Quasielastic scattering of neutrons and light on microemulsion droplets is considered. The droplets are modeled by double-layered fluid spheres immersed in another fluid. The surface layer thermally fluctuates in the shape. For the first time the scattering functions are consistently calculated taking into account all the contributions up to the second order of the fluctuations. The bulk fluids and the layer are characterized by different scattering length densities (or dielectric constants). The theory is applied for the description of neutron spinecho and dynamic light scattering experiments on microemulsions. From the fits to the experimental data the basic parameters of the droplets are extracted. The obtained values for the Helfrich's elasticity moduli differ markedly from those found in the original works. We include into the consideration the viscosity of the layer and give its estimation for the oetane-C~0Es-water microemulsion.
In droplet microemulsions, the surface-active molecules are spread at the oil-water interface and form a dense monolayer. The characteristics of the layer determine to a large degree the properties of the microemulsion. Within the Helfrich's phenomenology [1], the interface is characterized by a few parameters: the bending and saddle-splay moduli ~ยข and tr, the spontaneous curvature Cs, the surface tension coefficient a, and the equilibrium radius of the droplet, R0. The determination of these constants has been attempted by several methods [2-4]. However, different approaches yield very different parameters of the surface film. So, the neutron scattering associated with spin echoes (NSE) [5] gives significantly larger values ic than indirect macroscopical or optical techniques [6]. The hydrodynamic radius of the droplets determined from dynamic light scattering (DLS) is essentially larger than the mean radius found from NSE and small-angle neutron scattering (SANS) [4,7] that can only be explained by the presence of 3 to 4 hydration layers. There are also difficulties with the fit of the intermediate scattering function to the NSE data [5]. To our opinion, the above problems are