Rheological behavior in the didodecyldimethylammonium bromide/water system

Abstract

The phase behavior in the dilute region of the didodecyldimethylammonium bromide (DDAB)/water system is studied with a battery of techniques. The critical vesicle concentration (cvc), measured by tensiometry, conductimetry, ion-selective-electrode potentiometry and dye solubilization, is similar to the value reported in the literature. Moreover, the combination of surfactant-ion-selective-electrode and bromide-ion-selective-electrode potentiometry indicates that the vesicles are substantially ionized (α approximate 0.5) in the proximity of the cvc. The transition from small unilamellar vesicles to larger multilamellar liposomes was detected at 0.2 wt% by viscometry, conductimetry and dye solubilization measurements. The theology of the DDAB/water system was studied as a function of surfactant concentration and temperature. Non-Newtonian behavior, viscoelasticity, yield stresses and time-dependent flow behavior were observed. Maxima and minima in the dynamic moduli and in conductivity are related to structural changes and phase transitions. Moreover, in time-dependent shear flow, the microstructure is modified and the theological response shifts from thixotropic to antithixotropic or vice versa, depending on the DDAB concentration and the level and duration of the final applied stress. The conductivity behavior in the Lam1 phase region can be qualitatively explained by the capillary superconductivity theory. This conductivity behavior occurs when the thickness of the aqueous lamella is of the same order of magnitude as the Debye length.