Some experimental evidence in favour of connections in elongated surfactant micelles

Abstract

In dilute aqueous solutions of surfactants a succession of phases is generally observed, namely, solutions of globular then elongated flexible micelles (worm-like micelles) followed by the phases of fluid membranes (successively a lamellar phase and the L₃ (sponge) phase). This is well accounted for by the continuous decrease of the spontaneous curvature of the surfactant film upon variation of some control parameter (temperature or concentration of added salt or of cosurfactant). However, a number of experimental facts are still open to question, e.g.: i) The solutions of worm-like micelles undergo a separation in two distinct isotropic phases upon variation of the control parameter; ii) Upon increasing the concentration of aggregates in the micellar phase, a nematic phase is found to exist in a narrow range of the control parameter. For lower and higher values of this parameter the isotropic micellar phase persists; iii) The unexpected evolution of the viscosity of the solutions of worm-like micelles. We argue that these facts can be interpreted assuming that in the course of the continuous decrease of the curvature of the amphiphilic film the cylindrical aggregates (worm-like micelles) form increasingly more connections allowing for favorable regions of smaller curvature.