Hydrophobicity
Definition and meaning of Hydrophobicity in chemistry.
Hydrophobicity is the tendency of a nonpolar substance or molecular surface to avoid contact with water, causing it to clump together or separate from an aqueous phase rather than dissolve in it.
In more detail
Hydrophobicity is not caused by any direct repulsive force between water and nonpolar molecules; rather, it arises because nonpolar molecules cannot form hydrogen bonds with water. Surrounding water molecules must reorganize into an ordered, cage-like arrangement around a nonpolar solute to preserve their own hydrogen-bonding network, which lowers entropy and costs free energy. Minimizing the nonpolar surface exposed to water (by clustering nonpolar groups together) restores water's freedom to hydrogen-bond, so the process is thermodynamically favored. This entropy-driven behavior, called the hydrophobic effect, underlies phenomena from oil-water separation to biological self-assembly.
Key facts
| Field | Physical Chemistry |
|---|---|
| Driving force | Hydrophobic effect (entropy-driven, not direct repulsion) |
| Opposite property | Hydrophilicity (water-attracting) |
| Typical hydrophobic groups | Alkyl chains, aromatic rings, waxes, oils |
Vegetable oil poured into water does not dissolve but forms separate droplets that coalesce into a single layer, minimizing the total oil-water interfacial area because the oil's hydrocarbon chains are hydrophobic.
Frequently asked questions
Is hydrophobicity caused by a physical repulsion between water and nonpolar molecules?
No. It results from water molecules maximizing their own hydrogen bonding by minimizing contact with nonpolar surfaces, which is entropically favorable, not from an attractive or repulsive force acting on the nonpolar molecule itself.
Why does hydrophobicity matter in biology?
It drives protein folding (nonpolar side chains bury themselves away from water in the protein interior) and the self-assembly of lipid bilayers and micelles, both essential to cell membrane structure.