Synergism in Mixed Anionic–Amphoteric Surfactant Solutions: Influence of Anionic Surfactant Chain Length

The influence of chain length on the composition of mixed micelles in binary mixtures of N,N-dimethyldodecylamine oxide (DDAO) and sodium decyl-, sodium dodecyl- and sodium tetradecylsulfate (abbreviated as SDeS, SDS and STS, respectively) has been determined at 30°C. From the surface tension measurements, the critical micelle concentration (cmc) data were measured as a function of mixing composition. Cmc-values were then analyzed according to regular solution model. The composition of mixed micelles, the interaction parameter (β) and the activity coefficients were evaluated from the regular solution model for the all anionic-amphoteric mixed systems. The interaction parameter values indicated a synergistic interaction between DDAO and the three anionic surfactants at all mole fractions to be due to an overall attractive interaction in the mixed micelles. The strength of the interaction between the amphoteric surfactant and the sodium alkyl sulfate in three mixed systems obeys the following order: SDeS/DDAO > SDS/DDAO > STS/DDAO suggesting that the decrease in the length of anionic surfactant alkyl chain results in a stronger interaction with DDAO. On the other hand, the viscosity data reveals a viscosity increasing trend of STS/DDAO > SDS/DDAO > SDeS/DDAO.