Behavior of Ionically Charged Lamellar Systems under the Influence of a Shear Field

Abstract

The influence of a shear field on differently charged bilayers is studied by means of rheology, anisotropy of the electrical conductivity, freeze−fracture transmission electron microscopy, and small-angle neutron scattering (SANS). The charge density in the aqueous system tetradecyldimethylamineoxide (TDMAO)/tetradecyltrimethylammonium bromide (TTABr)/ n -hexanol was varied by changing the content of TTABr from 0 to 10 mol %. At low TTABr content, preferentially planar lamellae are formed, and at higher TTABr content, multilamellar, polydisperse vesicles (1 μm and larger), which are densely packed and therefore possess elastic properties and a yield stress value. It is shown that the effect of the shear field is such that at first the planar lamellae are transformed into vesicles. Once multilamellar vesicles are present, further increase of the shear rate causes vesicle shells to be stripped off until, at high shear rates, unilamellar vesicles are formed. This leads to an increase of the elastic properties and the yield stress value. The formed unilamellar vesicle system is found to be stable and does not relax back into its original state. Thus shearing is prooved to be a suitable method for control of the morphology of vesicles. The results can be rationalized in terms of the bending elasticity of the bilayer and the position of the investigated sample in the phase diagram.