Hydrophobicity

Hydrophobicity describes unfavorable mixing with water, as illustrated by long hydrocarbon regions. The hydrophobic regions of membrane-forming phospholipids face the interior of a lipid bilayer. This organization is explained by the hydrophobic effect, rather than by imagining that chains actively repel water. See IUPAC Gold Book: hydrophobic interaction.

The hydrophobic effect is the solvent-mediated tendency of nonpolar regions to associate in water. Exposing a hydrocarbon region changes how surrounding water molecules can arrange and interact; reducing that exposure can lower the total free energy. Both water and lipids enter the accounting. The effect is not an additional covalent bond between chains. See IUPAC Gold Book: hydrophobic interaction.

At fixed temperature and pressure, compare two candidate organizations by the change in Gibbs free energy, $\Delta G=\Delta H-T\Delta S$, where $H$ is enthalpy, $T$ is absolute temperature, and $S$ is entropy. An organization with lower $G$ is favored in this comparison. Reducing exposed hydrocarbon area is one contribution; changes in chain packing and the entropy of the lipids also matter. This explains why hydrophobicity alone cannot predict every aggregate shape. See 16.4 Free Energy.

A solvent-mediated tendency of appropriately prepared water-repelling surfaces to associate. Surface chemistry, dissolved gas and the experimental geometry can strongly affect the measured force; it is not a universal replacement for every other colloidal interaction.