Cation selectivity at air/anionic surfactant solution interfaces

The selective binding of alkali metal and quaternary ammonium ions to anion ic surfactant films at the air/solution interface has been investigated usi ng the ion flotation technique. Surfactant anions studied were the dodecyl phosphate monoanion, the hexadecyl phosphate dianion, bis(2-ethylhexyl) pho sphate, dodecanoate, and dodecyl sulfate. Alkali metal ions display only we ak selectivity at adsorbed films of all surfactants studied. The binding st rength increases in the order Na+ < K+ < Rb+ < Cs+, with the exception of f ilms of dodecanoate, for which the ions have equal affinity, and the hexade cyl phosphate dianion, for which the selectivity sequence is Cs+ < Rb+ < K < Na+. By contrast, the binding of quaternary ammonium counterions to adso rbed films of anionic surfactants has a striking dependence on the surfacta nt headgroup. Results are interpreted using a model which includes solvatio n of the headgroups and competing counterions and is compared with competit ive binding of phosphate, sulfate, and acetate ions at cationic surfactant films.