DISINFECTANTS 99 



The lower acids, up to 10 carbons, are efficient only 

 in acid media, at pH 6 or less. In this range, the dis- 

 infecting power increases with the molecular weight up to 

 C12H24O2, lauric acid. Myristic acid, C14H28O2, is not 

 quite as active in acid media, and the higher acids have no 

 disinfectant properties at all. The fact that the effi- 

 ciency increases with the acidity indicates that death is 

 brought about by the undissociated part of the molecule. 

 (For a discussion of this point and for the computation 

 of the undissociated fraction see p. 157). The dissocia- 

 tion constant is 1.8 X 10 '^ for acetic, 1.5 X 10"^ for butyric, 

 and 1.4 X 10'^ for caproic and caprylic acid. Klar- 

 mann's data show that death is brought about by 40-60 

 mg. undissociated caproic acid in 100 cc. Assuming 1.4 

 X 10^ as dissociation constant for all the higher homo- 

 logues, the corresponding values for the higher acids are 

 20-40 mg. for Cs, 1-1.5 mg. for Cio, 0.2 - 0.8 mg. for C12 

 and about 2 mg. for Cu. Palmitic and stearic acid are 

 so insoluble (0.00092 and 0.00038% respectively at 37"^ 

 according to Ralston and Hoerr, 1942) that they could not 

 be expected to act as disinfectants. 



At neutrality, capric, lauric and myristic acids are the 

 only acids capable of killing staphylococci. The lower 

 acids are practicallj^ completely dissociated, and the high- 

 er acids remain insoluble. But with increasing alkalin- 

 ity, these latter dissolve, and at pH 9 to 10, palmitic and 

 stearic acid become weak disinfectants and myristic acid 

 becomes stronger again, resembling the higher acids, 

 w^hile capric acid loses its power entirely. 



The detergent properties of soaps have been sur- 

 passed by those of new synthetic detergents developed 

 during the last decade. These so-called ' * wetting agents ' ' 

 are discussed here in connection with the soaps not only 

 because they are substitutes for soap, but also because 

 many of them contain fatty acids. The ''cationic de- 

 tergents" have the fatty acids bound on ammonium de- 

 rivatives in the cationic part of the molecule, the "ani- 



