Effect of added oil (heptane or squalane) on the microemulsion structures i
n 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 solutio
n, 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. Wit
h squalane, the solubilization of oil in D-2 phase is very low or almost ze
ro, whereas the oil solubilization in Win phase is relatively large. These
structural changes in microemulsions are discussed based on the self-diffus
ion coefficients of water, oil, and surfactant measured by the PGSE-NMR met
hod. The difference in the phase behavior may be attributed to the differen
ce in the penetration tendency of oil in the surfactant palisade layer.