Micelle to Vesicle Transition Induced by Cosurfactant: Rheological Study and Direct Observations

We report a micelle−vesicle transition induced by the addition of a neutral cosurfactant, hexanol, into a wormlike micellar solution of gemini cationic surfactant. This transition was investigated with rheology measurements as well as direct imaging techniques. It was observed by cryo transmission electron microscopy that micelles first become elongated with an increase of hexanol, which increases the viscosity. As the hexanol concentration is increased, another more fluid phase appears, where micelles are highly branched and coexist with small vesicles. One can describe this transition as a decrease of the spontaneous curvature caused by the cosurfactant molecule, which induces the elongation, and then the branching of micelles as well as the formation of small unilamellar vesicles. With a further increase of hexanol, these vesicles grow in size, which allows their observation by optical microscopy with differential interferential contrast. Some of these vesicles show long tubule-like shape, typically a few micrometers thick and up to several hundred micrometers long.