We present an experimental study of an AOT-decane-aqueous 0.5% NaCl microemulsion in which the oil-to-water ratio can be arbitrarily varied. This allows an investigation of the structural inversion from an water-in-oil microemulsion into an oil-in-water microemulsion which happens completely continuously without any phase separation. Conductivity, viscosity, and electrooptical Kerr effect data confirm the presence of two percolation processes. This structural inversion takes place in two stages: With increasing oil content, first a water continuous microemulsion transforms into a bicontinuous structure at roughly 20% oil (oil percolation threshold) and then at roughly 80% oil (water percolation threshold) and bicontinuous microemulsion turns into an oil continuous structure. These two percolations margin three structural regimes of a microemulsion: oil-in-water regime, water-in-oil regime, and a bicontinuous or oil and water continuous regime.

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The occurrence of a negative birefringence in water-in-oil (WjO) microemulsions has been substantiated and analyzed. The analysis is based on the well-established aqueous nanodroplet model of W/O microemulsions. In the particularly investigated waterjsodium bis(2-ethy1hexyl)sulfosuccinate (AOT)/aliphatic oil systems, the AOT monolayer exhibits a negative induced intrinsic birefringence, which is brought about by the pronounced polarizability of the alkyl moiety of the AOT molecule approximately perpendicular to the direction of its permanent dipole moment.