Formation of cubic structures in microemulsions containing equal volumes of oil and water

Abstract

Microemulsions are dispersions of oil and water stabilized by an interfacial film of surfactant and co-surfactant1. Frequently a short-chain alcohol, such as butanol or pentanol, is used as the co-surfactant. Stable dispersions can often be formulated over a wide range of oil and water volume fractions. In the regions of the phase diagram containing low concentrations of either oil or water, the structure has frequently been found to be one of oil droplets in water or water droplets in oil2. In many cases by simply changing the volume fractions of oil and water, it is possible to effect a gradual transition from an oil-in-water microemulsion to a water-in-oil microemulsion. This raises the question of the structure of the microemulsion in the intermediate region where there are approximately equal volumes of oil and water, and also that of the mechanism of the structural inversion. Microemulsions of such a type are frequently thought of as being constituted by ill-defined oil and water regions arranged with an almost total absence of long-range order3. However, I now report neutron small-angle scattering measurements4 on one such microemulsion containing tetradecyl trimethylammonium bromide, butanol, octane and water which shows a well defined diffraction pattern consistent with a cubic arrangement of oil and water domains. This result shows that microemulsions of this type can be considerably more structured than presently thought and can sometimes resemble cubic liquid crystals.