Morphology of ionic microemulsions: comparison of SANS studies and the net-average curvature (NAC) model

Abstract

Microemulsion (µE) literature presents numerous scattering studies (neutron, X-ray, light) of ionic and nonionic surfactants that have elucidated the morphological transitions that µEs experience upon changes in formulation conditions such as electrolyte concentration and temperature. Unfortunately, up to now, there is no way to predict the morphology of these µEs, and only after the µE is prepared can the morphology be determined using scattering techniques. In this work we compare the average curvature and drop size predicted by the net-average curvature (NAC) model to the Porod radii and characteristic length obtained from neutron scattering of toluene µEs prepared with sodium dihexyl sulfosuccinate (SDHS) and electrolyte. While the drop sizes predicted by the NAC model do not exactly correspond to the aggregate size obtained after applying a Porod analysis of the SANS profiles, the inverse of the average curvature does match the characteristic size obtained from SANS. This observation is consistent with previous comparisons made for nonionic µEs. The difference between the drop size predicted by the NAC model (that matches the solubilization curves) and SANS morphology suggests that the area per molecule of the surfactant (in contact with the internal phase) changes with the curvature of the system. The area per molecule obtained from Porod plots of Type I and II µEs and from the analysis of the scattering profiles of film-contrast Type III µEs show that as the system approaches net zero curvature the area per molecule increases to a maximum value. The data presented in this work suggests that the NAC model can be used to predict essential elements of the morphology of µEs, which may help in the design of µE-based templated structures (nanoparticles, nanoporous materials, etc.).