Water theory is watertight, researchers say
James E. Kloeppel, Physical Sciences Editor
217-244-1073;
kloeppel@uiuc.edu
1/17/2006
Click photo to enlarge
University of Illinois photo
Steve Granick, professor of materials science and engineering, of chemistry and of physics, has solved the mystery of hydrophobic surfaces.
There may be tiny bubbles in the wine, but not at the interface between
water and a waxy coating on glass, a new study shows.
The behavior of water when placed in contact with hydrophobic (water-repellent)
surfaces, such as raincoats and freshly waxed cars, has puzzled scientists
for a long time. According to a controversial theoretical prediction,
water near a hydrophobic surface will pull away and leave a thin layer
of depleted water at the surface – that is, water molecules at
the interface will pack less tightly than usual.
Now, a team of researchers at the University of Illinois at Urbana-Champaign
and Argonne National Laboratory has resolved the controversy. Using
near-perfect hydrophobic surfaces and synchrotron X-ray measurement
techniques, the researchers found the theoretical prediction to be correct.
They report their findings in the Dec. 31 issue of the journal Physical
Review Letters.
“Previous experiments have been interpreted sometimes in favor
of a depletion layer, sometimes against, and sometimes as indicating
intimate solid-water contact in places and ‘nanobubbles’
in others,” said
Steve Granick, a professor of
materials
science and engineering,
chemistry
and
physics at Illinois.
“Part of our study was to help understand why there was so much
disagreement in the scientific literature,” said Granick, who
also is a researcher at the
Frederick
Seitz Materials Research Laboratory on campus and at the university’s
Beckman Institute for Advanced
Science and Technology.
To study the nature of hydrophobicity, the researchers first prepared
a nearly ideal hydrophobic surface – a self-assembled methyl-terminated
octadecylsilane monolayer. Then they made synchrotron X-ray measurements
of the interface between water and monolayer.
The measurements revealed a depletion layer, about one water molecule
in thickness. The depletion layer was present with and without air dissolved
in the water. Because no nanobubbles were seen, bubbles must not play
a significant role in hydrophobicity, the researchers conclude.
The synchrotron X-ray data “unambiguously confirm the theoretical
expectation that water, when it meets a planar hydrophobic surface,
forms a depletion layer,” the researchers write.
“We found that in a real system — more complicated than
the theory assumes — the theory does capture the essence,”
Granick said. “The next time I see water beading on a raincoat,
my vision of how the water molecules experience that raincoat is going
to be different.”
The research team included Illinois graduate student and lead author
Adelé Poynor, graduate student Liang Hong, physicist Ian Robinson
(now at University College London); and synchrotron X-ray expert Paul
A. Fenter and postdoctoral researcher Zhan Zhang, both at Argonne
National Laboratory.
The National Science Foundation, through the Center of Advanced Materials
for the Purification of Water Systems at Illinois, and the U.S. Department
of Energy funded the work.
