Small-Angle Neutron Scattering of Ca(AOT)2/D2O/Decane Microemulsions

Abstract

Small-angle neutron scattering (SANS) studies were performed in order to investigate the structural properties of the monophasic domain of the Ca(AOT)2/water/decane phase diagram, which shows a consistent reduction of the microemulsion region as compared to the parent sodium system. Microemulsions of Ca(AOT)2/D2O/decane and Ca(AOT)2/H2O/decane-d22 have been studied at 25 °C, for an oil dilution line at constant [water]/[Ca(AOT)2] = 25.1, as a function of decane concentration and of surfactant volume fraction, φS, in the range 0.0186 ≤ φS < 0.0937 for the D2O system, or 0.0249 ≤ φS ≤ 0.0886 for the corresponding decane-d22 system. SANS spectra were analyzed according to a self-consistent method proposed by Sheu1 and support, in analogy to the sodium AOT microemulsion system, the presence of polydisperse droplets in the diluted region studied, while the system shows a pecolative behavior for higher concentrations. These droplets have a 21.3 Å average radius with polydispersity around 22%. The mean area per polar head group and the principal geometrical parameters are also reported.