amphiphile

An amphiphile is a molecule that contains both hydrophilic (water-attracting) and hydrophobic (water-repelling) parts. The hydrophilic region is typically polar or ionic, while the hydrophobic region is usually a hydrocarbon chain. Amphiphiles can be ionic (anionic or cationic), nonionic, or zwitterionic. Common examples include surfactants such as sodium dodecyl sulfate and cetyltrimethylammonium bromide, as well as natural molecules like phospholipids and bile acids.

In aqueous environments, amphiphiles tend to self-assemble into structures that minimize the contact of hydrophobic moieties with water. The most familiar are micelles, in which the tails are sequestered inside and the polar heads face outward. This assembly occurs above a concentration threshold known as the critical micelle concentration (CMC); below the CMC the molecules mostly exist as monomers. Amphiphiles can also form bilayers and vesicles, which are common in biological membranes, as well as various lyotropic liquid-crystalline phases at higher concentrations. In nonpolar solvents, amphiphiles may form reverse micelles with a polar core. The tendency to form particular structures depends on tail length, head-group size and charge, temperature, and salt concentration.

Applications of amphiphiles are broad and include detergency, emulsification, and dispersion in cleaning and formulation processes. They are also central to drug and gene delivery, cosmetics, and the design of polymeric amphiphiles and nanostructured materials. In biology, membrane phospholipids are natural amphiphiles essential to cell structure and function.