Chemistry

It's Not Just Another Phase

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Science  15 Mar 2002:
Vol. 295, Issue 5562, pp. 1975
DOI: 10.1126/science.295.5562.1975d

Many aspects of the microscopic structure of water in its many phases remain incompletely understood. For example, the structure of the liquid water surface influences key properties, such as surface tension and the mobility of molecules across the liquid-gas interface, but it has been difficult to determine the surface hydrogen bond configurations of water molecules. Wilson et al. have used near-edge x-ray absorption fine structure measurements to characterize the liquid surface of water jets in thermal equilibrium with local water vapor. Total ion-yield spectra, which reveal free OH bonds, were combined with simulations to identify a new interfacial species: acceptor-only water molecules that have two free OH bonds and are in dynamic equilibrium with the vapor. Condensation of these molecules is likely to occur through the initial formation of an acceptor hydrogen bond, in contrast to the condensation dynamics of amorphous ice, where vapor molecules are captured by forming donor and acceptor bonds.

Using a related technique, Myneni et al. have investigated the electronic structure of bulk liquid water, which differs substantially from that of the solid and of the vapor. Theoretical calculations indicate that this difference arises from a substantial fraction of water molecules with free OH bonds. On average, molecules have 2.4 to 2.8 hydrogen bonds, significantly less than the value of 3.5 found in previous molecular dynamics simulations.

Finally, although the bulk structure of ice is better understood than that of the liquid, the properties of the liquid layer that exists at the surface of ice near its melting point has been a topic of controversy. Bluhm et al. have developed an electron spectrometer that can operate in the presence of water vapor. Their results show that “premelting” occurs between -20° to 0° Celsius and that organic contaminants can strongly enhance the premelting of ice. — JU

J. Phys. Condens. Matt.14, L221; L213; L227 (2002).

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