Porod’s limit of small angle x-ray scattering from AOT-H2O isooctane micro-emulsions

Water-oil micro-emulsions consisting of AOT-H2O isooctane of different water-surfactant, W 0, and oil-surfactant, S 0, ratios, have been studied by SAXS over a wide rang eof scattering vectors. The shape of the high angle portion of the scattered intensity is found to be independent of the microemulsion composition and furthermore, does not follow the classical 1/q 4 Porod Law. Rather an additional 1/q 2 contribution to the scattering curve is observed. These results are explained in terms of the distance correlation function for a pseudo two-phase system, water and oil, with a diffuse interface formed by a monomolecular layer of surfactant molecules. The observed 1/q 2 behavior is shown to arise from a 1/r contribution to the correlation function. A quantitative analysis shows that the total interfacial area of the microemulsion is proportional to the total number of surfactant molecules in solution. Thus the average area per surfactant molecule is independent of the microemulsion composition.