466 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 1.0 0.8 0.6 .N_ 0.4 E o z 0.2 _ .,. ,, ........., \. •p. 12.0 "......... ' \'\- pH 13.0 'x. 0.0 - • ' •, tx.,,, I I I .......... t ............ I ...... I I 0 2 4 6 8 10 12 14 16 18 Time (minutes) Figure 4. Effect of pH on stress decay in 0.5 M TGA. reaction, as neither the pH 11 buffer or peroxide at pH 7 (where little oxidation occurs) had a significant effect on the subsequent rate of reactant-induced stress decay. THE EFFECT OF PH ON REDUCTION RATES The rate of reduction of hair by thiol-containing reducing agents is strongly dependent on pH (2,3,5). SFTK curves for TGA in the range of pH 9 to 11 have been reported previously (3). The largest increase in reduction rate was between pH 9 and 10, prob- ably due to increased ionization of the thiol group, which has a pKa of about 10.0. The rate was found to Continue to increase from 10 to 11. Figure 4 shows that this increase in reduction rate with pH continues, at least to pH 13. Increases in rate at pH values well above the pKa of the thiol may be due to swelling of the hair at high pH. COMPARISON OF REDUCING AGENTS T95 % data from several different reducing agents at three different pH's are compared in Table III. The relative ranking of these reducing agents depends upon the pH of the comparison. Previously, dihydrolipoate (DHL) was found to be faster than thioglycolate at pH 10.0 and equivalent at pH 11.0, and the rate of reduction with DHL did not increase between pH 10.0 and pH 11.0 (3). In the present study, DHL was found to be faster than TGA at pH 10.5 and slower at pH 11.5. Ammonium sulfide, on the other hand, was slower than TGA at pH 10.5 and faster at pH 11.5. Sodium sulfide at pH 13.0 was the fastest system tested. EFFECT OF SOLVENTS ON REDUCTION RATE WITH TGA N-propanol has been reported to increase the extent of reduction of wool protein by

SINGLE FIBER DEPILATORY STUDIES 467 Table III Effects of Various Reducing Agents on T95 % Reducing Agent pH T95 % (min.) Na TGA 10.5 14.0 Na dihydrolipoate 10.5 9.2 [3 mercaptoethanol 10.5 11.0 (NH4)2S 10.5 20 min. Cysteine Methyl Ester 10.5 20 min. Dithiothreitol 10.5 4.5 min. Na TGA 1 l. 5 8.0 Na dihydrolipoate 11.5 10.0 [3 mercaptoethanol 11.5 5.0 (NH4)2S 11.5 4.5 Na TGA 13.0 3.0 Na2S 13.0 1.0 Saturated BaS (0.5 M) 13.0 5.0 All reducing agents 0.5 M except BaS. thioIs at near neutral pH (7). Table IV shows the effect of n-propanol on T95 % with 0.5 M TGA at pH 11.5. Under these conditions, T95 • was increased slightly at 10-20% n-propanol. At higher concentrations, the rate of stress decay was clearly slowed, sug- gesting a marked decrease in reduction rate. The effects of several other solvents on T95 • are shown in Table V. Ethanol and acetone did not have a significant effect on T95%, and triacetin, glycerin, and propylene glycol all increased T95 %. These decreases in reduction rate with solvent may be due to de- creased water activity, leading to less swelling of the hair and slower penetration of the reducing agent. EFFECT OF EMULSION STATE ON REDUCTION KINETICS A cosmetically acceptable depilatory is likely to be in the form of an emulsion. Thus it is important to understand the effect of emulsion state on depilatory performance. Figure 5 shows SFTK data obtained using emulsion systems containing mineral oil, water, sodium thioglycolate, and the necessary surfactants to produce the desired emul- sion state (see Appendix), all at pH 10.5. System 1, a W/O emulsion, showed very little activity. Perhaps the oil-continuous phase coated the hair surface and prevented the TGA from penetrating from the aqueous phase. The highest activity was exhibited Table IV Effect of n-propanol on T95•o System T95% A. 0.5 MNaTGA, pH 11.5 B. A + 10% n-propanol C. ,A + 20% n-propanol D. A + 40% n-propanol E. A + 50% n-propanol 6 min. 9 min. 8 min. 12 min. 20 min.