Microemulsions and Their Precursors

Abstract

Amphiphiles are generally defined with reference to water. An amphiphilic molecule possesses a hydrophilic (water soluble) moiety and a hydrophobic (water insoluble) moiety separable by a mathematical surface. In this paper the only hydrophobic moieties considered are those composed of hydrocarbon chains (as opposed, for example, to fluorocarbon chains). According to IUPAC [1], surfactants are substances which lower the surface tension of the medium in which they are dissolved and/or the interfacial tension with other phases. This definition is too broad as it would include substances such as benzene which lower the surface tension when dissolved in water but have none of the other properties expected of a surfactant. A generally accepted restriction of the definition is that surfactant molecules must be amphiphilic [2–4]. We adopt this restriction and, furthermore, share the view of Laughlin [5] that those properties associated with surfactancy (e.g., detergency, co-solubilization of oil and water, emulsification, foaming, and the like) are not common to all amphiphiles. He reserves the term “surfactant” for molecules which form association colloids such as micelles and liquid crystals. Thus, amphiphiles such as long-chained fatty acid salts (soaps), sodium alkyl sulfates and sodium alkyl sulfonates (detergents), and lecithins are clearly surfactants, whereas monohydric straight-chained alcohols and amines are mere amphiphiles. A special property of the association colloids formed by amphiphiles is that the amphiphiles associate into monolayer or sheet-like structures with the water soluble moieties on one side of the sheet and the water insoluble moieties on the other side (a bilayer is a pair of opposed monolayers). In the spirit of Laughlin, we define surfactants as amphiphiles which form association colloids distinguished by sheet-like surfactant microstructure.