Mid-infrared spectroscopy sheds new light on hydrophobic effect

The hydrophobic effect-the way oil and water refuse to mix or how water beads on certain surfaces, such as waxy leaves-is intricately linked to the manner in which apolar (water-fearing) compounds are solvated by water.

The hydrophobic effect-the way oil and water refuse to mix or how water beads on certain surfaces, such as waxy leaves-is intricately linked to the manner in which apolar (water-fearing) compounds are solvated by water. In the 1940s, this phenomenon was first explained by the “iceberg model,” which asserts that water molecules in hydrophobic groups form rigid, icelike structures in a shell around apolar molecules. Subsequent experiments have been unable to substantiate the iceberg model, however, because they could not image the icebergs.

In an effort to provide a more consistent picture of the effect of hydrophobic groups on the structural dynamics of water, Yves Rezus and Huib Bakker of the Institute for Atomic and Molecular Physics (Amsterdam, the Netherlands) used polarization-resolved mid-IR pump-probe spectroscopy and various hydrophobic solutes to track the orientational dynamics of HDO molecules. A femtosecond pulse excited vibrations in oxygen-deuterium (OD) bonds aligned along the beam’s polarization direction. A second pulse counted the excited OD bonds that vibrated in both parallel and perpendicular directions. The results-reported in Phys. Rev. Lett. 99, 148301 (Oct. 5, 2007)-showed that a significant fraction of the water molecules rotated after 2.5 ps, while the remaining fraction had yet to rotate after more than 10 ps. The slower-rotating fraction appeared to be related to the apolar molecules, increasing as the apolar concentration was increased. They concluded that their method provides a molecular picture of the icebergs, which comprise four strongly immobilized water OH groups for every methyl group in solution. Contact Y. L. Rezus at rezus@amolf.nl.

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