Azobenzene dye induced micelle to vesicle transition in cationic surfactant aqueous solutions.

A photo-responsive azobenzene dye, sodium 4-[(E)-phenyldiazenyl] benzoate (AZONa), was employed to induce microstructural changes with cetyltrimethylammonium bromide (CTAB) in aqueous solution. A spherical micelle to vesicle transition via wormlike micelles was investigated by employing turbidity, viscosity, and dynamic light scattering measurements, as well as cryo-transmission electronic microscopy. At a certain molar ratio [AZONa]/[CTAB] of 0.65 the zero-shear viscosity achieve a maximum, showing clearly the formation of wormlike micelles that obey the classic Maxwellian behavior at low shear frequencies. At this critical molar ratio, an increase in temperature resulted in the breakdown of wormlike micelles, whereas the viscosity eta(0) showed three domains of growth characterized by scaling laws as the surfactant concentration was varied between 10 and 40 mM. The rapid increase in eta(0) could be explained by an incomplete screening of electrostatic repulsions of the solution. The decrease in eta(0) was due to the formation of branched wormlike micelles above once a concentration [CTAB]=20 mM.