It is usually thought that inorganic salts have little effect on the surface activity of equimolarly mixed cationic-anionic surfactant mixtures.This work studied the effect of sodium halide (NaX, X=F^-, Cl^-, Br^-, I^-) on the surface activity of equimolarly mixed decyltriethylammonium bromide-sodium decanesufonate.It was showed that the added salts with high concentration had obvious effect on the surface activities, and the critical micelle concentration (cmc) was decreased, while the lowest surface tension of the aqueous solution (γ_cmc) was increased.Such effects were increased with ion radius of X^- of NaX.The results were explained in terms of the “screening” effect of inorganic counter ions on the attraction between oppositely charged headgroups of mixed cationic-anionic surfactants and the “salt out” effect.

Key words: surfactant, cationic2anionic surfactant mixture, surface tension, surface activity