SMALL-ANGLE NEUTRON-SCATTERING INVESTIGATION OF STRUCTURAL INVERSION IN A 3-COMPONENT IONIC MICROEMULSION

We have made an extensive investigation of the phase diagrams and the associated micro-structure of a pseudo-ternary system AOT/water (0.6 wt% NaCl)/decane. This micro-emulsion system shows, for a surfactant concentration in excess of 6 wt%, a characteristic phase progression from a water-in-oil (W/O) micro-emulsion in coexistence with excess water at low temperatures to an oil-in-water (O/W) micro-emulsion in coexistence with excess oil at high temperatures through a one-phase micro-emulsion in the intermediate temperatures. Thus, one expects a structural inversion to occur somewhere in the one-phase channel. We have performed extensive small angle neutron scattering (SANS) measurements to study the pathway of such a structural inversion in this micro-emulsion system when there are comparable amounts of oil and water in the system, and when the volume fraction of water is much higher than that of oil. In the former case, we observed the inversion of the W/O to the O/W micro-emulsions through an intermediate bicontinuous one-phase micro-emulsion. In the latter case, we show that the inversion takes place through a pathway of L3-L-alpha-L1 as temperature increases. Particular emphasis is put on analyses of the interfacial structure in different micro-emulsion phases.