28 JOURNAl. OF THE SOCIETY OF COSMETIC CHEMISTS arsenic and mercury poisoning. It has always appealed to me that the cosmetic industry has been the first user of many materials. Here again, 'the cosmetic industry can truthfully boast that it was first to commercialize for peace-time use a material from a new group of com- pounds-the mercaptans. Of the two discoveries which have led to the use of mercaptans in cos- metics, one has revived a dying market the other has revived a field which had already been pro- nounced dead, and this mercaptan cosmetic has grown to be the single biggest income-producer in our entire industry. The former refer- ence is to the cosmetic product, the cream depilatory the latter, to a product, the cold waving lotion, and the new methods of cold waving growing from it. MERCAPTANS IN DEPILATORIES Cosmetic depilatories containing sulfides have been known and used for centuries. The •trabian Nights and many other ancient records mention them, yet the same prob- lems which were so picturesquely described by Koeune(7) in 1937 have existed unsolved for centuries. In Koeune's own words The use of sulphides in depila- tories brings in its t3ain a number of vexed problems which have, and probably can be, only par- tially solved. For example, we have, first and forel•ost, the problem of minimising and dis- guising the distinctly insalubrious odour. Then, of scarcely less im- portance, we have to consider the serious caustic action ()n the skin. And in case by some mysterious alchemy we should dissipate the smell of the sulphide, a malign fate draws across the track some very red herrings in the form of drying out of the depilatory in the tube, deterioraticm of the active ingredients, discoloration of.the paste, while the good perfume which one has reluctantly sacri- ficed to the God of Sulphide turns in its grave and rises up in horrible forms which stink of the very pit itself. Like so many problems crying for solution, "the mysterious alchemy" called for by Koeune was forthcom- ing independently and almost at the same time from two research groups. In Holland a Dutchman, Karel Bohemen, found that thi,•)glycolic acid and other mercapto-carboxylic acids could be used in certain alka- line media. His discovery resulted in the issuance of patents in cer- tain foreign countries to him o• his agent--Fletcher. In America, Evans and McDonough discovered that mercaptans could be used in cosmeti.c cream depilatories, and after a priority contest with Bohe- men, a U.S. Patent was issued only to them. In foreign countries be- cause the conflict with Bohemen patents was governed by different laws, the patents issued to Evans and McDonough cover the use of mercaptans other than mercapto- carboxylic acid in depilatories. Briefly summarized, Evans and McDonough found that: 1. All merdaptans in an alka- line solution, preferably at a pH greater than 9.0 and less than 12.5

MERCAPTANS IN COSMETICS 29 will remove hair without irritating the skin. To illustrate this Table I has been prepared. TABLE I--EFFECT OF PH oN RATE OF DEPILATION BY MERCAPTAN pH Hair Removal in Time to Irritate 8.0 Not in 30 min. None in 30 min. 8.5 Not in 30 min. None in 30 min. 9 0 30 min. None in 30 min. 9.5 24 min. None in 30 min. 10.0 19 min. None in 30 min. 10.5 15 min. None in 30 min. 11.0 12 min. None in 30 min. 11.5 9 min. None in 30 min. 12.0 7 min. None in 30 min. 12.5 5 min. Slight in 30 min. 13.0 3 min. Severe in 15 min. Thioglycolic acid (0.5 molar) was used in this experiment because we had found that substituted mer- captans were not only better because of their odor but also more effective . and less irritating. Of the sub- stituted groups we had found the ionizing groups, particularly the acidic groups such as --COOH, to be preferred over the non-ionizing groups such as the ether groups. 2. The concentration of the mer- captan should be greater than 0.1 mole per liter of solution and need not be greater than 1.5 moles per liter. To illustrate this 'Fable II has been prepared. 3. Any non-volatile alkaline ma- terial can be used provided that a) its dissociation constant is greater than 2 X 10 -s and b) it does not form an insoluble salt with the particular mercaptan. For ex- ample, monoethanolamine alone even when used in sufficient amount to give a pH of 11 will not make a satisfactory depilatory and barium hydroxide is not usable with thio- glycolic acid because it forms an in- soluble salt. 4. Although it is preferable to have the concentration of the alka- line material slightly greater than the total acidic equivalency of the mercaptan, it should never be greater than twice. 5. The filler or cream-forming materials serve an important func- tion in mercaptan depilatories other than just the ease of localizing the depilatory in application. As mer- captans in an alkaline medium oxi- TABLe II--EFF•CT OF CONCENTRATION OF MERCAPTAN (THIoO, LYCOLIC ACID) ON RATE OF DEPILATION Molar Concentration pH by Excess Irritation of Thioglycolic Acid Ca(OH)2 Time to Remove in 30 Min. 0.05 12.35 None in 30 min. None 0.10 12.30 Partial in 20 min. None Complete in 30 min. 0.20 12 28 Partial in 10 min. None Complete in 15 min. 0.50 12.29 Partial in 6 min. None Complete in 9 min. 0.80 12.26 Partial in 6 min. None Complete in 9 min. 1.50 12.26 Partial in 8 min. None Complete in 10 min. 2.00 12.28 Partial in 8 min. None Complete in 10 min.