SMALL-ANGLE LIGHT-SCATTERING-STUDIES OF DENSE AOT-WATER-DECANE MICROEMULSIONS

We have performed extensive studies of a three-component microemulsion system composed of AOT-water-decane (AOT = sodium-bis-ethylhex-sulfos uccinate is an ionic surfactant) using small-angle light scattering (S ALS). The small-angle scattering intensities are measured in the angul ar intel val 0.001-0.1 radians, corresponding to a Bragg wave number r ange of 0.14 mu m(-1) < &< < 1.4 Irm-l The measupements were made by c hanging temperature and volume fraction phi of the dispersed phase (wa ter + AOT) in the range 0.05 < phi < 0.75. All samples have a fixed wa ter-to-AOT molar ratio, w = [water]/[AOT] = 40.8, in order to keep the same average droplet size in the stable one-phase region. With the SA LS technique, we have been able to observe all the phase boundaries of a very complex phase diagram with a percolation line and many structu ral organizations within it. We observe at the percolation transition threshold, a scaling behavior of the intensity data. This behavior is a consequence of a clustering among microemulsion droplets near the pe rcolation threshold. In addition, we describe in detail a structural t ransition from a droplet microemulsion to a bicontinuous one as sugges ted by a recent small-angle neutron scattering experiment. The loci of this transition are located several degrees above the percolation tem peratures and are coincident with the maxima previously observed in sh ear viscosity. From the data analysis, we show that both the percolati on phenomenon and this novel structural transition are derived from a large-scale aggregation between microemulsion droplets.