Structure and Flow in Surfactant Solutions

A review is given on rheological data of viscoelastic surfactant solutions. The viscoelastic properties in the discussed systems are due to entangled threadlike micelles. In such solutions that have this microstructure the zero shear viscosities depend strongly on conditions like the charge density of the micelles, the salt and cosurfactant concentration, and the chainlength of the surfactant. For a 1% solution the viscosity can vary between one and 106 mPas. Over extended concentration ranges the viscosities show simple power law behaviour (η° = (c/c*)x with x = 1.5 - 8.5). The largest values for x are observed for systems with charged unshielded micelles and the smallest values for shielded or neutral systems which are close to the L1/Lα-phase boundary. The various exponents are explained on the basis or different scission mechanisms as proposed by M. Cates. Many viscoelastic surfactant solutions show simple Maxwell behaviour.