ACYLATED AMINO ACIDS IN SHAMPOOS 409 like products somewhat more soluble than the corresponding lauric acid soaps. In many respects lauroyl sarcosine occupies an intermediate posi- tion with respect to soap and ]auryl sulfate. Lauroyl safcosine is a stronger acid than lauric acid, but not as strong as the mono-lauryl ester of sulfuric acid. Titration curves for these materials appear in Fig. 1. The pKvalue for lauroyl sarcosine is approximately $.$, for lauric acid about 7.2 and for dodecyl acid sulfate, less than 3. The alkali salts of lauroyl safcosine are only very slightly alkaline unlike the soaps they remain clear in solution when the pH is reduced to as low as 5. The calcium and magnesium salts of lauroyl safcosine are more water soluble than the corresponding laurares of special importance--when they do precipitate--they crystallize nicely with none of the sticky curd forming tendencies of the soaps. Sodium lauroyl sarcosinate resembles sodium laurate markedly in general surface-active properties, which are peculiarly pH sensitive. The surface tension of aqueous solutions of sodium lauroyl sarcosinate decreases rapidly with increasing acidity to a value as low as 23 dynes/centimeter (Fig. 2). Wetting action on protein fiber (wool) parallels surface tension Figure 2 40 30 • o O " •35 20 • 30 ß • 25 10 5 6 7 8 9 10 pH closely, although the product has practically no wetting action on cellulose at pH's above 8. Foaming action also improves with decreasing pH, reaching a maximum at pH 5.5 (Fig. 3), also optimum for detergency. It would be noted that this pH corresponds closely with the pK value of lauroyl sarcosine. A complex of undissociated acid and carboxylate ion comprises the important surface-active species. Similar behavior has been noted in sodium laurate, where maximum foam height also coincides with pK (3).

410 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 300 200 1oo Figure 3. 4 6 8 10 pH Figure 4. SARCOSYL N L_ 100-"•'• 1.5 1.0 .5 4 6 8 lO pH The BIOS and FIAT reports (4) on the German detergent industry issued after the war contain repeated references to the high regard of Euro- pean dermatologists for the acyl sarcosines and particularly sodium oleyl sarcosinate, as skin detergents. The "feel" these products impart to the skin on washing is undeniably soft and similar to that obtained with mild toilet soap. Repeated washings with these products or prolonged immer- sion in them produces no significant irritation. Draize-Woodward ocular mucosa tests do not indicate, however, that the sarcosinates are particularly better than lauryl sulfate. There is some indication that the irritation re- sponse is decreased as the pH of the sarcosinate is lowered work is being continued along these lines. Although there is strong subjective evidence of the hair-conditioning