146 JOURNAL OF COSMETIC SCIENCE SDS and C12E6, each with about 0.5 iaCi/ml of •4C-radiolabeled SDS and 100 mM NaC1. We verified that the concentration of radiolabeled SDS in the contacting solution did not change appreciably during the five-hour exposure to the skin. A five-hour exposure was chosen because this time was sufficient to enable significant penetration of SDS into the skin, but short enough to prevent the saturation of the skin with SDS. The tem- perature of the diffusion cell was ambient, that is, 20 ø + 1 øC. After five hours of exposure, the contacting solution was removed, and the donor compartment was rinsed four times with 2 ml of PBS to remove any SDS that was not bound to the skin. The skin was subsequently heat-stripped by soaking it in a bath of water at 60 øC for two minutes, and the epidermis (SC and viable epidermis) was separated from the dermis. The exposed epidermis was then dried in a fume hood for two days and weighed. The dried epidermis was dissolved in 1.5 ml of Soluene-350 (Packard, Meriden, CT). Ten milliliters of Hionic Fluor scintillation cocktail (Packard) was added to the Soluene-350 after the epidermis was dissolved, and the concentration of radiola- beled SDS was determined using a Packard Tri-Carb 4350 scintillation counter. Know- ing the concentration of SDS in the contacting solution, Csos, the radioactivity of the contacting solution, Crad, do .... the dry weight of the epidermis, m, and the radioactivity of the epidermis, Crad, Jkin it was possible to determine the concentration of SDS in the dried epidermis, CsoS, skin using the following equation: Crad, skin ' CSDS Cszs, ,kin = C•, •on•' m (1) DYNAMIC LIGHT SCATTERING The SDS and the SDS/C•2E 6 solutions were prepared in Millipore filtered water with 100 mM NaC1. The 100 mM NaC1 was added to the surfactant solution to screen electrostatic intermicellar interactions in the DLS measurements (32-35). To prevent dust from interfering with the light-scattering measurements, the surfactant solutions were filtered through a 0.02-lam Anotop 10 syringe filter (Whatman International, Maidstone, England) directly into a cylindrical-scattering cell, and sealed until use. DLS was performed at 25 øC and a 90 ø scattering angle on a Brookhaven BI-200SM system (Brookhaven, Holtsville, NY) using a 2017 Stabilite argon-ion laser (Spectra Physics) at 488 nm. The autocorrelation function was analyzed using the CONTIN program pro- vided by the BIC dynamic light scattering software (Brookhaven, Holtsville, NY), which determined the effective hydrodynamic radius, Rh, using the Stokes-Einstein relation (36): kBT -- Rh - 6 •r•l• (2) where k•3 is the Boltzma__nn constant, T is the absolute temperature, x I is the viscosity of the salt solution, and D is the mean diffusion coefficient of the scattering species. In order to eliminate the effects of intermicellar interactions when measuring the hydro- dynamic radii of the micelies, the effective hydrodynamic radii were determined at several different total surfactant concentrations having a fixed solution composition, and the average effective hydrodynamic radii were extrapolated to a zero micelie concentra- tion (32-35,37).

PENETRATION OF MIXED MICELLES INTO THE EPIDERMIS 147 MICELLIZATION BEHAVIOR OF THE SDS/Ct2E 6 SURFACTANT MIXTURES In this paper, o• x denotes the fraction of the total surfactant that is SDS, referred to as the SDS composition, and is defined as follows: Otx = ismSix q_ [C12E6]x (3) where [SDS] denotes the concentration of SDS, [C•2E6] denotes the concentration of C12E6, and the subscript x refers to the monomeric fraction (x = 1), to the miceliar fraction (x = m), or to the overall solution (x = s). Accordingly, % -- 0.83 implies that 83% of the surfactant in the contacting solution is SDS, and that the remaining 17% (1 - % = 0.17) is C12E 6. Recently developed molecular-thermodynamic theories of mi- cellization (30,31) were used to predict the micellization behavior of the SDS/C12E6 surfactant mixtures. Specifically, the concentration and the composition of the surfactant monomers and of the mixed micelles were predicted as a function of the total surfactant concentration and solution composition. The resulting predicted values of oq, O•m, and the total surfactant monomer concentration, C•, for the contacting solutions examined are reported in Tables I and II. RESULTS AND DISCUSSION EFFECT OF ADDING Ci2E6 AT A FIXED SDS CONCENTRATION ON THE PENETRATION OF SDS INTO THE EPIDERMIS It is well known that when two surfactants that interact synergistically are mixed, the surfactant mixture often exhibits lower skin irritation than either of the individual surfactants (6,24,26). It is also known that SDS and C12E 6 interact synergistically to reduce the CMC of the surfactant mixture (30,31). SDS is a model skin irritant (10,13,15,16,26,38), while Cx2E 6 is thought to be a mild surfactant, although it may lead to skin dryness (3,39). The system of SDS and C12E 6 was chosen as a model surfactant mixture because of the synergy that it exhibits, as well as because the skin irritation potential of this surfactant mixture is expected to result primarily from the action of the irritating surfactant, SDS. This, in turn, allows us to relate the penetration of SDS into the epidermis to skin irritation, while neglecting the irritation potential of C•2E 6. Evidence for the relationship between the concentration of SDS in the epidermis and the skin irritation induced by SDS was presented in our recent paper (28), in which the concentration of SDS in the epidermis was observed to be dose-dependent for % = 1, Table I Predicted Values of oq and o• m for Mixtures of SDS and C12E 6 in 0.1 M NaC1 at the Various SDS Concentrations and Solution Compositions (O•s) Used for the SDS Skin Penetration Experiments (30,31) 25 mM SDS 50 mM SDS 100 mM SDS O• s 0•1• O• m 0•1, O• m 0•1• O•rn 1 1,1 1,1 1,1 0.83 0.96, 0.83 0.96, 0.83 0.96, 0.83 0.50 0.925, 0.50 0.925, 0.50 0.925, 0.50