By Lynn Yarris
There is a theory floating out in the scientific community which posits that the slipperiness of ice and the infamous hole in the ozone layer above the Antarctic are the result of the same phenomenon. According to this theory, despite the term "frozen solid," the surface of ice is actually "wet." This theory has received a strong boost with the discovery by Berkeley Lab researchers that even at temperatures far below those in the Antarctic atmosphere, the surface of solid ice remains covered by a thin film of quasi-liquid water.
Ice has a uniquely low coefficient of friction, which is why skaters can glide so swiftly across it. The long-time explanation for this anomaly was that pressure, for instance from the weight of a skater on a pair of sharp-edged blades, liquified the surface layers of water molecules. However, modern methods of testing proved that pressure was not the answer. Other studies provided evidence of solid ice being wet on the surface at 230 K (-43 degrees Celsius below the normal freezing point of water) but there was too little information on how these water molecules might be configured to draw any firm conclusions.
Now, researchers with the Materials Sciences Division (MSD), in collaboration with French and Dutch scientists, have produced the first detailed atomic-scaled pictures of the surface of crystalline ice. These pictures reveal that even at 90 Kelvin (-183 degrees Celsius), water molecules on the surface of ice continue to vibrate, giving the surface a "quasi-liquid" character that could account for catalytic chemistry as well as slipperiness.
"Our analysis of solid ice showed that at 90 K, water molecules on the surface are bound in a lattice but aren't frozen like those in the layers beneath them," says MSD chemist Michel Van Hove. "Water molecules in this surface layer have an unusually high degree of vibrational motion with amplitudes several times that seen in water molecules buried deeper within the bulk of the ice."
The high degree of vibrational motion exhibited by water molecules in the surface layers of ice is attributed to the absence of other water molecules above them, together with the weakness of the bonds between water molecules. This not only gives rise to a liquid-like vibrational motion, it also frees the water molecules to interact with other molecules to which they are exposed, such as those in the atmosphere.
Temperatures in the polar stratospheric clouds are around 200 K (-73 degrees Celsius). Van Hove and his colleagues believe there are enough vibrating layers of water molecules to create a thin film of quasi-liquid water on the ice crystallites in the clouds.
"This watery film could be the catalyst that provides a crucial link in a long chain of chemical reactions which leads to the creation of the ozone hole," says Van Hove.
At temperatures around 230 K and above, Van Hove thinks that the surface film of ice becomes a true liquid which would account for its uniquely low coefficient of friction.
The molecular-scale images of the surface of ice were produced at the Center for Advanced Materials by the research groups led by Van Hove and chemist Gabor Somorjai.
The researchers created a thin film of "ideal" ice (about ten angstroms thick) by condensing water vapor on a cold platinum surface in an ultrahigh vacuum. They then "bounced" low-energy electrons off the surface of this ice and studied the resulting diffraction patterns using a technique developed by Van Hove called "tensor-LEED (for low-energy electron diffraction). This technique provides researchers with a timely and practical means of determining the position of atoms on a given surface.
"With tensor-LEED, a simple surface structure that would have taken us months to resolve can now be determined in less than a day," says Van Hove.
Once models of the surface structure of ice were obtained, the results were analyzed by Christian Minot in Paris, using total-energy calculations, and Geert-Jan Kroes in Amsterdam, using molecular dynamic simulations.
"What we see is that the outermost molecular film of ice solidifies only up to a point," says Van Hove. "Large vibrational amplitudes continue to exist down to at least 90 K."
Now it appears those vibrating molecules not only allow ice skaters to glide with ease across a frozen lake, but may also provide a critical clue to the hole in the ozone layer.