PERMANENT WAVING: POST-YIELD SLOPE 697 0.800 • 0.60• I-- 0.400 o,. 0.200 o ORIENTAL HAIR pH 6.5 46 øC 12% ACTIVE WAVING AGENT 10 HairslMandrel =(1) NH 4 + MERCAI•TOPROPiC m(2) MEA + MERCAPTOPROPH (3) GLYCERYL MERCAPTOPR ..... (4) GLYCERYL MONOTHIOGL i ! i 'IONATE tOPIONATE .•LYCOLATE 10 20 30 40 50 TIME (MINUTES) Figure 8. Variation of post-yield slope with time for different mercaptans. not contrary to theory since equilibrium may not be attained in 20 min at these lower pH values, even at 17% GMTG concentration. For mercaptans of similar acidities, other parameters such as ionic nature and molecular size can become important. A comparison of four commercially available mercaptans on Oriental hair is presented in Figure 8. The ammonium salt and the monoethanolamine salt of mercaptopropionic acid show almost identical behavior at pH 6.5 and 46øC. Both are slower than the ester, glyceryl mercaptopropionate. The pKa of mercaptopropionic acid is 10.2, while the pKa ofgly- ceryl mercaptopropionate can be estimated at 7.8 (15). Herrmann (8) explains the variation in penetration rates between mercaptans as the result of the electrostatic interactions between ionizable groups in the mercaptan's structure and ionizable groups in the keratin fiber. For compounds such as glyceryl mercaptopropionate which contain no ionizable groups other than sulfhydryl, penetration is rapid at low and neu- tral pH values (6.5). However the mercaptopropionate salts have an ionizable carboxy- late group which is negatively charged at pH 6.5 as is the net charge on the keratin fiber. Electrostatic repulsion between these negative charges reduces the rate of fiber penetration. Herrmann found that the slowest rates of penetration for thioacids occur- red around pH 6. While glyceryl thioglycolate has a pKa comparable to glyceryl mercaptopriopionate and contains no other ionizable groups, it is a lower molecular weight than the propionate and causes a faster decay in the post-yield slope. These results further sup- port the dependence of the rate of decay of post-yield slope on the rate of penetration.

698 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS OPTIMIZATION OF TIME AND pH FOR 17% GMTG AT 50 ø C 10 Haim/Mandrel I-- 0.8. U,I 0.4 •: 0.6. MINUTES o MINUTES 2.0 4.0 6.0 8.0 10.0 pH WAVING SOLUTION Figure 9. Optimization ofdmc and pH •o• g]yccry] mono•h•og ycoJ•c. OPTIMIZING THE WAVING PARAMETERS The experiments illustrated in the above examples can be accomplished quickly and the formulation will rapidly target a few sets of conditions before beginning curl analysis or any salon testing. As an example, the data for 17% glyceryl mono- thioglycolate at three different times over the pH range of 2-9 is combined in Figure 9. Since we are trying to achieve a minimum post-yield slope in minimum time, this data would suggest that 17 % glyceryl monothioglycolate would yield satisfactory softening VARIATION OF POST YIELD SLOPE WITH TIME pH 6.5 46 ø C 10 HairslMandrel O 12% THIOGLYCOLI• ....,,,,,,•=• .b• ß 12% THIOGLYCOLI• • -'• •,•.• e12% THIOGLYCOLF ß 12% THIOGLYCOLI• o 12% GMTG • ACID • ACID + 2.1% GLYCERIN • ACID + 9.2% GLYCERIN - CACID + 17% GLYCERIN O 5 10 15 20 25 30 35 TIME (MINUTES) Figure 10. Variation of post-yield slope with time at different levels ofglycerin/ATG.