INTERACTIONS BETWEEN BETAINE-TYPE ZWITTERIONIC AND ANIONIC SURFACTANTS IN MIXED MICELLES

The interaction between zwitterionic and anionic mixed surfactants has been investigated by means of surface tension, static and dynamic light-scattering, H-1 NMR spectroscopy, rheological measurment, and probing methods. These systems are N-alkyl-N,N-dimethylbetaine (C12, dNB; C14, TNB) and sodium dodecyl sulfate (SDS) in the presence of 0.01 mol dm-3 NaCl. These systems show a composition dependency in micellar properties. When the molar fraction of DNB and TNB is about 0.6, the cmc values of these systems edhibit minima, whereas the solubilized amount of a water-insoluble dye, viscosity, and NMR line widths show a maxima. Furthermore, the size of a micelle is found to increase at the same fractain. The viscosity display a Newtonian flow at all compositions for the DNB-SDS system, while a strong shear dependence is observed at a certain composition for the TNB-SDS system. In particular, viscoelastic behavior appears at the intermediate composition region for the TNB-SDS system where a sphere-rod transition would occure in micellar shape due to the formation of intramolecular complexes by the electrostatic interaction of oppositely charged head groups between TNB and SDS.