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.
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.
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