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j. Soc. Cosmet. Chem., 46, 153-162 (May/June 1995) A spectroscopic method for the evaluation of the substantivity of N-lauroyl sarcosinate to human skin DURGA V. SUBRAMANIAN, Hampshire Chemicals Corporation, 2 East Spit Brook Road, Nashua, NH 03060-5737. Received.July 29, 1993. Presented at the Annual Scientific Seminar of the Society of Cosmetic Chemists, Baltimore, Maryland, May 6-7, 1993. Synopsis Substantivity is an important property of ingredients in cosmetic formulations. This paper describes a novel analytical technique for the study of adsorption of sarcosinate surfactants on live human skin. The perdeu- tero analogue of lauroyl sarcosinate was chosen as the model for the surfactant. The skin after contact with the deutero surfactant was rinsed and examined by ATR-infrared spectroscopy. The presence of spectral bands at frequencies characteristic of C-D bonds provides evidence for the adsorption of the surfactant on skin. It is shown that the surfactant adsorbs at 10-[xg/cm 2 levels and that the pH of the surfactant solution has a significant impact on its substantivity. The analytical method described in this paper may be useful for the investigation of the sorption of other materials on skin and other substrates. INTRODUCTION N-Acyl sarcosinates are anionic surfactants. Their surface-active properties, biodegrad- ability, mildness, and adsorption to substrates have rendered them valuable in a variety of applications (1). Several commercial personal care products, such as soap bars and shampoos, are formulated with acyl sarcosinates. It is reported that the sarcosinates in the shampoos form a thin film on the hair and thus aid wet and dry combing by providing lubrication (2). The adsorption of acyl sarcosinates to human hair and other proteinaceous materials has been demonstrated by Nelson and Stewart (3). Their research on the substantivity of sarcosinates employed radiolabeled N-acyl sarcosines as the surfactant. Substrates were soaked in the radioactive surfactant solution for a set period of time and then were thoroughly washed. The residual radioactivity in the substrate was measured and taken to indicate the amount of surfactant sorbed. Similar adsorption of the surfactant may be responsible for the skin feel experienced after the skin is washed with sarcosinates. The goal of our research was to establish that sarcosinates adsorb on live human skin. Analytical methodologies employing radioactive surfactants could not be used with live human skin as the substrate because of the hazardous nature of radioactive compounds. 153