ABSTRACT
Surfactants are naturally occurring or synthetic organic compounds with at least one lyophilic group and one lyophobic group in the molecules. Due to their amphiphilic nature, these compounds have a high afnity toward surfaces and interfaces. In fact, the term surfactant is an abbreviation for surface active agent. If the solvent in which the surfactant is to be used is water or an aqueous solution, as in biologic applications, the lyophilic and lyophobic groups of the molecules are termed hydrophilic and hydrophobic, respectively. In aqueous media, their dual preference for water presents a problem. On one hand, the hydrophobic groups (typically hydrocarbon tails) do not mix well with water and exhibit a strong tendency to minimize the contacts with water. The formation of a pure surfactant phase is also unfavorable, on the other hand, since the hydrophilic groups (typically charged, polar, or hydrogen bonded to water) prefer the contact with water instead of being immersed in a nonpolar medium provided by the hydrophobic portions of the molecules. Two phenomena result from these opposing forces: A small part of the molecules may migrate to and arrange as a monolayer at the air-water or solid-water interface (the process is referred to as adsorption), whereas the majority nd an alternative way of limiting the unfavorable contacts by aggregating in the bulk solution with the hydrophilic groups oriented toward the aqueous phase. The latter process, referred to as self-assembly, micellization, micelle formation, or self-association, can be viewed as an alternative to adsorption. It is a very important phenomenon since the behavior of particular self-assembled structures composed of many surfactant molecules is qualitatively different from that of the unassociated free surfactant molecules called unimers.
