Structure and Phase Behavior in Four-Component Nonionic Microemulsionst

Structure and droplet-to-bicontinuous structure transitions in a family of four-component microemulsions formed with pentaethylene glycol mono-n-dodecyl ether (C₁₂E₅), D₂O, sodium chloride, and the alkanes octane, dodecane, and hexadecane are probed with small-angle X-ray scattering, electrical conductivity, and NMR self-diffusion measurements. The structure and phase behavior of these microemulsions are linked as droplet-to-biocontinuous transitions precede the appearance of the three-phase region. The droplet-to-bicontinuous transitions in both oil- and water-rich two-phase microemulsions are weakly dependent upon the chain length of the alkanes. The scattering of the dilute water-in-oil and oil-in-water microemulsions is consistent with models treating these microemulsions as dispersions of spheroidal droplets that interact as hard spheres. Scattering from bicontinuous microemulsions with nearly equal oil and water contents is consistent with two models; one based on disordered lamellar structures and another based on the presence of a nearby tricritical point. Both the length scale and the degree of lamellar distortion of these bicontinuous microemulsions decrease with increasing alkane chain length. The surfactant headgroup areas decrease with increasing salinity and decrease slightly as the chain length of the alkane increases.