The effect of added decane, m-xylene, or squalane on the phase behavior of the polyoxyethylene dodecyl ether (C12EO n ) −water system was investigated as a function of polyoxyethylene (EO) chain length at 25 °C. When the surfactant is relatively lipophilic (more lipophilic than the balanced state), the type of liquid crystal is changed to the more lipophilic one upon addition of decane. The lamellar (Lα) to lipophilic reverse hexagonal (H2) transition takes place in the C12EO3 system. On the other hand, the hexagonal (H1) to hydrophilic discrete cubic (I1) liquid crystal transition occurs in the hydrophilic C12EO7 system. There are two kinds of effects of oil on the self-organizing structures. One is the “penetration effect”, in which oil molecules penetrate into the surfactant palisade layer and expand the effective cross-sectional area, a S. The other is “swelling effect”, in which oil molecules are solubilized in the core of aggregates and expand the volume of aggregates. In this case, the a S is almost constant. Due to the swelling effect, the H1−I1 phase transition takes place in the decane−C12EO7 system, whereas the Lα−H2 transition occurs in the decane−lipophilic C12EO3 system due to the penetration effect. This causes the opposite tendency of the change in the curvature of the surfactant layers in liquid crystals on each side of the balanced state. As a result, the phase behavior of surfactant is quickly changed from forming micelles to reverse micelles within a narrow range of the EO chain in the presence of oil. On the other hand, m-xylene tends to penetrate the surfactant palisade layer, and the H1−Lα transition occurs even in the hydrophilic C12EO7 system.

A phase diagram of a water−poly(oxyethylene) oleyl ether (POlE) system was constructed as a function of poly(oxyethylene) chain length at 25 °C. The POlEs contain a highly pure oleyl group, whose purity is above 99.7%. The POlEs are in a liquid state over a wide range of composition. The increase in the poly(oxyethylene) (EO) chain of POlE corresponds to the increase in the curvature of surfactant layer toward water or the increase in HLB (hydrophile−lipophile balance) number of the surfactant. Various self-organizing structures were found:  hexagonal and lamellar lquid crystals, four kinds of isotropic liquid crystals, a sponge phase, and reverse hexagonal liquid crystal. The phase transition between normal hexagonal and lamellar liquid crystals were investigated at constant volume fraction of the oleyl group in system by means of small-angle X-ray scattering. Correlation among the phase behavior, the packing of oleyl chain in self-organizing structures, and the HLB of POlE is discussed. The effect of temperature on the phase behavior in the present system is also discussed.

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Phase behavior of a mixed surfactant, sodium dodecyl sulfate + lipophilic poly(oxyethylene) dodecyl ether, in a brine-decane system was investigated at a constant brine/decane weight ratio equal to 1. Solubilization capability of the mixed surfactant reaches its maximum and microemulsion is formed when the surfactant is changed from hydrophilic to lipophilic in a given system. In the present system, lamellar liquid crystal (LC) intrudes in the single-microemulsion region, and three-phase microemulsions are not formed. The mixing fraction of nonionic surfactant in the total surfactant in the midst of the LC present region increases with increasing oil content due to the high solubility of nonionic surfactant in oil. The partition of nonionic surfactant molecules between the oil and the bilayer in the LC phase is analyzed by using the geometrical relationship of the phase equilibria in the phase diagrams, taking into account the solubility. The monomeric solubility of nonionic surfactant in oil is much less than that of an ordinary cosurfactant like hexanol, and the mixing fraction of nonionic surfactant in the bilayer decreases with increasing salinity. The interlayer spacing of the midlamellar liquid crystal between the two microemulsion regions was measured by small-angle X-ray scattering. The average effective cross-sectional area per surfactant is about 0.37 nm2 and is unchanged upon dilution. It is considered that there is a strong attractive interaction between the ionic group and the nonionic hydrophilic moiety of surfactants.