Vesicle to micelle transition: Rheological investigations

Rheological experiments were carried out on aqueous dispersions of cetyltrimethylammoniumhydroxynaphthalenecarboxylate (CTAHNC) as a function of temperature. The results indicate the formation of very long elongated wormlike micelles at temperatures higher than about 50 degrees C, conferring to the system a very high viscosity. This behavior is explained by the combined effect of a large end cap energy and a low ionization degree resulting from a strong binding of the weakly soluble counterions. At lower temperature the surfactant forms a much more fluid vesicle phase, which is observed by videomicroscopy. Experiments performed 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 behavior of the micellar phase obtained by mixing CTAB and CTAHNC is similar to that obtained for other charged micellar solutions.