Paracrystal model of the high-temperature lamellar phase of a ternary microemulsion system

Small-angle neutron scattering was used to investigate the microstructure of the high-temperature L_α phase in the AOT/water/decane system [AOT = sodium bis(2-ethylhexyl) sulfosuccinate]. The system was modeled as randomly oriented lamellar stacks with a one-dimensional paracrystalline distortion. For samples injected into cells at high temperature, the two-dimensional Q maps show anisotropic scattering consistent with partial alignment of the phase, while samples injected near room temperature do not exhibit this feature. From the latter samples, it was found that a stack typically contains about 15 lamellae, about 80 Å apart. Each layer is coated by a diffuse surfactant interface with a characteristic thickness of approximately 9 Å. The fact that the scattering spectra do not exhibit maxima beyond the first order is explained by the observation that the Hosemann g factor gives about a 20% variation in the interlamellar spacing. The mean spacing is inversely proportional to the surfactant concentration, as expected for a lamellar phase. Over-estimation of the zero-order scattering predicted by the paracrystal model is attributed primarily to improper treatment of the contrast between each layer and the space-filling lamellar matrix.