JOURNAL OF COSMETIC SCIENCE 224 MICELLE/MACROMOLECULE SIZE ANALYSIS A pilot study of micelle, polymer, and/or surfactant/polymer complex size in the test formulations was conducted by dynamic light scattering. Approximately 1.5 ml of test formulation (50 mM SLS with or without added polymer) was fi ltered through a 0.1-μm syringe fi lter. The fi rst 0.25 ml was discarded to avoid contamination. The fi ltered solu- tion was poured into a disposable polystyrene cuvette and analyzed via a 173°-backscatter measurement using a Zetasizer (Malvern Instruments Ltd., Malvern, UK). Results were reported as scattering intensity versus hydrodynamic radius, rh. Because the SLS–polymer formulations contained an excess of each polymer relative to its ability to bind SLS [cf. Cabane (11) for the SDS/PEO system see also Goddard (6)], and the SLS concentrations were high relative to its tendencies to self-aggregate and bind to polymers (see next sec- tion), it is probable that the rH values obtained for the SLS–polymer compositions repre- sent a weighted average of polymer and SLS/polymer aggregate radii. In the absence of polymers, the measurements represent the characteristic size of SLS micelles for a repre- sentative commercial SLS sample. CMC AND RELATED SURFACE TENSION MEASUREMENTS The surface tension, γ, of each test formulation as a function of concentration was deter- mined using a Krüss K100 tensiometer (Krüss USA, Matthews, NC) fi tted with a Wil- helmy plate. The measurements were made by sequentially diluting the test compositions with DI water thus, the ratio of SLS and polymer was maintained constant. Apparent CMC for SLS and critical aggregation concentration (CAC) for the SLS–polymer mixtures were determined from infl ection points on a plot of γ versus log concentration as de- scribed later. These methods are approximate and should not be construed to replace more precise measurements made using pure SDS and constant polymer concentrations (11,12). Test formulation solutions were prepared with Millipore water (17.8 M) and studied 1–3 times. RESULTS 14 C-SDS PENETRATION INTO PORCINE SKIN Figure 1 shows the 14 C-SDS penetration results on porcine skin. Total recovery of radio- label in the SC after a 10-min exposure to the 50 mM SLS control was ~100 μg/cm2 14 C-SDS equivalents, corresponding to 3.7% of the applied radioactive dose. Addition of 2% PEO to this formulation reduced penetration by ~80%. This result is consistent with values previously reported by Moore et al. (1). Addition of 2% PVA yielded a similar result. 14 C-SDS PENETRATION INTO HUMAN SKIN Figure 2 shows 14 C-SDS penetration into and through human skin after a 10-min exposure. Nearly all of the recovered radioactivity was found in the tissue, with 1% penetrating into

PRECLINICAL SURFACTANT SKIN PENETRATION ASSAY 225 the receptor solutions. Tabular results showing the distribution are given in the Appendix. The 50 mM SDS control showed ~16 μg/cm2 14 C-SDS equivalents was recovered from the human skin membrane–receptor solution, an approximately sixfold reduction from the value measured in porcine skin. The treatment with 2% PEO was observed to signifi cantly reduce skin penetration. Unlike the porcine skin result, reduction of 14 C-SDS skin pene- tration by 2% PVA was not statistically signifi cant. To test an exposure time closer to consumer use, a 2-min study was conducted. Figure 3 shows the penetrated 14 C-SDS dose the distribution is given in the Appendix. The SLS control formulation yielded an average of 7.3 μg/cm2 14 C-SDS equivalents in the human skin membrane–receptor solution, with only 0.1% of this total found in the receptor so- lution. In the presence of 2% PEO, the amount of SDS recovered in the skin was signifi - cantly reduced. The presence of 2% PVA did not yield a statistically signifi cant reduction in penetration. MICELLE/MACROMOLECULE SIZE ANALYSIS The 50 mM SLS composition yielded a scattering intensity distribution peaked at a hydrodynamic radius (rh) of 1.14 ± 0.27 nm (mean ± SD, n = 6). SLS + 2% PEO yielded a bimodal intensity distribution with the smaller component centered at rh = 1.52 ± 0.10 nm (n = 3). SLS + 2% PVA yielded a bimodal size distribution with the Fi gure 1. Penetration of 14 C-SDS into porcine skin after a 10-min exposure. (A) Total SDS recovered in the porcine SC (B) Penetration profi le into SC obtained from D-Squame tape strips. The applied dose corre- sponded to 2740 μg/cm2 of 14 C-SDS equivalents.