>The Clathrate Structure of Water

Hydrocarbons, which are nonpolar and nonionic and cannot form hydrogen bonds, show only limited solubility in water. However, energy is not the only consideration. When such hydrophobic ("water fearing") molecules do dissolve, they do not form hydration shells as hydrophilic substances do. Instead, the regular water lattice forms icelike clathrate structures, or "cages," about nonpolar molecules (Figure 2.13).

This ordering of water molecules corresponds to a decrease in the entropy, or randomness, of the mixture (see here). The decrease in entropy contributes to the low solubility of hydrophobic substances in water. It also accounts for the well-known tendency of hydrophilic substances to form aggregates in water - oil forms droplets when shaken with vinegar. Surrounding two hydrophobic molecules with two separate cages requires more ordering than surrounding them with a single cage, so the hydrophobic molecules tend to cluster.

See also: Hydrogen Bonds, Structure and Properties of Water, Entropy