The era of modern cosmetics emerged in the 1940s with the widespread use of synthetic surface-active agents. These materials, commonly called surfactants, modifi ed the surface tension of the oil and water phases and enabled formulators to mix them together in order to form a composition that was stable for at least the commercial shelf life of the product. These preparations were called emulsions, and the surface-active materials used to form them were emulsifi ers.
Such emulsions are generally prepared by heating the oil and water phases to a temperature of 70°C or greater before combining them. The oil and water phases are mixed together until uniform, then slowly cooled to ensure the formation of the appropriate-sized droplets or micelles, which control the stability of the emulsion. The emulsions typically have a homogeneous, opaque, white appearance and a smooth, pleasant feel upon application to the skin, hair or other epithelial surface. In fact, emulsion chemistry has become a major disciplinary area that a competent cosmetic chemist must master.
However, the introduction of surfactants to the cosmetic industry may have been a double-edged sword, because although surfactants have yielded cosmetics with desirable aesthetic properties, they have also created issues associated with their use. To the formulator, developing emulsion-based products may be problematic. The sometimes time-consuming process can be incompatible with key actives. They can limit aesthetic properties, cause thermodynamic instability and yield non-reproducible results. Additionally, some formulators fi nd scale-up for manufacturing diffi cult. Surfactants can also be skin irritants. They strip the lipid barrier of the skin or the lipid bilayer of epithelial cell membranes, leaving the tissue vulnerable. Occasionally, surfactants themselves can evoke adverse skin reactions.