences Laboratory, tested the theory with atechnique called rare gas physisorption that enlists krypton to probemetal surfaces and water layers on those surfaces. They found that thefirst single layer of water, or monolayer, wetted the platinum surfaceas they had expected but "that subsequent layers did not wet the firstlayer," Kimmel said. "In other words, the first layer of water ishydrophobic."

The results jibe with an earlier Stanford University study that usedX-ray adsorption to show that rather than being fixed pointing outwardin the dangling position, wet and ready to receive the next waterlayer, the arms of a water monolayer on a metal surface aredouble-jointed. They swivel back toward the surface of the metal tofind a place to bind. To the water molecules approaching thisbent-over-backward surface, the layer has all the attractiveness of afreshly waxed car's hood.

The second layer beads up, but that's not all: Additional water'sattraction to that first hydrophobic water monolayer is so weak that 50or more ice-crystal layers can be piled atop the first until all theso-called non-wetting portions are covered--akin to "the coalescence ofwater drops on a waxed car in a torrential downpour," said Bruce Kay,PNNL laboratory fellow and co-author with Kimmel and PNNL colleaguesNick Petrik and ZdenekDohnálek.

Kimmel said that self-loathing water on metal is more than a curiosityand will come as a surprise to many in the field who assumed that waterfilms uniformly cover surfaces. Hundreds of experiments have been doneon thin water films grown on metal surfaces to learn such things as howthese films affect molecules in which they come into contact and whatrole heat, light and high-energy radiation play in such interactions.

PNNL is a DOE Office of Science laboratory that solves complex problemsin energy, national security, the environment and life sciences byadvancing the understanding of physics, chemistry, biology andcomputation. PNNL employs 4,000 staff, has a $700 million annualbudget, and has been managed by Ohio-based Battelle since the lab'sinception in 1965.