The interactions between nonpolar substances are enhanced by van der Waals forces, which occur when the atoms of two molecules are in close proximity. These brief interactions result from random variations in the electron distribution in one molecule, which can create opposite charge distributions in the adjacent molecule. So there will be a weak, temporary δ⁺ to δ^– attraction. van der Waals forces occur in polar and nonpolar molecules. In the latter case, they can induce cohesion between molecules that do not form ionic attractions of hydrogen bonds. Although a single van der Waals interaction is brief and weak, the sum of many such interactions over the entire span of a large nonpolar molecule can result in substantial attraction. This is important when hydrophobic regions of different molecules such as an enzyme and a substrate come together (see Chapter 8).

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