SURFACTANTS AND STRATUM CORNEUM 255 of rate of sorption. Again the rate function for surfactant uptake vs concentration at one hour is biphasic and intersects at the CMC of SLS. (Figure 2). The CMC of SLS is between 7 and 10 raM, depending on the conditions in various studies. Faucher and Goddard (3) also examined binding of sodium lauryl ether sulfates con- taining different levels of ethoxylation to bleached human hair, a keratinaceous substrate similar to stratum corneum (Figure 3). They found that increasing the degree of ethox- ylation concomitantly decreased binding. Various explanations are possible for this phenomenon. As ethoxylation increases, the molecular size increases, making it more difficult to penetrate the keratin matrix. Additionally, the CMC decreases, and thus the surfactant monomer level decreases, rendering less surfactant available to interact with the substrate, assuming the monomer is the interactive species. The relationship of CMC to skin reactivity of surfactants will be addressed in detail in a later section. More recent studies of Ananthapadmanabhan et al. (4) examined binding of surfactants to stratum comeurn in greater detail. They looked at a broad concentration range, along with time, temperature, and pH effects. Figure 4 shows binding of SLS and cocoyl isethionate at doses up to 80 mM, and the findings again confirm a change of slope at the CMC. Like the results of Faucher and Goddard, binding continues to increase but at a slower rate above the CMC. Thus while monomer concentration is important to binding, it does not totally dictate the binding parameters, a phenomenon that will be discussed later. Not unexpectedly, binding also increases with increasing temperature. The relationship of binding to time of exposure was surprising (see Figures 4 and 5) (data 60 ul 50 o • 40 ß .-, 30 • 20 lO Stratum Corneum Bleached Hair o o 1 2 3 4 5 6 7 8 9 lO CONCENTRATION OF SIS IN PERCENT Figure 2. Concentration dependence of sorption rate: uptake of sodium lauryl sulfate by stratum comeurn and bleached hair. Absorption of SLS is measured as percent increase over the weight of the dried, rinsed substrate. Data taken from ref. 3.

256 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS I- 6 I&l •3 3 SLS ES-2 ES-3 I 130 E o o 2 4 6 8 lO 12 HOURS Figure 3. Sorption of sodium lauryl ether sulfates by bleached hair from 10% solutions. Absorption of SLS is measured as percent increase over the weight of the dried, rinsed substrate. ES is ether sulfate. Data taken from ref. 3. not shown). Binding of SLS increased up to six hours but decreased at 24 hours compared to the other time points, mostly at doses beyond 40 mM (4). This suggests changes in stratum corneum integrity at the longer incubation times. It is well known that SLS has a profound denaturing effect on protein secondary structure (1). Indeed, both stratum corneum and synthetic collagen membrane swell and denature in the presence of SLS and other surfactants, and for the former it appears reversible up to 35 mM. At longer time points, 1% soap irreversibly swells the membrane and the membrane begins to fall apart (5). The surfactant may interfere with the intercorneal cohesion (6,7), perhaps at the corneocyte envelope, where it may bind hydrophobically to the covalently bound lipid, thereby causing the lameliar lipid to detach. Indeed, Imokawa has shown in electron micrographs of stratum corneum treated with 5% SLS that the multilamellar structures were no longer visible in the intercellular spaces. One can speculate that SLS has blocked the attachment of the lameliar structure to the corneocyte envelope, causing the mac- rostructure to disintegrate. Direct effects of SLS and other surfactants on actual removal of stratum corneum lipids from the membrane per se, which is another damaging effect that might account for the altered integrity of the stratum corneum, are not well substantiated (5). Most studies show minimal effects of SLS on lipid removal both in vitro (8) and in vivo (9-14), although some methods of exposure (11), e.g., cup shake on the skin surface, may release corneocytes and associated lipid, presumably because SLS disrupts the intercorneal cohesion as described above this results in somewhat more apparent lipid removal along with corneocytes.