Synergism in mixtures of zwitterionic and ionic surfactants

We have studied the effect of the hydrocarbon length, electrical charge and the addition of inert electrolytes on the mixed aggregation process of alkyl ammonium propane sulfonate and the ionic surfactants, sodium dodecyl sulfate (SIDS) and dodecyl trimethyl ammonium bromide (DTAB). Using conductivity and fluorescence probing, we determine the cmc of the mixed micelles and their variation with micelle composition. Results indicate that in the absence of electrolytes the cationic DTAB and the zwitterionic surfactants mix ideally. The binary mixtures of the anionic surfactant SDS and the zwitterionic DDPS present a non-ideal behavior with a weak interaction between the surfactants, as a result of the locations of the positive and negative charges on the zwitterionic surfactants, while no interactions are detected for TDPS + SDS and HDPS + SDS mixtures. The addition of inert electrolytes favors the interactions between the ionics and the zwitterionic surfactants, probably because more ionic surfactant can be accommodated in the micelles. Viscoelastic behavior is detected in mixed micelles of anionic-zwitterionic surfactants of equimolecular composition. (c) 2005 Elsevier B.V All rights reserved.