PERMANENT WAVING OF HUMAN HAIR: THE COL•) PROCESS 115 the keratin surface would be reduced by 50%. In deriving these equations it was assumed that all hair fibers are of uniform diameter and are 'wrapped in a uniform close packed arrange- ment. It need not be emphasized that these' conditions are idealized and are seldomly realized under actual waving conditions. In hair undergoing a cold wave treatment in which the tress is saturated with ammonium thiogly- colate, the following reaction takes place: Hair--S--' S--Hair 2 Hair-SH n t- NH4OOCCH•S--S-- CH2COONH• In the above equation Hair--S-- S--Hair is used to represent a nor- mal segment of a keratin molecule as shown in Fig. 9. "Backbone" NH CO I CHCH2S SCH•CH (CystineLinkage) CO NH NH CO R--CH CH--R CO NH NH . OC (Hydrogen Bond) / R--CH CH--R / CO NH / N/H CO / CHCH2CH2CH•NHa* -OOCCH•CH (Polar ß or Salt Linkage) Polypeptide Chain Polypeptide Chain Figure 9 Hair--SH is used to indicate this segment after the reductive splitting of the disulfide bonds. Several methods have been used. to follow the reduction of the disulfide bonds in hair undergoing a cold waving treatment. The Sullivan cystine method (5) as modified for this problem in this laboratory (6) has been very helpful in following the action of thioglycolates on hair. Table 2 shows the results obtained. in one of these studies. TASTE 2--EffECT or TIME o• THE DEOREE OF REDUCTION OF HUMAN HAIR USING A COMMERCIAL COLD WAVINO SOLUTION (DCR-3) AT ROOM TEMPERATURE Reduction Time, Cystine Min. Found,,% Av. 0•Con trol 17.0 O--Control 16.0 16.5 4 3.0 4 2.9 3.0 8 2.6 8 2.6 2.6 14 2.4 14 2.6 2.5 20 2.5 20 2.4 2.5 It should be pointed out that in these studies small tresses of hair (about 1 gm.) were reduced for the time stated by immersing them in a beaker containing a commercial cold wave lotion (--SH=0.72 N pH = 9.2, at room temperature). The results show that under these rather drastic conditions the re- duction is practically complete within four minutes and further treatment has very little effect upon the residual disulfide linkages. It should not be concluded from this experiment, however, that the same degree of reduction is obtained dur- ing the process of cold waving, for in this particular experimeni a huge excess of ammonium thioglycolate

116 JOURI•AL OF THE SOCIETY OF COSMETIC CHEMISTS was available, whereas under normal waving conditions the amount of thioglycolate is limited by the ratio of capillary air space to the amount of hair on the curling rod, as pointed out before. It should also be real- ized that this amount of reduction is not necessary or even desirable in actual practice. One-fourth the amount of reduction observed in Table 2 is probably the degree of reduction obtained in an average cold wave. In Fig. 9 linkages other than the covalent disulfide bonds are indi- cated. These hydrogen bonds and salt linkages must play some role in the process of cold waving. To them must be added the van der Waals' attractive forces•which exist between the non-polar side chains. In studying the process of cold wav- ing it becomes of importance to determine how much each of these forces contribut6s to the ability of a fiber to resist deformation. It has been pointed out by Sookne and Harris (7) in their study of wool fibers that van der Waals' forces vary inversely as the sixth or higher power, whereas Coulomb forces will vary inversely as the square of the distance. It is to be expected, therefore, that in the process of de- forming a fiber several different forces will .have to be overcome. In exp.eriments carried out in this laboratory this subject was investi- gated. Single hair fibers were stretched in water, 0.1 h r HC1, and 5 N monochloracetic acid, and the load-elbnga•ion curve plotted. Since it is known that water will not. affect salt linkages or hydrogen bonds significantly, dilute HC1 will affect salt linkages only, and 5 N monochloroacetic acid will •:upture both salt linkages and hydrogen bonds (8, 4), the load-elongation curves of fibers thus treated reveal the magnitude of these 'individual forces in keratin. Figures 10 and 11 show the re- sults obtained when fibers were elon- gated under these conditions. 'It is quite evident from Fig. 10 that in the case of 0.1 N HC1 the load required to stretch thff fiber has been reduced, indicating the .amount that the salt linkages contributed to the fiber strength. Much more pronounced is the effect of the hy- drogen bonds, for when these are broken, as with 5 N monochloro- acetic acid, the resistance of the fiber to deformation drops precipi- tately (Fig. 11). That no perma- nent change has been introduced in the fiber by the chloroacetic acid A B ELONGATION Figure 10.--Stress strain curve. 20% elongation in 0.1 N HCI. ,,/--original water curve, B--in 0.1 N HC1