A light scattering study of spinodal decomposition in systems containing surfactant molecules

Abstract

We have performed a series of spinodal decomposition measurements on two critical systems containing surfactant molecules, i.e. (i) a three-component microemulsion made up of a surfactant (aerosol-OT), water and decane, and (ii) a water - butoxyethanol micellar-like mixture. The measurements were performed, via a temperature jump crossing the coexistence line along the critical isovolume fraction line, using the time-resolved light scattering intensity technique. All of the three stages of the evolution were studied. The time evolution of intensities for the initial stage closely follows the linearized theory. The time evolution of the characteristic wavevector, the maximum scattered intensity, and the intensity distribution in the intermediate and late stages are discussed in the framework of dynamical scaling theories and compared with the findings of recent molecular dynamics simulations. Such a comparison of the data obtained with the numerical analysis carried out for binary mixtures containing surfactant gives a clear indication that in the systems investigated the phase-separation process occurs through the formation of different structural coalescing domains. Whereas in the AOT microemulsion system the separation occurs through the formation of irregular bicontinuous-like domains, in the water - mixture the separation is dominated by a process of formation of micellar domains. In addition, in the latter case thermal fluctuation processes have a significant effect on the coarsening of the domain structures.