Posted: January 17, 2007 |
Water theory is watertight |
(Nanowerk News) 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.
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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.
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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.
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"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.
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"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.
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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.
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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.
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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.
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"We found that in a real system – more complicated that 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."
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