Water interacts especially favorably with charged and polar groups because it can orient its partial charges and form hydrogen-bonding networks around them. A nonpolar molecular surface cannot participate in those interactions in the same way.

The hydrophobic effect is the tendency of nonpolar molecular surfaces in water to reduce their total contact with the aqueous environment. It is not a new attractive force called a “hydrophobic bond.” It emerges from the free-energy balance of water and solute interactions.

When an isolated nonpolar group is exposed to water, nearby water molecules must arrange their hydrogen-bonding network around a surface that cannot hydrogen-bond with them. That interfacial environment restricts some solvent configurations relative to bulk water. If several nonpolar surfaces cluster together, the total nonpolar area exposed to water can decrease and some interfacial water returns to the bulk.

This can make association thermodynamically favorable even though the nonpolar groups do not strongly attract one another directly.

Example

Imagine two small hydrocarbon groups, molecular regions built only from carbon and hydrogen, dispersed separately in water. Each presents a nonpolar surface to the solvent. If the groups approach and touch, the area of hydrocarbon-water interface becomes smaller than the sum of the two separate interfaces. Water molecules formerly associated with the buried surfaces are released back into the bulk solvent.

The same principle helps explain why:

• hydrocarbon-rich oils separate from water;
• nonpolar side chains tend to become buried inside many soluble proteins;
• molecules containing both polar and nonpolar regions can assemble so that their nonpolar surfaces become buried.

The hydrophobic effect depends on temperature, molecular size, solvent conditions and the detailed balance of enthalpy and entropy. It should therefore not be reduced to the slogan “nonpolar molecules repel water.” Water does not exert a special repulsive force on hydrocarbons; rather, states that reduce unfavorable nonpolar-water interfacial exposure can have lower Gibbs free energy.

Hydrophobic association is one of the central ways that ordinary intermolecular interactions and thermodynamics generate organized structures in water without requiring covalent bonds between all of their components.