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j. Soc. Cosmet. Chem., 48, 253-274 (September/October 1997) Review of properties of surfactants that determine their interactions with stratum corneum LINDA D. RHEIN, SmithKline Beecham, 1500 Littleton Rd., Parsippany, NJ 07054. Accepted for publication November 1 O, 1997. INTRODUCTION Surfactants have unique properties in solution that impact their interaction with sur- faces, with skin being one of those surfaces frequently contacted. Numerous investigators have studied the interaction of surfactants with intact stratum corneum, the horny (keratinaceous) membrane on the outermost part on the skin. Since roughly 70% of the dry weight of stratum corneum is proteinaceous, one therefore might foresee the op- portunity for considerable interaction with sites on the protein. Certain surfactants have been known to denature the secondary structure of proteins (1). Interactions of surfac- tants with insoluble proteins comprising stratum corneum are less well studied. Two parameters are considered in this review in relation to surfactant interactions with stratum corneum: (a) binding to the membrane and (b) membrane swelling. Within these contexts, the impact of surfactant solution properties, the role of pH, and inter- actions in mixed surfactant systems with stratum corneum are discussed with relevance to skin irritation. BINDING Because surfactants produce certain undesirable effects such as irritation in the tradi- tional sense (erythema, edema, and scaling) and perceived effects on the skin such as tightness and dryness, it is plausible that they are in fact binding in some manner to the structure. The binding of alkyl sulfate type of surfactants to human callus was investi- gated by Imokawa and Mishima (2). Figure 1 shows the surfactant sorption isotherms of the homologous series of alkyl sulfates to callus measured in vitro. It is apparent that they follow Langmuir's model, suggesting a limited number of saturable binding sites. However, it was further observed that the apparent saturation of the isotherms occurs at approximately the critical micelie concentration (CMC) for each surfactant. Because the concentration of surfactant monomers becomes constant above the CMC, this would limit the number of monomers (or submicellar species) available to bind to the callus. 253