An amphiphilic (also called amphipathic) compound that adsorbs at interfaces to form oriented monolayers and shows surface activity. An amphiphilic compound is a molecule that has a hydrophilic (polar) head and a hydrophobic (nonpolar) tail. Common synonyms for the term “surfactant” include amphiphile, surface-active agent, and tenside. If a surfactant is placed into contact with both a polar medium, such as water, and a nonpolar medium, such as an oil, one part of its molecule has an affinity for the polar medium and one part that has an affinity for the nonpolar medium. An example is an organic molecule such as sodium dodecyl sulfate (SDS), which can be thought of as having a hydrocarbon (dodecyl) tail and a highly polar (sulfate) head group. If the molecule is placed into a system containing water and oil, the sulfate head group will have an affinity for the water, while the dodecyl tail will have an affinity for the oil. The energetically most favorable orientation for such molecules is at the interface between the polar and nonpolar media, so that each part of the molecule can reside in an environment for which it has the greatest affinity. In the case of SDS in a mixture of oil and water, the SDS molecules will preferentially adsorb at the water/oil interface, with the polar sulfate group oriented into the water and the dodecyl tail group oriented into the oil. Three consequences of the amphiphilic nature of surfactants are their ability (1) to adsorb and form layers at interfaces, (2) to reduce the interfacial tension between fluids, and (3) to associate to form clusters, called micelles.