80 JOURNAL OF COSMETIC SCIENCE Table II Fluorescein Tests With Human Stratum Corneum: Absence of Fluorescein Deposition (485 nm) From Bar Slurries and Most Deposition From Water Absorbance System 280 nm 485 nm Water 0.56 0.48 Bar A a 0.20 0.01 Bar B a 0.18 0.01 Bar C a 0.18 0.01 a 280-nm peak due to protein extraction by solvent. Experimental conditions: 2 cm x 1.5 cm human stratum corneum pieces, 10% soap slurry, 500 ppm fluo- rescein, 15-sec soak, rinse with ample amount of water. Absorbance values 0.01 are within experimental error. The peak at 280 nm is entirely due to skin components extracted by the solvent system. Interestingly, Table II also indicates that only extracts taken from skin treated with an aqueous dispersion of fluorescein in tap water exhibited any absorbance at • = 485 nm. The latter suggests that some fluorescein deposition occurred from water, and this was also consistent with the visual observation of color remaining on the corneum after rinsing. The mechanism of deposition of fluorescein from water is not clear at present. Although molecular binding may occur, retention of fluorescein may also arise from small particles being trapped in crevices in the skin and not being rinsed away, since fluorescein is not soluble in water at the 50-ppm level employed. This trapping process may be further enhanced by the presence of calcium in the tap water. Fl•orescein retention and s•rfactant binding from p•re s•rfactant soh•tions. Fluorescein reten- tion on human stratum corneum was also determined in the presence of several pure surfactant solutions whose molecular binding to stratum corneum was already known from previous measurements employing •4C-labeled species (3,21). The surfactants se- lected were sodium dodecyl sulfate (SDS), sodium lauroyl isethionate (SLI), sodium/TEA oleate, and sodium/TEA laurate (TEAL). The binding isotherms for a one-minute equili- bration followed by a rinse in room-temperature distilled water [back-and-forth motion (5x)] are shown in Figure 9. These results show that oleate binds in high levels at low concentrations, but that at concentrations above 20 raM, laurate binds more than even oleate. Binding of SDS and TEAL appear to be comparable at high concentrations. SLI, on the other hand, appears to bind the least at surfactant levels above 20 mM. The delta absorbance values of methanol-water extracts from human stratum corneum treated with 40 mM surfactant solution spiked with 50 ppm fluorescein are collected in Table III (one-minute treatment at room temperature and rinse with an ample supply of distilled water). The references in this study were methanol-water extracts from stratum comeurn treated under identical conditions as the same surfactant solutions that did not contain fluorescein. The measured surfactant binding by radioisotope analysis is in- cluded for comparison. Several points should be noted from the results in Table III. First, there is no correlation between the extent of surfactant binding to the stratum corneum and the level of fluorescein extracted (see also Figure 10). In fact, in none of the surfactant systems

ANIONIC SURFACTANT RINSABILITY 81 E 0.03 o 0.025 E 0.02 E • 0.015 i• O.Ol "" 0.005 TEA-Na Laurate TEA-Na Oleate SDS SLI 03 0 , I , I , 0 10 20 30 40 50 60 Surfactant Concentration, mM Figure 9. Surfactant binding to human stratum corneum at 37øC in one minute. Table III Fluorescein Deposition From Pure Surfactant Solutions--No Correlation With Surfactant Binding (Most Fluorescein Deposition From Water) 485 nm Treatment (A Absorbance) Surfactant binding (pg surfactant/mg HSC) Water 0.052 0 40 mM Na laurate 0.01 20 (0.00056) 40 mM SDS 0.01 15 (0.0012) 40 mM SLI 0.01 9 (0.0022) Experimental conditions: Human stratum corneum, 1-min contact with surfactant solutions/water with 50 ppm fluorescein, rinse with water. Absorbance values 0.01 are within experimental error. Binding values are accurate to about 1 lag surfactant/mg HSC. studied could any fluorescein residue be visually detected on the stratum corneum. If at all, the absorbance at X -- 485 nm suggests an inverse correlation between fluorescein retention and surfactant binding, i.e., most deposition from water and least from Na laurate. This is again direct evidence that fluorescein does not partition into surfactant aggregates that are strongly bound to the stratum corneum proteins. Thus, fluorescein is essentially completely rinsed from the stratum corneum by the reasonably mild rinsing conditions employed, even though significant bound-surfactant residues exist especially for SDS and NaL. As in the earlier discussed results, the only case where fluorescein remains bound to the corneum is when it is present alone in water. Clearly, fluorescein retention on corneum has no relationship to surfactant binding to corneum.