DYNAMICAL PROPERTIES OF SALT-FREE VISCOELASTIC MICELLAR SOLUTIONS

Rheological experiments were carried out on aqueous micellar solutions of the ''dimeric'' surfactant ethanediyl-alpha,omega-bis(dodecyldimet hylammonium bromide) as a function of surfactant volume fraction and t emperature. The aim was to investigate the effect of electrostatic int eractions on the micellar size through the linear viscoelastic propert ies of the system. The zero-shear viscosity results indicate a sharp c rossover from a dilute regime in which the micelle growth is very weak to a regime of rapid growth, in agreement with the theoretical predic tions. However, upon increasing further the volume fraction, a maximum of the zero-shear viscosity is observed which suggests that the micel lar length also goes through a maximum. This is confirmed by the analy sis of the viscoelastic spectra on the basis of the recently developed models taking into account both reptation and Rouse processes. The de crease of micellar length observed in the high volume fraction range h as been interpreted as due to the decrease of the effective end-cap en ergy associated with the theoretically predicted increase of the micel le ionization degree with volume fraction. The effect of the electrost atic interactions also appears in the behavior of the plateau modulus that exhibits a larger volume fraction dependence than in highly scree ned micelles.