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The effect of molecular weight of oil on the three-phase behavior and maximum solubilization was investigated in microemulsion systems of mixtures of di-and octaethylene glycol dodecyl ethers and mixtures of hexanol and octaethylene glycol dodecyl ether at constant temperature (25 or 35 °C). It was found that the minimum weight fraction of surfactant to make equal weights of water and oil to a single phase CXb) increases with increasing molecular weight of oil (hydrocarbon). Nonionic surfactant mainly distributes between micro-oil domains and the interface between microwater and oil domains inside the microemulsion phase. Surfactant molecules at the interface are responsible for the solubilization. Assuming that the monomeric solubility of each surfactant in oil is the same as that in the micro-oil domains of the microemulsion and the microwater domains consist of pure water, the weight fractions of each surfactant at the interface in the microemulsion (Ci or C2) were obtained by a mass-balanced equation. The mixing fraction of surfactant at the interface was also determined by another method based on the geometrical relation of the three-phase tie triangle in the composition tetrahedron. In the hexanol systems, the monomeric solubility of hexanol in water should be taken into account. Ci + C2 indicates the net solubilizing power and it dramatically decreases with increasing the molecular weight of oil.

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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.