SYNERGISM IN MIXTURES CONTAINING ZWITTERIONIC SURFACTANTS

Synergism can exist in mixtures of two or more surfactants if they interact attractively with each other and meet various other conditions which are reviewed. Attractions between surfactants are dominated by any electrostatic interaction of their hydrophilic head groups. Zwitterionic surfactants with no net formal charge show only weak interaction with nonionic surfactants. When the zwitterion is capable of accepting or donating a proton to form a species with a net charge, then interaction with an oppositely charged surfactant may be strong enough to exhibit synergy, even at neutral pH. Long-chain amino acids, capable of both accepting and donating protons to form charged species, show more or less equal attractive interaction with both anionic and cationic surfactants; those such as betaines, that are capable only of accepting a proton, interact much more strongly with anionic than with cationic surfactants. Materials normally not considered zwitterionic, such as amine oxides, sulfoxides, and pyrrolidones, but capable of accepting a proton to form a cationic species, also show much stronger interaction with anionic than with cationic surfactants.