Characterization of a Sucrose Ester Microemulsion by Freeze Fracture Electron Micrograph and Small Angle Neutron Scattering Experiments

A microemulsion system, containing biodegradable sugar surfactants and a nontoxic cosurfactant for pharmaceutical purposes, is characterized in a sucrose ester/ethyl or cetyl 2-(hexylethyl)-2-hexanoate (EHEC8 or EHEC16)/diethyleneglycol monoethyl ether (DME)/water system. The phase behavior and structure of these systems are intimately linked by the nature of the amphiphiles and the chain length of the oily ester. By adjusting the nature of the oil and the composition of the surfactant mixture, Winsor IV systems containing low amounts of a nonaggressive surfactant composition and equal amounts of water and oil could be formulated. For a given surfactant + cosurfactant-to-oil ratio, a dilution line is investigated in the sucrose monolaurate + sucrose dilaurate/DME/EHEC8/water system, in which numerous systems are prepared by adding from 20 up to 40% water (Φ w ). Along this dilution path, the microstructure of the system is characterized by rheological, small angle neutron scattering (SANS), and freeze fracture electron micrograph experiments (FFEM). The microemulsion nature of the samples is assessed by SANS. The SANS results are analyzed by means of the Teubner−Strey model, and the microemulsion nature of the three selected samples (Φ w = 25, 30, and 32%) is confirmed. The investigated samples exhibit a shear thinning process related to a reorganization mechanism, which occurs under shear. However, in the samples containing more than 30% water, a second rheological transition occurs, which is related to the steep decrease of the apparent viscosity around γ = 1000 s - 1 . This second shear thinning is explained by the presence of two structures in these samples. The microstructures of the two samples have not yet been completely determined with the FFEM technique. The specific bicontinuous structure of one sample has been confirmed and characterized by Φ w = 25%.