Oil-induced structural change in nonionic microemulsions

Effect of added oil (heptane or squalane) on the microemulsion structures in polyoxyethylene dodecyl ether (C12EOn) systems was investigated by means of phase behavior and NMR diffusion experiments. In the binary water-C12EOn systems, an isotropic fluid, D-2 (or L-3), and an aqueous micellar solution, Win, phases are successively formed with increasing the EO-chain length. Upon addition of heptane, D-2 and Wm phases are merged and a microemulsion of large solubilization is produced at a low surfactant concentration. With squalane, the solubilization of oil in D-2 phase is very low or almost zero, whereas the oil solubilization in Win phase is relatively large. These structural changes in microemulsions are discussed based on the self-diffusion coefficients of water, oil, and surfactant measured by the PGSE-NMR method. The difference in the phase behavior may be attributed to the difference in the penetration tendency of oil in the surfactant palisade layer.