SURFACTANT SYSTEMS WITH CHARGED MULTILAMELLAR VESICLES AND THEIR RHEOLOGICAL PROPERTIES

A general method is presented for the preparation of a viscoelastic su rfactant 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 tetradecyltri methylammonium bromide (C-14-TMABr) or sodium dodecyl sulfate (SDS). T he structure of the vesicles and their dimensions were determined from freeze-fracture electron micrographs (FF-TEM). For a 100 mM surfactan t solution the multilamellar vesicles had a diameter in the range of 1 mu m and an interlamellar spacing of around 800 Angstrom. For these c onditions 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 oscilla ting theological measurements. The storage modulus was about 1 order o f magnitude larger than the loss modulus and was independent of freque ncy. The moduli were determined as a function of the concentration and chain length of the surfactant and cosurfactant, the charge density a nd ionic strength, the amount of solubilization of hydrocarbon, and th e temperature. For a constant charge density the yield stress values a nd shear moduli increase with the surfactant concentration according t o 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 concentrati on 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 exc ess salt. The storage moduli depend strongly on the chain length of th e surfactant. Theoretical calculations show that the shear moduli of t he phases are much smaller than the osmotic pressure of the systems. S everal models are proposed for the explanation of the shear moduli. Th e values of the moduli can best be understood on the basis of a hard-s phere model in which the multilamellar vesicles are treated as hard-sp here particles.