Micelle-vesicle transition of oleyldimethylamine oxide in water

We studied the effects of the degree of ionization(α) and the surfactant concentration (C _d) on the micelle-vesicle transition in salt-free oleyldimethylamine oxide (OlDMAO) aqueous solutions by the dynamic light scattering (DLS), the hydrogen ion titration, the small angle neutron scattering (SANS), the electrophoretic light scattering (ELS) and viscoelastic measurements. From the study of ionization effects, the micelle-vesicle transition was recognized as a change of aggregate size by the DLS measurement; however, the micelle-vesicle transition was not detected both in the ELS measurement and the hydrogen ion titration, suggesting that the electric properties of the worm-like micelles and the vesicles are very similar despite a large difference of shapes between them. From the results of the SANS, the DLS and the viscosity measurements, it was suggested that a concentration-dependent micelle-vesicle transition took place around C _p = 10 mmol kg ^-1 for the solutions at α = 0.5. In the concentration-range 10 mmol kg ^-1 < C _d < 150 mmol kg ^-1, the micelles and the vesicles coexisted. In the concentration region (C _d = 10-50 mmol kg ^-1), the vesicle size increased with the surfactant concentration.