OBSERVATION OF SURFACE-INDUCED LAYERING IN BICONTINUOUS MICROEMULSIONS

Neutron-specular-reflectivity measurements were made on liquid surfaces of disordered one-phase microemulsions consisting of mixtures of nonionic surfactant C10E4, D2O, and octane, with equal volume fractions of water and oil, in the region of the phase diagram where the bulk microemulsion structure was known to be bicontinuous. From these measured reflectivity date, the one-dimensional scattering-length-density profiles in the vertical direction of the air-microemulsion interfaces were reconstructed directly by using a model-independent inversion scheme developed recently by one of us. From the reconstructed scattering-length-density profiles, we constructed the one-dimensional Debye correlation functions. From these correlation functions, we deduced the surface-induced layering with periods that correlate well with the bulk water-water domain average repeat distance known from a previous small-angle neutron-scattering study. We discuss in particular the effect of a finite-Q range of data on the height of the first D2O layer.