The nature of the boundary between water and oil is crucial to many nanometre-scale assembly processes, including protein folding. But until now, what the interface really looks like remained in dispute. At the boundary between liquid water and water vapour, an interface forms that is marked by an area of lower-than-average density. The same sort of 'depletion layer' also occurs when water comes into contact with a sufficiently large hydrophobic surface — oil, in the most notorious instance, and various other organic molecules. According to theory, this happens because hydrophobic surfaces provide no opportunity for water molecules to establish their usual hydrogen bonds. Without this adhesive force, the molecules move away from the surface to seek such bonds in the bulk of the liquid.
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