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.

82 JOURNAL OF COSMETIC SCIENCE 0.06 0.05 0.04 0.03 0.02 0.01 Human Stratum Corneum 37 C, 1 minute contact SDmS TEA-_Na lau,rate 0 0 5 10 15 20 25 Surfactant Binding, ug/sq.cm of corneum Figure 10. Fluorescein deposition vs surfactant binding on human stratum corneum. Deposition in ab- sorbance units at 485 nm. More deposition from water than from surfactant solutions. FLUORESCEIN RETENTION ON PORCINE SKIN F/uorescein retention from water. The results of fiuorescein deposition from water on porcine skin using 1% fluorescein are given in Table IVa. The absorbance values represent the difference between the methanol:water extracts of skin samples treated with water containing fluorescein and those without the fluorescein. Interestingly, significant fluo- rescein deposition was observed at pH 5 (the native pH of the 1% fluorescein dispersion). At pH 8, where the fluorescein was made completely soluble with base (TEA) addition, the extent of deposition was relatively low compared to that at pH 5. These results indicate that the extent of fluorescein deposition is inversely related to its solubility in the medium in which it is dispersed. F/uorescein retention from bar s/urries. The spectra of methanol:water extracts from porcine skin treated with slurries of the cleansing bars A, B, and C are shown in Figure 11. The Table IVa Delta Absorbance Values Showing Fluorescein Deposition (485 nm) From Water Product 280 nm 485 nm 1% Fluorescein (native pH -5) 0.28 0.47 1% Fluorescein (pH = 8) 0 0.25 Experimental conditions: Porcine skin, 15-rain treatment, rinse with 100 ml water (260-ppm hardness), extraction with 2 x 10 ml 80:20 MeOH:water. Delta absorbance is the difference in absorbance between methanol:water extracts of skin treated with water in the presence and absence of fluorescein.