USC associate professor and senior author Alexander Benderskii |
Air
and water meet over most of the earth’s surface, but exactly where one
ends and the other begins turns out to be a surprisingly subtle
question.
A
new study in Nature narrows the boundary to just one quarter of water
molecules in the uppermost layer – those that happen to have one
hydrogen atom in water and the other vibrating freely above.
Such
molecules straddle gas and liquid phases, according to 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 “bulk” water.
The
finding matters for theoretical reasons and for practical studies of
reactions at the water’s surface, including the processes that maintain 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,”
Benderskii said. “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.”
He
added that the study provided a new way for chemists and biologists to
study other interfaces, such as the boundary between water and
biomembranes that marks the edge of every living cell.
“Water
interfaces in general are important,” Benderskii said, calling the
study “an open door that now we can walk through and broaden the range
of our investigations to other, perhaps more complex, acqueous
interfaces.”
In
their study, Benderskii and his colleagues used techniques they
invented to test the strength of hydrogen bonds linking water molecules
(from the hydrogen of one molecule to the oxygen of another). These are
the bonds that keep water a liquid at room temperature.
Specifically,
the researchers inferred the bond strength by measuring the
hydrogen-oxygen vibration frequency. The bond gets stronger as the
frequency decreases, similar to the pull one feels when slowing down a
child on a swing.
In
the case of straddling molecules with one hydrogen in water, when
compared to bonds below the surface, “the hydrogen bond is surprisingly
only slightly weaker,” according to Benderskii.
Likewise, the bond for the hydrogen atom sticking out of the water is similar in strength to bonds in the gas phase.
The researchers concluded that the change between air and water happens in the space of a single water molecule.
“You recover the bulk phase of water extremely quickly,” Benderskii said.
While
the transition happens in the uppermost layer of water molecules, the
molecules involved change constantly. Even when they rise to the top
layer, molecules for the most part are wholly submerged, spending only a
quarter of their time straddling air and water.
The study raises the question of how exactly to define the air-water boundary.
If
the straddling molecules constitute the boundary, it would be analogous
to a wood fence where three of every four boards are missing – except
that since water molecules always are moving between submerged and
straddling positions, the location of the fourth board would change
millions of times per second.
If
the boundary were the entire top layer of water molecules, the analogy
would be a fence where one in four boards is sticking out at any one
time.
A
physical chemist, Benderskii began the study at Wayne State University
in Detroit before joining the USC Dornsife College of Letters, Arts and
Sciences in 2009 as an associate professor.
Benderskii’s
collaborators were lead author Igor Stiopkin, formerly at Wayne State
and now at the University of Wisconsin-Madison; Champika Weeraman, also
previously at Wayne State and now at Canada’s National Research Council
in Ottawa; Piotr Pieniazek and James Skinner of the University of
Wisconsin-Madison; and Fadel Shalhout, formerly at Wayne State and now
at USC Dornsife College.
The National Science Foundation and the U.S. Department of Energy funded the study.
