Foaming activity and pK(a) of some surface active compounds

A novel method to determine the pK(a) of surface active compounds is presented. Since the ionized and unionized forms of amphiphiles have distinct surface and foaming properties, foamability as a function of pH can be used to obtain an approximate pK(a). Since foam is easily produced and measured, this method requires no specialized equipment. The foamability-pH profile is sigmoidal for both weak acids and bases, and a change of the foam activity is consistently observed in the region of their pK(a). The above relationship is confirmed using the following surface active compounds: amitriptyline, cyclohexylamine, dexverapamil, 3,5-diiodothyropropionic acid (DITPA), diphenhydramine, heptanoic acid, n-heptylamine, ibuprofen, laurylcarnitine, naphthoic acid, oleic acid and phenytoin.