The skin is covered with a hydrolipid film that, depending on the area of the body, comprises secretions from sebaceous glands and from apocrine and eccrine sweat glands. Decomposition products from cornification (cellular debris and stratum corneum lipids) and corneocytes in the process of being shed are also present. This film provides a degree of waterproofing to the skin’s surface, traps water to help maintain skin pliability, and provides a natural defense against pathogenic organisms. But this film also attracts
and holds dirt and pollutants from the environment. The skin’s surface is also home to a variety of microorganisms. In most cases these organisms, the so-called resident flora, cause no harm and provide an additional defense against overgrowth by potential pathogens. But these organisms can act on components of the surface film and create undesirable by-products, such as those resulting from the metabolism of compounds found in apocrine sweat that create body odor. Thus, while the surface hydrolipid film is an important skin integument, periodic cleansing to remove dirt, debris, and odor is essential to maintaining skin health and in many cultures, social acceptance. Additionally, periodic cleansing is necessary to remove soil (including bacteria) from the skin surface that is acquired by incidental contact or by intentional application, e. g., medications or makeup and other cosmetic products.
Water alone is capable of removing much of the soil from the skin’s surface (5). However, water has a limited ability to dissolve and remove oils; as the old adage goes, “oil and water don’t mix.” The surfactants that make up the bulk of most personal cleansing products aid this process. A surfactant, or surface active agent, is a material that lowers the interfacial tension of the medium it is dissolved in, and the interfacial tension with other phases. Said more simply, a surfactant increases the affinity of dissimilar phases for each other. This ability is based on surfactants’ unique structure, which combines both hydrophilic and hydrophobic moieties at opposite ends of the surfactant molecule. In a dilute aqueous solution, surfactant molecules will arrange themselves such that the hydrophilic portion of the molecule is oriented toward the bulk solution while the hydrophobic portion orients itself in the opposite direction. For water in contact with skin the presence of surfactant molecules at the interface lowers the interfacial tension and aids wetting, which improves water’s ability to spread over the skin’s surface. This, along with the mechanical action of applying the cleanser, helps to remove soil. As the concentration of surfactant in solution increases a point is reached at which the surfactant molecules begin self-association into micellar structures. This point is known as the critical micelle concentration (CMC). Surfactants in aqueous micelles have their hydrophilic end oriented toward the bulk (water) phase and their hydrophobic end oriented toward the interior of the micelle. The hydrophobic interior provides a good environment for dissolving lipids, and micellar solubilization is an important mechanism by which surfactants remove oily soils from the skin’s surface and help keep the soils suspended until they are rinsed away. Other factors may aid this process. For example, the skin’s surface possesses a net negative charge at physiological pH and repulsive forces between the skin and anionic surfactants or their associated micelles help keep suspended soils from redepositing, making these surfactants particularly good cleansers.