Illustrated Glossary of Organic Chemistry

Surfactant: A contraction of surface active agent. A molecule that lowers the surface tension of a liquid, or the interfacial tension between two liquids. To be a surfactant a molecule must have both a polar (hydrophilic or lipophobic) region (not necessarily large) and a nonpolar (hydrophobic or lipophilic) region (which must be large) in its structure.

Potassium stearate, a salt of stearic acid (a fatty acid), can function as a surfactant because it has both a long hydrophobic alkyl group tail (shown in red) and a hydrophilic carboxylate group (in blue).

Pulmonary surfactant contains phospholipids whose fatty acid chains are mostly derived from palmitic acid. This surfactant molecule lowers the surface tension of mucous in the lung, making it easier for oxygen to pass from the atmosphere into the bloodstream.