ANIONIC SURFACTANT RINSABILITY 83 spectra obtained using bar slurries with and without fluorescein are shown along with the difference spectra. The three difference spectra are also compared in Figure 11. It is clear from these figures that the solvent extracts from skin treated with all three bar slurries and rinsed with 200 ml of water at room temperature contain an essentially negligible level of fluorescein as measured by the absorbance at k = 485 nm. The actual absorbance values at 280 nm and 485 nm are given in Table IVb for easy comparison. In the 280 region, the isethionate-based Bar A does not give any measurable peak, with Bar B giving some positive value and Bar C, in fact, giving a negative value. If the absorbance at k = 280 nm were due to fluorescein, then a corresponding peak at k = 485 nm should also have been present. However, the absence of absorption at 485 nm suggests that the small absorption that was observed at 280 nm is due to contamination from skin components extracted into the methanol-water solvent system. IN VIVO STUDIES ON HUMAN SKIN Binding of fluorescein to human skin i, vivo from tap water and bar slurries is shown qualitatively in the form of photographs in Figure 12. As can be seen, there is more fluorescein deposition from water than from the bar slurries. Interestingly, the deposi- tion from water at pH 7 was substantially higher than that at pH 9.2, indicating the inverse relationship between solubility and deposition. The deposition from Bar A was higher than that from Bar B under the mild rinsing conditions with 100 ml water. Noting that the pH of Bar A slurry is 7 and that of Bar B is 9.2, the extent of deposition is a likely reflection of the behavior of fluorescein at different pH values. Importantly, no fluorescein deposition was observed from Bar A slurries when the rinsing was done under running tap water for 30 seconds. 280 nm Solvent System: 4:1 MeOH: water Bar ^ __ Bar A/dye __ •. Bar A diff / 0.2 0.• 0 43.1 -O2 250 300 350 400 450 500 550 Wavelength 0.4 0.3 0.2 Bar B 280 m Solvent System: 4:1 MeOH: water __ Bar.B./dye '...-' Bar B d•ff 0 -O.1 -O'•5 300 350 400 450 500 550 Wavelength • 0 -O,1 280 nm Solvent System: 4:1 MeOH: water Bar G __ Bar C/dye __ Bar C d•ff __ . _. 0.3 0.2 -O'•50 300 350 400 450 500 550 Wavelength 280 nm Bar A diff __ Bar B dlff ... Bar C d•ff -0.4 250 300 350 400 450 500 550 Wavelength Figure 11. U¾/¾IS spectra of methanol:water (80:20) extracts of porcine skin treated with bar slurries with and without fluorescein and the delta absorbance spectrum. Bottom figure on the right shows all three difference spectra for comparison.

84 JOURNAL OF COSMETIC SCIENCE .v. ß ,,, pH 9 pH 7 Figure 12a. In vivo deposition of fluorescein on human forearm from water at two pH values, showing that more deposition occurs at pH 7 than at pH 9. Fluorescein concentration 50 ppm, contact time 15 sec, wash with 100 ml tap water. .: "Bar B: 100ml ß . --•. tap water rinse ß ,, ., Bar A: 100ml tap water rinse " Bar A: 30 sec tap water rinse Figure 12b. In vivo deposition of fluorescein on human forearm from 10% slurries of Bar A and Bar B. Fluorescein concentration 50 ppm, contact time 15 sec, wash with 100 ml tap water or 30 sec of running tap water. Under realistic rinsing conditions (30 sec of running tap water), fluorescein does not deposit from Bar A slurries.