VESICLE TO MICELLE TRANSITION - RHEOLOGICAL INVESTIGATIONS

Rheological experiments were carried out on aqueous dispersions of ety ltrimethylammoniumhydroxynaphthalenecarboxylate (CTAHNC) as a function of temperature. The results indicate the formation of very long elong ated wormlike micelles at temperatures higher than about 50 degrees C, conferring to the system a very high viscosity. This behavior is expl ained by the combined effect of a large end cap energy and a low ioniz ation degree resulting from a strong binding of the weakly soluble cou nterions. At lower temperature the surfactant forms a much more fluid vesicle phase, which is observed by videomicroscopy. Experiments perfo rmed on mixtures of CTAHNC and of cetyltrimethylammonium bromide (CTAB ) show also a vesicle to micelle transition for a ratio of CTAB/CTAHNC that decreases upon increasing the temperature. The rheological behav ior of the micellar phase obtained by mixing CTAB and CTAHNC is simila r to that obtained for other charged micellar solutions.