Small-angle x-ray and neutron-scattering ͑SAXS and SANS͒ measurements are made of a three-component isometric microemulsion, C 10 E 4 -D 2 O-octane, in the one-phase channel around the hydrophile-lipophile balance temperature of the system. A previous SANS contrast variation experiment indicated that the microstructure of this isometric microemulsion is bicontinuous in water and oil, with the surfactant film having a zero mean curvature. We analyze the SAXS and SANS data taken with a bulk contrast in terms of a modified Berk's random wave model. We choose a spectral function which is an inverse sixth-order polynomial, with three parameters a, b, and c, as introduced by Lee and Chen earlier. This three-parameter spectral function is then used in conjunction with Cahn's clipping scheme to obtain the Debye correlation function for the microemulsion. The analysis gives an excellent agreement with the intensity data in an absolute scale. We then use the three parameters so obtained to calculate the mean Gaussian curvature of the surfactant film. We also show a three-dimensional-reconstructed morphology of the microemulsion.

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Microemulsions are isotropic mixtures of water, oil, and surfactant which self-assemble to form interfacial monolayers with a characteristic length scale on the order of hundreds of Angstroms. Small angle neutron scattering was used to determine the bulk structure of a wateroctane-C10E4 bicontinuous microemulsion. The mean curvature of the surfactant film was verified to be near zero by independently measuring the scattering contributions from the water-surfactant and oil-surfactant interfaces through hydrogen-deuterium contrast variation. The exponentially damped, oscillatory scattering length density profile of the microemulsion near a hydrophobic silicon surface was also observed with neutron reflectivity. The surface reflectivity results as well as the bulk small angle scattering measurements are explained using a simple Ginzburg-Landau theory.