June 9, 2011
Surface Of Water May Not Be All Wet
Where does water end and air begin? Scientists found that some water molecules exist as both a gas and a liquid by splitting the difference between the two, according to a new study in Nature.
In the uppermost layer of water, the boundary between gas and liquid occurs in one quarter of water molecules, where one hydrogen atom is in the water and the other vibrates freely above it.
These molecules straddle between gas and liquid phases, says senior author Alexander Benderskii of the University of Southern California.
"The free hydrogen behaves like an atom in gas phase, while its twin below acts much like the other atoms that make up "Ëbulk' water."
Benderskii and his colleagues invented techniques in their study to test the strength of hydrogen bonds that link water molecules (from the hydrogen of one molecule to the oxygen of another). These bonds keep water in its liquid stage at room temperature.
The strength of the bond was inferred by measuring the hydrogen-oxygen vibration frequency. Researchers found that as the frequency decreases, the bond becomes stronger, similar to what a person feels when they try to slow down a child on a swing, the study says.
According to Benderskii, the straddling molecules were similar in frequency to the molecules in which they exist. For example, the molecules with the one hydrogen in water had a bond that was "surprisingly only slightly weaker" than the hydrogen bonds below the surface; while the bond for the hydrogen atom above the water has similar frequency to the bonds in its gas phase.
Researchers therefore concluded that the change between air and water occurs with a single water molecule.
The molecules involved in the transition between air and water changes constantly and most likely occur in the uppermost layer of water molecules, according to the study.
For the most part, molecules are submerged in water, spending only about one quarter of their time straddling between air and water, even as they rise to the top layer, the study says.
Why is this important? According to Benderskii, the findings are important for both theoretical reasons as well as practical studies of reactions at the water's surface that include the processes involved in maintaining a vital supply of nitrogen, oxygen and carbon dioxide in the atmosphere.
"The air-water interface is about 70 percent of the earth's surface," he says. "A lot of chemical reactions that are responsible for our atmospheric balance, as well as many processes important in environmental chemistry, happen at the air-water interface."
In addition, Benderskii says that the findings allow chemists and biologists to study other interfaces, especially the boundary that exists between "water and bio membranes that marks the edge of every living cell."
"Water interfaces in general are important," he says. The study is "an open door that now we can walk through and broaden the range of our investigations to other, perhaps more complex, aqueous interfaces."
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