Small-angle neutron scattering study of droplet density dependence of the water-in-oil droplet structure in a ternary microemulsion

Droplet density, ϕ, dependence of the water-in-oil droplet structure of microemulsion was investigated by means of small-angle neutron scattering (SANS) at room temperature in a ternary system consisting of AOT (Aerosol-OT; dioctyl sulfosuccinate sodium salt), water, and n-decane. SANS profiles from 2 different contrast samples of the bulk contrast (AOT/D₂O/C₁₀H₂₂) and of the film contrast (AOT/D₂O/C₁₀D₂₂) were collected. By introducing a relative form factor as the ratio of scattering functions of the bulk and film contrast samples, we evaluated the ϕ dependence of the shape and size of droplets without the influence of the structure factor, S(Q). The mean radii of droplet core and shell were constant, while the polydispersity index decreased for ϕ ≤ ϕ^* ≃ 0.62. The evaluated structure factors S(Q) from the bulk-contrast samples were in good agreement with those from the film contrast. From the ϕ dependence of S(Q), we confirmed that the droplet density fluctuations were dominant at low-ϕ regime and the inter-droplet correlations at high-ϕ regime. The crossover concentration of these two regimes was estimated to be about 0.25. It is demonstrated that this relative form factor method is quite useful for SANS experiments.