Three cationic surfactants, N-dodecyl-N-(2-hydroxyethyl)-N,N-dimethylammonium bromide, N-tetradecyl-N-(2-hydroxyethyl)-N,N-dimethylammonium bromide and N-cetyl-N-(2-hydroxyethyl)-N,N- dimethylammonium bromide have been synthesized. The aqueous two-phase systems of surfactant mixtures from such N-alkyl-N-(2-hydroxyethyl)-N,N-dimethylammonium bromides or N-cetyl-N,N,N-trimethyl- ammonium bromide (CTAB) with sodium dodecyl sulfate (SDS) have been investigated at 313.15 K. The results indicate that two aqueous two-phase regions can be observed in these mixed cationic-anionic surfactant systems. The two regions are nearly symmetric to the equimolar line. With the increase of chain length in the cationic surfactants, the aqueous two-phase regions with abundant cationic surfactants shift slightly to the lower molar ratio of anionic surfactant versus cationic one.

Key words: aqueous two-phase system, cationic/anionic surfactant, quaternary ammonium salt, sodium dodecyl sulfate