Hydrophobic

Hydrophobic describes a property of molecules or portions that repel water or fail to dissolve in it. The term comes from Greek hydor "water" and phobos "fearing." In chemistry and biology, hydrophobic substances are typically nonpolar or only weakly polar, such as alkanes, many lipids, and certain polymers. By contrast, hydrophilic substances readily interact with water.

The hydrophobic effect arises because water molecules form a highly ordered network around nonpolar groups; aggregation of nonpolar moieties reduces this disrupted structure, increasing overall entropy. This effect is not a property of the molecules alone but emerges from interactions with water, and it drives phenomena such as micelle formation and protein folding. The strength of hydrophobicity can be inferred from measurements of contact angle: surfaces that repel water exhibit contact angles above 90°, with superhydrophobic surfaces often above 150°.

Biology: cell membranes are formed by amphipathic lipids with hydrophobic tails facing inward away from water. Many proteins contain hydrophobic regions that stabilize structure in the aqueous environment. Some substances are miscible in nonpolar solvents but not in water, illustrating hydrophobic character.

Applications: detergents and surfactants use hydrophilic heads and hydrophobic tails to emulsify oils; in chromatography, reversed-phase columns separate compounds by hydrophobic interactions; coatings and microfluidic devices exploit hydrophobic or hydrophilic surface properties.

Hydrophobic is often used interchangeably with lipophilic, though the terms are not identical: hydrophobic describes water-repelling behavior, while lipophilic describes affinity for lipids or nonpolar solvents.