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J. Cosmet. Sci.! 58, 229-244 (May/June 2007) Why is sodium cocoyl isethionate (SCI) mild to the skin barrier?-An in vitro investigation based on the relative sizes of the SCI micelles and the skin aqueous pores SASWATA GHOSH and DANIEL BLANKSCHTEIN, Department of Chemical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139. Accepted for publication March 6, 2007. Synopsis Sodium cocoyl isethionate (SCI) is an important surfactant ingredient in mild, syndet (synthetic detergent) cleansing bars. In vitro and in vivo studies have demonstrated that SCI is mild and less damaging to the skin barrier than soaps and surfactants such as sodium dodecyl sulfate (SDS). We have recently shown that SDS forms small micelles in aqueous solutions contacting the skin relative to the aqueous pores in the stratum corneum (SC), and as a result, the SDS micelles can contribute to SDS skin penetration and induce skin barrier perturbation. In this paper, we attempt to explain the well-documented skin mildness of SCI by examining the size of the SCI micelles relative to that of the aqueous pores in the SC. For this purpose, we have conducted in vitro mannitol skin permeability and average skin electrical resistivity measurements upon exposure of the skin to an aqueous SCI contacting solution in the context of a hindered-transport aqueous porous pathway model of the SC. These in vitro studies demonstrate that an SCI micelle of radius 33.5 ± 1 A (as determined using dynamic light-scattering measurements) experiences significant steric hindrance and cannot penetrate into the SC through aqueous pores that have an average radius of 29 ± 5 A. We believe that this inability of the SCI micelles to contribute to SCI skin penetration and associated skin barrier perturbation is responsible for the observed skin mildness of SCI. Through in vitro quantitative skin radioactivity assays using 14 C-radiolabeled SCI and pig full-thickness skin (p-FTS), we also show conclu­ sively that SCI skin penetration is dose-independent, an important finding that provides additional evidence that the larger SCI micelles cannot penetrate into the SC through the smaller aqueous pores that exist in the SC, and therefore, cannot induce skin barrier perturbation. INTRODUCTION Sodium cocoyl isethionate (SCI) is an important surfactant ingredient in personal wash­ ing bars, and is known to be milder, or less irritating, to the skin than harsher skin agents, such as other anionic surfactants like sodium dodecyl sulfate (SDS) and soap (sodium salts of fatty acids such as stearic and palmitic acids) (1-8). In vivo and in vitro studies have shown that SCI does not significantly reduce the barrier properties of the Address all correspondence to Daniel Blankschtein. 229