J. Soc. Cosmet. Chem., 36, 287-296 (July/August 1985) ESCA studies on skin lipid removal by solvents and surfactants M. K. BAHL, Colgate-Palmolive Co,, Research Center, 909 River Road, Piscataway, NJ 08854. Received February 14, 1985. Synopsis Electron spectroscopy for chemical analysis (ESCA) provides valuable in vitro information about the ele- mental and chemical composition of the skin surface to a depth of about 50 Angstroms. ESCA is especially useful for evaluating the removal of skin lipid from epidermis due to treatments with solvents and surfactants. This is done by measuring changes in the skin's atomic percentage of nitrogen which increases with lipid removal. Extraction of structural lipid from the skin surface depends on the polarity of the solvent. Ethanol extracts the most lipid, followed by acetone and then ether. Skin lipid removal due to solvents and surfactants is always greater from the inner side of the epidermis compared to the exterior side. Skin lipid removal by sodium lauryl sulfate (SLS), is greater than for sodium laureth sulfate--6.5 (AEOS EO-6.5). This lipid removal activity is consistent with the greater skin irritation and the greater swelling produced by SLS. INTRODUCTION An important function of the stratum corneum of skin is to limit the penetration of substances into the living layers (1,2). Various studies (3,4) have suggested that the barrier property of epidermis is associated with the filamentous protein and the lipids. Indeed, electron microscopic observations (5) have indicated a lipid layer surrounding the fibrous keratin in the stratum corneum. On the other hand, no evidence for such layers could be found in hard keratin such as hair and feathers (6). However, recent studies indicate that stratum corneum lipid is intercellular in location rather than associated with the intracellular filamentous protein (7,8). Requirement of the lipids for the barrier property of the epidermis was further demonstrated by Onken and Moyer (9) who restored the barrier property of the epidermis by adding the extracted lipids back to the extracted original preparation. The barrier property of the skin is generally evaluated from permeability measurements (4). The barrier property of lipid-protein complex in epidermis can be disturbed by a variety of polar organic solvents and surfactants (4, 10). Treatment of the skin with polar organic solvents, surfactants, or detergents removes lipids from the stratum cor- neum (the skin's major rate-limiting permeability barrier) and radically increases its permeability. However, an increase in permeability is not always necessarily due to the 287

288 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS extraction of lipids, e.g., the permeability of skin increases due to hydradon of the skin. An increase in permeability of the skin can therefore be accounted for on the basis of various factors such as extraction of lipid, membrane expansion, and protein denaturation. We report here for the first time ESCA studies on skin samples with the objective of estimating the relative amount of lipid removal from the skin surface due to subjecting it to certain solvents and surfactants. Electron spectroscopy for chemical analysis (ESCA) (12, 13) involves irradiation of the sample with monochromatic X-rays which causes core electrons to eject from various atoms present in the sample. Only electrons originating from atoms lying within 50fk of the surface have a high probability of escaping from the sample without any energy loss. The binding energy of the core electrons EBE provides information about the chemical composition of the sample surface: EBE = Ex ray -- EKE + Constant The number of electrons with a specific EKE is directly proportional to the concentration of a particular atom within about 50• of the surface. The present investigation demonstrates that ESCA is a sensitive and useful method to study the lipid removal from the surface of the epidermis, due to treatment with solvents and surfactants. It is further shown that ESCA differentiates skin lipid removal from the two sides of the epidermis. It is suggested that skin lipid removal by these solvents and surfactants correlates with changes in skin permeability, skin swelling, and skin irritation due to these solvents and surfkctants. EXPERIMENTAL SKIN SAMPLES Frozen, abdominal cadaver skin samples from hospital autopsy procedures were ob- tained. After partial thawing, the fat layer was completely removed with dissecting scissors. The remaining dermis and epidermis were placed in a water bath, 54 -- 2øC, for 30-60 seconds. The sample was removed from the water and laid with the epidermis side facing up. Carefully the stratum corneum was scraped-rolled-separated from the dermis using a soft, fiat polyethylene spatula. This epidermal membrane sheet was then floated on warm water (20øC) and gently manipulated until it was a single fiat sheet. This sheet was lifted out on screening and air dried. Strips of epidermal mem- branes (1/2 inch X 1/2 inch) were cut from this dried epidermis for treatment and ESCA studies. ESCA MEASUREMENTS ESCA measurements were performed in a Perkin-Elmer 560 ESCA/SAM analysis system. This system is equipped with a dual anode Mg/A1 X-ray tube, differential pumped ion gun and a SIMS II system. The skin samples were clamped between indium foils and in most cases both sides of the epidermal membrane were studied for elemental analysis. Sample size was approximately 1/2 inch X 1/2 inch. All ESCA spectra were taken with a PHI Model 25-270 AR double pass cylindrical mirror analyzer operating in the large aperture mode and at a pass energy of 100 eV for survey scans. High resolution spectra were taken at pass energies of 25 and/or 50eV. The area of the skin