FIGURE 1. Configurations of liquid water molecules near hydrophobic cavities in molecular-dynamics simulations.

The blue and white particles represent the oxygen (O) and hydrogen (H) atoms, respectively, of the water molecules. The dashed lines indicate hydrogen bonds (that is, O-HO within 35° of being linear and O-to-O bonds of no more than 0.35 nm in length). The space-filling size of the hydrophobic (red) particle in a is similar to that of a methane molecule. The hydrophobic cluster in b contains 135 methane-like particles that are hexagonally close-packed to form a roughly spherical unit of radius larger than 1 nm. In both cases, the water molecules shown are those that are within 0.8 nm of at least one methane-like particle. For the single cavity pictured in a, each water molecule can readily participate in four hydrogen bonds. (Owing to thermal motions, hydrogen bonding in liquid water is disordered.) Water molecules in a are typical of the bulk liquid where most molecules participate in four hydrogen bonds. The water molecules shown in b, however, are not typical of the bulk. Here, the cluster is sufficiently large that hydrogen bonds cannot simply go around the hydrophobic region. In this case, water molecules near the hydrophobic cluster have typically three or fewer hydrogen bonds.