Cryo-TEM Imaging the Flow-Induced Transition from Vesicles to Threadlike Micelles

Transition of vesicles to micelles induced by flow of dilute aqueous cetyltrimethylammonium chloride (CTAC)/ sodium 3-methylsalicylate (CH(3)Sal(-)) dispersion was directly imaged by rapidly vitrifying small ultrathin specimens after varying periods of postflow on-the-grid relaxation, and then examining them by cryogenic-temperature transmission electron microscopy (cryo-TEM). Vesicles prevail in quiescent equimolar 5 mM dispersions of CTAC and NaCH(3)Sal at equilibrium. When the dispersions are deformed and strained by rapid drainage of the excess from the specimens, the vesicles transform into an entangled network of threadlike micelles. The micelles revert to vesicles with time after drainage flow ceases. Intermediate structures during the transition were also captured by cryo-TEM. We hypothesize that the straining actions of flow disrupt vesicles, distort the distribution of, or strip, the counterions around vesicle fragments, shift the binding and dissociation equilibrium of counterions, alter the local preferred curvature of the surfactant assemblies, and thus induce structural instability of the fragments that leads to their reconstruction into networks of branching threadlike micelles.