Amphiphilic molecule

What is an amphiphilic molecule?

The word comes from Greek roots meaning “loving both.” An amphiphilic molecule has two regions with opposite preferences:

• A hydrophilic head, which is polar or charged and is attracted to water.
• A hydrophobic tail, usually a hydrocarbon chain, which is repelled by water and attracted to oils.

Also called amphipathic molecules or amphiphiles, they include all surfactants, soaps, biosurfactants such as glycolipids, and the phospholipids that make up cell membranes.

Why amphiphiles behave the way they do

A molecule that is half water-loving and half water-repelling is never fully comfortable dissolved in water or in oil. It lowers its energy by arranging itself so that each part is in the environment it prefers:

• At interfaces, amphiphiles line up with heads in the water and tails in the oil or air. That is why surfactants lower surface tension and interfacial tension.
• In solution, above a threshold concentration, they self-assemble into structures that hide their tails from water: micelles, rod-like aggregates, bilayers and vesicles.

What decides the structure they form

The shape of the assembly depends largely on the shape of the molecule. A widely used guide is the packing parameter, which compares the volume of the tail with the area of the head and the length of the tail. Molecules with large heads relative to their tails tend to form spherical micelles; as the tail becomes relatively bulkier, they form cylindrical micelles, then bilayers such as membranes and vesicles. Temperature, salt and other ingredients shift those structures.

Amphiphiles at Locus

The name of Locus’s Amphi ingredient line refers to the amphiphilic structure of its glycolipid biosurfactants: a sugar head group joined to a fatty acid tail.

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