PERMANENT WAVING OF HUMAN HAIR: THE COLD PROCESS 119 of this cystine had been oxidized further. In other experiments re- building of cystine linkages up to 100% of the original has been ob- tained, although the average amount of rebuilding (in many experiments in the beauty shop) is about 85%. TABLE 3--EvvECT Or T•ME ON OF RoeOXIDATION OF REOUCeO HUMAN HAIR Cystine Oxidation Time, Min. Found, % Av. 0--Control not reduced 16.1 0--Control not reduced 15.1 15.6 3 10.2 3 10.3 10.3 6 11.9 6 12.0 12.0 9 12.1 9 12.4- 12.3 15 13.2 15 13.2 13.2 Considerable work has been done on the oxidation of sulfhydryl com- pounds and disulfides by various workers. Thus, Stoves (11) has contended that cystine links of kera- tin fibers are first hydrolyzed 'by aqueous solutions of oxidizing agents and the sulfur is then oxidized to sulfuric and sulfonic acid residues. Rutherford and Harris (12) working with wool, came to the conclusion. that the action of hydrogefi peroxide on keratin consists of the oxidation of the disulfides with the formation of disulfoxides and similar com- pounds. In another paper these authors (13) suggest that the di- sulfide sulfur of wool, exposed tb the irradiation of a carbon arc, even in an atmosphere of nitrogen will be oxidized in part to sulfuric acids. Lemin and Vickerstaff (14) also agree with Harris and co-workers that the action of hydrogen per- oxide on keratin leads,to the oxida- tion of disulfide bonds. It seems, therefore that oxidation of hair or keratin does not necessarily stop at the cystine stage but will frequently go beyond it. This is a point well worth remembering when evaluating the efficacy of fixative or "neutral- izing" agents. Experimentally, it is possible to follow the oxidation of sulfhydryl groups in hair quantitatively much more easily than to follow the other changes which occur in hair as a re- suit of the oxidation step. It has been pointed out before that one Of the actions of ammonium thiogly- colate is to swell the hair. In such hair it would seem that som• of the pairs of sulfhydryl groups are sepa- rated too far to allow their reoxida- tion to disulfides. The work of Phillips and his coworkers (15) on the oxidation of thioglycolate re- duced wool by oxygen also indicates that oxidation of sulfhydryl groups to disulfides may not be as smooth a process as is generally conceived. It can be visualized that when re- duced hair is treated with an excess of oxidizing agent, be it hydrogen peroxide, bromate, or iodate, some of the sulfliydryl groups will be in a fortuitous position so that they can be oxidized immediately to disul- fides. Others will not be so situated and either will not be oxidized at all or will be oxidized to sulfuric or sulfonic residues. The number of disulfide bonds formed may be suffi- cient to give the hair a more or less permanent set. Yet due to insuffi- ciency of--S•S-- bonds formed

120 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS and the effect of the secondary valence bonds which eventually will be re-established in the fiber plus the tendency to a'higher state of entropy, considerable strain is placed on the reformed disulfide bonds and it is therefore not sur- prising that in actual beauty shop practice the relaxation of a "perma- rierit" wave is riot uncommon. Certain commercial waving proc- esses have been designed to over- come this difficulty. In these proc- esses an absorbent pad is placed around each curl after the waving lotion (ammonium thioglycolate) has acted sufficiently long and be- fore the fixative (oxidizing) solution is applied. These pads withdraw the excess waving lotion from the spaces between the fibers. by cap- illary action. By removing the ex- cess thioglycolate from the outside of the fiber without removing sig- nificant amounts of solution from within the fiber, they tend to reverse the action of the reducing agent. A little reflection will readily show that at the end of the reducing step the ratio of dithiodiglycolate to thioglycolate is higher within the fiber than it is outside of the fiber. Going back to the equation showing the reaction of hair with thioglycol- ares given at the beginning of this paper, it can certainly be expected that the reaction will not go any further to the right, but since it is an equilibrium reaction and it so happens that the Redox potentials of cysteine/cystine and thioglycolic acid/dithioglycolic acid systems are the same (16), it may be reversed. Under the influence of the ad- sorbent pads we thus may have a partial reoxidation, which under optimum conditions may be suffi- cient to give some permanent set to the hair. The contribution of the absorbent pad to the success of the cold wave is more than inducing the reoxidation of sulfhydryl group. By withdrawing the waving lotion from the air spaces surrounding the hair fibers and rebuilding some di- sulfide linkages, the adsorbent pads initiate the deswelling of the hair fibers. Since this is a gradual and controllable process, it allows the amino acid side chains to reorient themselves in positions which are more favorable to the formation of desirable salt linkages and hydrogen bonds. But beyond this, the slow deswelling action af the absorbent pads permits the sulfhydryl groups which were spacially too far sepa- rated to be oxidized to disulfides to be brought into juxtaposition so that they can be thus oxidized. Briefly stated, the function of the absorbent pad is therefore the ini- tiation of the rebuilding of the co- valent --3--S-- bonds, the gradual deswelling of the hair fibers thus allowing the establishing of desir- able coordinate valence bonds, and the bringing together spacially '---SH groups, which would otherwise be resistant to oxidation to disulfides. In the discussion of the chemistry of the cold wave process the picture has been intentionally oversimpli- fied. In the treatment of various steps it has been assumed that the hair fiber possesses a homogeneous