COMPLEX FLUIDS WITH A YIELD VALUE - THEIR MICROSTRUCTURES AND RHEOLOGICAL PROPERTIES - MULTILAMELLAR VESICLE SYSTEMS WITH A YIELD STRESS VALUE

A general method is presented for the preparation of viscoelastic surf actant phases that consist of densely packed multilamellar vesicles in water. The phases form spontaneously when ionic surfactants are added to L(alpha)- or L(3)-phases from uncharged surfactants and cosurfacta nts. The dimensions and the structure of the vesicles were studied fro m FF-TEM micrographs for 100 mM surfactant solutions. The average diam eter of the vesicles is in the range of 1 mu m, the interlamellar spac ing is around 800 Angstrom. Under these conditions the vesicles are de nsely packed and cannot pass each other. The systems are highly viscoe lastic and have a yield value. The storage and the loss moduli were de termined by oscillating theological measurements. The storage modulus was about one order of magnitude larger than the loss modulus and inde pendent of frequency. Both parameters were determined as a function of the concentration and the chain length of the surfactant and cosurfac tant, the charge density and ionic strength, the solubilization of hyd rocarbon and temperature. The yield values and shear moduli increase l inearly with the surfactant concentration. For constant surfactant con centration the moduli increase in a S-shaped form with the charge dens ity and reach saturation for a mole fraction of about 7% of ionic surf actant. The storage moduli and yield values decrease with the addition of excess salt. The storage moduli depend strongly on the chain lengt h of the surfactant. The values of the moduli are explained on the bas is of a hard-sphere model in which the multilamellar vesicles are cons idered as hard-sphere particles.