SURFACTANT PENETRATION 95 earlier, the toxicity of the cationic surfactants enables them to be mainly served as disinfec- tants. In addition, the amphoteric surfactants are usually used in combination with other surfactants (e.g., a standard shampoo formulation contains one anionic and one ampho- teric surfactant ( 6)). Nevertheless, anionic surfactants and nonionic surfactants are the most common agents used in general skin-care products. The anionic surfactants mainly serve as detergents, emulsifi ers, and foaming agents, whereas nonionic surfactants can be used alone or are usually mixed with anionic surfactants to form mixed micelles or some structured phases, which, to some degree, reduce the irritation of some anionic surfac- tants to the skin barrier. This review article emphasizes on the anionic and the nonionic surfactants mainly used in the facial care products. Skin cleaning is the fi rst step in modern skincare routines and is essential to the skin hygiene. Surfactants have been used in this fi eld for a long period (2). They undoubtedly have the ability to clean the skin. On the other hand, the surfactants in the facial care products, occasionally, may cause some undesirable effects to skin, including dryness, pruritus, red- ness, and even infl ammation. When it comes to the cause of these irritation symptoms, besides some genetic reasons in particular populations—the groups with the sensitive skin—and the environmental factors (12), they are probably connected with the penetra- tion of surfactants into the skin, resulting in adverse impacts to the skin substances such as proteins and lipids (4), which eventually lead to skin irritation. To avoid this, consum- ers prefer to choose mild facial cleansing products. Nevertheless, there is still a lack of Figure 3. Relationship of the surfactant monomers and micelles. Table II Four Categories of Surfactants and Typical Examples Category Examples Anionic surfactants Soap surfactants (RCOO¯)n M Sodium stearoyl lactylate Sulfate surfactants RO-SO3-M SDS Sulfonate surfactants R-SO3-M Sodium dodecyl benzene sulfonate Amino acid–derived surfactants Acylated amino acids (N-acyl l-glutamates and N-acyl glycinates) Nonionic surfactants Fatty alcohol ethoxylate surfactants Poloxamer Polyhydric alcohols surfactants Span class Cationic surfactants Benzalkonium bromide Dialkyldimethylammonium surfactants Amphoteric surfactants Lecithin Betaine

JOURNAL OF COSMETIC SCIENCE 96 fundamental understanding in the mechanism of surfactant penetration into skin. Since the beginning of the 21st century, a number of original researches have investigated various surfactants’ penetration and proposed the relevant penetration models to explain how these surfactants pass through skin. This review article summarizes the current under- standings and progress of the surfactant penetration into skin, mainly focusing on anionic and nonionic surfactants because they are frequently used in skin-care products. SKIN STRUCTURE Study of the human skin structure is essential to have a better understanding of how sur- factants penetrate and interact with skin. The skin is the largest organ of the human body. It works as a barrier to isolate and protect the internal organs and tissues from the exter- nal environment (13,14). The skin comprises three basic layers: subcutaneous tissue, der- mis, and epidermis. The basic structure of the human skin is presented in Figure 4. The subcutaneous tissue is the innermost layer of the skin structure. The average thick- ness of this layer is about 4–9 mm, but the actual thickness depends on the individual. The subcutaneous tissue contains some fat tissues which provide elasticity and work as a shock absorber for nerve endings and blood vessels including arteries and veins. The der- mis, a connective tissue layer between the epidermis and the subcutaneous tissue, is full of collagen and elastin fi bers, which provides a tough cell matrix for the human skin (15). In this layer, there are some arrector pili muscles, artery and vein vessels, and nerves extended from the subcutaneous tissue (16). In addition, some sweat glands, sebaceous glands, and hair follicles pass through the skin surface, originating from the subcutaneous layer and the dermis layer (17). The density of the hair follicles is approximately 10–70 on every centimeter square of the skin area. The outmost layer of skin termed the epidermis, which Figure 4. Human skin diagram (Reprinted from (16) with permission. Copyright 2012 Elsevier).