Water's Unexpected Self-Loathing
"Water, water everywhere and all the boards did shrink; Water, water, everywhere, nor any drop to drink," lamented Coleridge's famed ancient mariner as he was mesmerized by endless months at sea. Today, water teases scientists in a similar fashion. "Despite over a century of intensive investigation, some fundamental properties of water and aqueous systems are still not understood," explains Greg Kimmel of Pacific Northwest National Laboratory. He and his colleagues recently demonstrated that platinum is an awkward base for ice crystals, disproving the conventional wisdom that some noble metal surfaces create perfect atomic platforms. Water mixes favorably with other water molecules in most systems, but against platinum, water wants to minimize contact with itself.
It was previously believed that if ice were added to a platinum surface, the first layer would bond to the metal and also provide perfect molecular attachment points for the next layer. Kimmel's experimental work follows a hypothesis from Cambridge University suggesting the first layer of ice is strongly influenced by the platinum substrate, preventing good attachment points for a subsequent layer.
Ice is a hexagonal double-layer of water molecules. The lower bi-layer's hydroxyl (-OH) groups point down, while the top bi-layer's hydroxyl groups point straight up (see Figure 1 for the orientation). Meanwhile, atomic platinum is like "cannon balls" placed one on top of the other to obtain the closest three-dimensional fit. (The technical name for the structure of platinum is face-centered cubic.) The old visualization of ice on platinum included downward-pointing OH groups fitting nicely into the crevices between platinum atoms and the second bi-layer's OH groups pointing straight up. The structure of ice on platinum was expected to be exactly like regular ice (see Figure 2).
But that is not right! The old picture ignored the energy cost of OHs pointing out into space and away from the substrate. The Cambridge hypothesis says there are no "dangling OH groups," but instead, the optimal energy configuration has all the OHs pointing in toward the platinum (see Figure 3). The first layer of ice is essentially bent over the inward pointing OHs create zero attachment points for the next layer. Since the first layer is hydrophobic, this example threatens the old picture plus our everyday notions that "like dissolves like" and "water likes water."
This is a nice idea, but testing can be difficult. How can someone know exactly how many layers of ice cover a metal surface? Kimmel decided to use rare gas physisorption, which enlists a temperature-triggered release of krypton to detect the number layers of ice covering a percentage of the surface. Additionally, he rigged a printer cartridge to spray individual layers of water onto platinum. "It can release liquid on a millisecond timescale," he explains. After spraying, the surface is gradually cooled to 35 K and the desorption patterns of krypton reveal the number of ice layers. The magnitude of the desorption peaks give the percentage of platinum covered by any given number of ice layers.
Kimmel found that the first layer of ice wets the entire platinum surface, but additional layers fail to accomplish that feat. When two layers of water are added to platinum and frozen, 80% of the surface is still covered by just one layer of ice.
Subsequent layers ball up like drops on the hood of a waxed car. The first layer is so hydrophobic that it requires 50 sprayed layers to cover the entire surface with more than one layer. To Kimmel this amounts to "putting the waxed car into a lake." This is a case where water hates water!
The results agree with the Cambridge hypothesis and an X-ray absorption study from Stanford University, all of which discount the old picture of dangling OHs. These were successful projects, but important questions remain unanswered. Rare gas physisorption is a test of broad trends (how many layers of ice cover what percentage of a surface), which leaves in-depth molecular questions in the realm of speculation. Future work may look carefully at the properties of the "balled up" ice crystals or ask to what extent the hexagonal structure of ice is maintained by the first layer. Water-metal interactions are important in other frontiers of science, including catalysis and corrosion, and new questions keep appearing. Kimmel affirms "research on water and aqueous systems remains extremely active."
References and Further Reading
Kimmel, GA, NG Petrik, Z Dohnalek, and BD Kay (2005). Crystalline Ice Growth on Pt(111): Observation of a Hydrophobic Water Monolayer. Phys. Rev. Lett., (95)166102.
Michaelides, A, A Alavi, and DA King (2004). Insight into H2O-ice adsorption and dissociation on metal surfaces from first-principles simulations. Phys. Rev. B, (69)113404.
Ogasawara, H, et al. (2002). Structure and Bonding of Water on Pt(111). Phys. Rev. Lett., (89)276102.
- By James Krier.