No abstract

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 cryogenictemperature 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.

Drag reduction, shear and extensional rheometry, and cryogenic transmission electron microscopy (cryo-TEM) were used to study aqueous solutions of one cationic surfactant, Arquad 16-50 (5 mM), with three isomeric counterions, 2-, 3-, or 4-Cl-benzoate at 12.5 mM. Each isomer showed different types of rheological and drag reduction behavior and different micellar structures. The 4-Cl system showed good drag reduction, high apparent extensional viscosity, and a thread-like micellar network, while the 2-Cl system showed no drag reduction, low apparent extensional viscosity, and only spherical micelles. The 3-Cl system was drag reducing and had high extensional viscosities at 30 °C. However, at 20 °C, the 3-Cl solution precipitated at high shear or extensional rates, leading to loss of drag reduction and low apparent extensional viscosity. The 3-Cl at 20 °C showed threadlike structures in some pictures and vesicles in others, presumably because of variations in the level of shear the samples were subjected to during sample preparation. The differences in behavior are explained by the position of the chlorine group on the benzoate ring. The hydrophobic chlorine in the 2-Cl system must reside in an unfavorable position in the aqueous phase and hence only spherical micelles are formed, which leads to no drag reduction and very low apparent extensional viscosity. The chlorine groups in the 3-Cl and 4-Cl counterions reside in the nonpolar hydrocarbon core of the micelles and, hence, stable elongated micelles can form. A schematic micellar phase diagram is proposed to summarize the transformations of surfactant molecular aggregates at different temperatures and external forces.