Complex fluids with a yield value; their microstructures and rheological properties

Summary A general method is presented for the preparation of viscoelastic surfactant phases that consist of densely packed multilamellar vesicles in water. The phases form spontaneously when ionic surfactants are added to L_α-or L₃-phases from uncharged surfactants and cosurfactants. The dimensions and the structure of the vesicles were studied from FF-TEM micrographs for 100 mM surfactant solutions. The average diameter of the vesicles is in the range of 1 μm, the interlamellar spacing is around 800 ?. Under these conditions the vesicles are densely packed and cannot pass each other. The systems are highly viscoelastic and have a yield value. The storage and the loss moduli were determined by oscillating rheological measurements. The storage modulus was about one order of magnitude larger than the loss modulus and independent of frequency. Both parameters were determined as a function of the concentration and the chain length of the surfactant and cosurfactant, the charge density and ionic strength, the solubilization of hydrocarbon and temperature. The yield values and shear moduli increase linearly with the surfactant concentration. For constant surfactant concentration the moduli increase in a S-shaped form with the charge density and reach saturation for a mole fraction of about 7% of ionic surfactant. The storage moduli and yield values decrease with the addition of excess salt. The storage moduli depend strongly on the chain length of the surfactant. The values of the moduli are explained on the basis of a hard-sphere model in which the multilamellar vesicles are considered as hard-sphere particles. Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4–8, 1994.