Synergism in mixed zwitterionic-anionic surfactant solutions and the aggregation numbers of the mixed micelles

Synergistic interaction between sodium dodecyl sulfonate (C(12)AS) and three zwitterionic surfactants was measured at 40.0 degrees C in aqueous 0.1 M NaCl solution. The three zwitterionic surfactants were alkyldimethylammoniopropanesulfonates with different alkyl chain length -12, 10 and 8 (abbreviated as C(12)DPAS, C(10)DPAS and C(8)DPAS, respectively). Surface tension results showed that a certain efficiency exists both in mixed micelle formation and surface tension reduction owing to the electrostatic attraction between the anionic and zwitterionic surfactants. The strength of the interaction between the zwitterionic surfactant and C(12)AS in the three mixed systems obeys the following order: C(12)AS/C(12)DPAS >C(12)AS/C(10)DPAS > C(12)AS/C(8)DAPS. The synergism is greatest when the molar ratio of anionic surfactant to zwitterionic surfactant in the solution is around 7:3. Fluorescence studies on the C(12)AS/C(12)DPAS system showed that the mixed micellar aggregation number becomes smaller than that of the zwitterionic surfactant with the addition of a small amount of anionic surfactant to the zwitterionic surfactant. However, after a small amount of C(12)DPAS was added to C(12)AS, the mixed C(12)AS/C(12)DPAS micellar aggregation number is much larger than that of C(12)AS. The mixed micellar aggregation numbers become largest at the composition point where the efficiency of mixed micelle formation is greatest. (C) 1998 Elsevier Science B.V. All rights reserved.