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Geraldine L. Richmond
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May 7, 2001
Volume 79, Number 19
CENEAR 79 19 pp. 13
ISSN 0009-2347
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'Like dissolves like' doesn't hold for water at hydrophobic surfaces


A tan oil/water interface, water molecules give up strong hydrogen bonding with other water molecules to interact with the organic phase, a new study shows [Science, 292, 908 (2001)].

CLICHÉ REFUTED Richmond (center), Brown (top), and Scatena find that oil and water do mix.
Using total internal reflection vibrational sum frequency spectroscopy, chemistry professor Geraldine L. Richmond and coworkers Larry F. Scatena and Mac G. Brown at the University of Oregon, Eugene, selectively observed water molecules at the interface with hydrophobic liquids. They deduced the orientations of water molecules interacting with the hydrophobic phase from analysis of O–H signals.

Only a small fraction of water molecules at the interface are strongly hydrogen bonded to other water molecules, they find. Most straddle the interface, with one O-H bond in the aqueous phase and the other in the organic phase. Some even have both O-H bonds in the hydrophobic phase.

This last type of orientation is affected by pH. At low to neutral pH, the water molecules point their oxygen atoms toward the aqueous phase. Oriented in this way, they can be hydrogen bond acceptors.

At high pH, the molecules flip 180°, pointing the oxygen atoms away from the aqueous phase. The researchers believe this flipping is caused by changing electrostatics at the interface caused by the higher hydroxide ion concentration.

"Contrary to the view that oil and water don't mix, the water and hydrophobic molecules interact," Richmond says. The interactions at the interface are such that the hydrophobic phase has nearly as much pull on the interfacial water molecules as the aqueous phase, she explains.

The findings have implications for processes in which the interaction of water with a hydrophobic surface is key, such as protein folding and membrane formation. They also will provide much needed data for what Richmond says has been a long-running debate about how water hydrogen bonds to other water molecules at a hydrophobic liquid/water interface. "Because there have not been ways of examining interfacial water molecules, the discussion has been largely dominated by theorists," she notes.

The observations are consistent with the lower interfacial tension at organic/water interfaces compared with the high surface tension at an air/water interface. But they run counter to the hydrophobic effect--that is, the observed increase of hydrogen bonding among water molecules in the presence of a small nonpolar solute.

This effect is commonly invoked in modeling how water induces protein folding, Richmond says. Often in such models, water molecules are treated as if they were next to an inert noninteracting surface. "What we're showing is you can't ignore the hydrophobic surface because water interacts with that, too," she says.

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