That old truism about mixing oil and water can apply to water and water, according to researchers at Pacific Northwest National Laboratory in Washington State.
Greg Kimmel and colleagues have thrown out the textbook description of how water molecules readily come together with open hydrogen bonding arms to show that “wetness” is not a universal property of water. They observed how a single layer of water-ice grown on a platinum wafer gives incident water molecules the cold shoulder, putting up a barrier to the formation of additional layers.
“Water-surface interactions are ubiquitous in nature and play an important role in many technological applications such as catalysis and corrosion,” explains Kimmel, “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 the next layer of water.” Last year, however, Cambridge University theorists suggested that these “dangling OH groups” don’t actually exist, and that instead of dangling, the OH groups cling to the hexagonal noble-metal surface.
Kimmel and his colleagues wanted to test this theory using rare gas physisorption, which enlists krypton to probe metal surfaces and water layers on those surfaces. They found that the first single layer of water, or monolayer, did indeed “wet” the platinum surface as would be expected but subsequent layers did not wet the first layer. “In other words, the first layer of water is hydrophobic,” says Kimmel.
Experimentalists at Stanford University has previously 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. This contrasts starkly with Kimmel’s findings and subsequent verification of the work will demonstrate which model is right.
Hydrophobic water on a metal surface 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.
Kimmel, G., Petrik, N., Dohnálek, Z., & Kay, B. (2005). Crystalline Ice Growth on Pt(111): Observation of a Hydrophobic Water Monolayer Physical Review Letters, 95 (16) DOI: 10.1103/PhysRevLett.95.166102