Small angle neutron scattering was used to measure a series of water-octane-${\mathrm{C}}_{10}$${\mathit{E}}_4$ mixtures in the disordered microemulsion and lamellar phases. Varying the hydrogen-deuterium contrast in oil as well as water enabled us to isolate and determine the scattering contributions from the surfactant monolayers and the water-surfactant and oil-surfactant interfaces. Analyses of the resulting scattering patterns that include the effects of water and oil peneration allow us to demonstrate and quantify the inversion of the surfactant film’s small mean curvature as a function of temperature and its relation to the overall phase behavior.

• Received 15 March 1994

DOI:https://doi.org/10.1103/PhysRevLett.73.106