Where Water Holds Still

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Science  12 Oct 2007:
Vol. 318, Issue 5848, pp. 171-173
DOI: 10.1126/science.318.5848.171e

When polar molecules dissolve in water, the solvent's accommodation of groups that resemble its own structure (such as hydroxyls and amines) is unsurprising. Less clear is how the network of hydrogen-bonded H2O molecules accommodates a solute's nonpolar alkyl components. Rezus and Bakker explored this question through ultrafast vibrational anisotropy measurements of nitrogen-based aqueous solutes with varying degrees of N-alkylation. After tagging water molecules in the solvation shell with low-level vibrational excitation, the authors used the femtosecond time resolution of a laser probe to monitor rotational mobilities of these molecules before diffusional exchange with the surrounding bulk region. The data revealed two distinct reorientation rates, and the population of more slowly rotating water molecules steadily increased as the number of hydrophobic methyl groups on the solute rose. A correlation of four immobilized OH groups with each methyl group suggests a strong disruption of the solvent dynamics in the hydrophobic vicinity, though the authors emphasize that the structure in which the waters are briefly locked remains disordered, rather than ice-like. — JSY

Phys. Rev. Lett. 99, 148301 (2007).

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