Weaker in Tighter Groups

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Science  01 Nov 2002:
Vol. 298, Issue 5595, pp. 927
DOI: 10.1126/science.298.5595.927a

The surfaces of liquids such as water and methanol are the place where key processes such as dissolution, evaporation, and condensation occur. The surface tension of liquid water is much greater than that of methanol, and one might therefore expect intermolecular distances (for example, those between oxygen atoms) to be shorter at the water surface. Wilson et al. show that the opposite is the case. In an extended x-ray absorption fine structure study of both surfaces, they find that the O-O distance expands by 5.9% at the water surface and contracts by 4.6% at the methanol surface.

This counterintuitive result may be explained by the different hydrogen-bonding networks in the two liquids. Water molecules can form a large number of interfacial bonds in a variety of configurations, not only parallel but also perpendicular to the surface; the water molecules are highly mobile but still relatively tightly bound, increasing the surface tension. In contrast, methanol molecules form highly ordered surfaces, but lateral interactions between molecules at the interface are weak, which decreases the surface tension. — JFU

J. Chem. Phys.117, 7738 (2002).

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