SURFACTANT SYSTEMS WITH CHARGED MULTILAMELLAR VESICLES AND THEIR RHEOLOGICAL PROPERTIES

A general method is presented for the preparation of a viscoelastic surfactant phase that consists of densely packed multilamellar vesicles in water. The vesicle phase forms spontaneously when ionic surfactants are added to a dilute L(alpha)- or L(3)-phase, the bilayers of which consist of mixed uncharged single-chain surfactants and cosurfactants. The investigated phases were prepared from alkyldimethylaminoxides (C(x)DMAO), n-alcohols (C-6-C-9), and the ionic surfactant tetradecyltrimethylammonium bromide (C-14-TMABr) or sodium dodecyl sulfate (SDS). The structure of the vesicles and their dimensions were determined from freeze-fracture electron micrographs (FF-TEM). For a 100 mM surfactant solution the multilamellar vesicles had a diameter in the range of 1 mu m and an interlamellar spacing of around 800 Angstrom. For these conditions the vesicles are densely packed and cannot pass each other. The vesicle phase is highly viscoelastic and has a yield stress value. The viscoelastic properties of the phase were determined from oscillating theological measurements. The storage modulus was about 1 order of magnitude larger than the loss modulus and was independent of frequency. The moduli were determined as a function of the concentration and chain length of the surfactant and cosurfactant, the charge density and ionic strength, the amount of solubilization of hydrocarbon, and the temperature. For a constant charge density the yield stress values and shear moduli increase with the surfactant concentration according to a linear relation G' proportional to (c(0) - c(e)) where c(0) is the total surfactant concentration and c(e) the surfactant concentration for dense packing of the vesicles. For constant surfactant concentration the moduli increase in an 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. Theoretical calculations show that the shear moduli of the phases are much smaller than the osmotic pressure of the systems. Several models are proposed for the explanation of the shear moduli. The values of the moduli can best be understood on the basis of a hard-sphere model in which the multilamellar vesicles are treated as hard-sphere particles.