Cosolvent Effects on the Spontaneous Formation of Nanorod Vesicles in Catanionic Mixtures in the Rich Cationic Region

The aggregation behavior of cation-rich catanionic mixtures of cetyltrimethyl ammonium bromide (CTAB) and sodium dodecyl sulfate (SDS) was investigated in water ethylene glycol (EG) solutions by performing surface tension, electrical conductivity, pulsed field gradient nuclear magnetic resonance, transmission electron microscopy, and cyclic voltammetry measurements. Different physicochemical properties such as the critical micelle concentration, degree of counterion dissociation (alpha), interfacial properties, aggregation numbers, morphology of aggregates, and interparticle interaction parameters were determined. Cosolvent effects on the interactions between the two surfactants CTAB and SDS were analyzed on the basis of regular solution theory, both for mixed monolayers at the air/liquid interface (beta(delta)) and for mixed micelles. It was shown that an excess of cationic surfactant resulted in the formation of nonspherical vesicles. These were predominantly nanorod vesicles in water EG mixed solvents. The interparticle interactions were assessed in terms of cosolvent effects on the micellar surface charge density, the sphere-to-rod morphology change, and the phase transition from vesicles to mixed micelles. Moreover, the variation of the repulsive electrostatic potential energy between two pairs of aggregates was investigated for two modes of nanostructural transition, namely the transition between spherical and rod-like micelles and the transition between rod-like micelles and nanorod vesicles.