Solubilization of Micellar Cubic Phases and Their Structural Relationships in Anionic−Cationic Surfactant−Dodecane−Water Systems

The morphology and solubilization of micellar cubic phases are investigated as a function of the cationic weight fraction in the mixture of two surfactants, the surfactant-to-water ratio, and the oil weight fraction in the water−dodecane−sodium dodecyl sulfate (SDS)−dodecyltrimethylammonium bromide (DTAB) or didodecyl-dimethylammonium bromide (DDAB) systems at 25 °C. One phase transition H 1 → I 1 appears at low cationic weight fraction, Y, and the other Lα → I 1 takes place at high Y upon addition of oil. Solubilization for dodecane of the micellar solution and of the micellar cubic phase increases with an increase in the cationic surfactant weight fraction Y from 0 to 0.30. SAXS measurements index the structure of the cubic phase as the discontinuous Pm 3 n cubic phase for single surfactant systems and mixed surfactant systems at different levels of oil. The cross-sectional area of the hydrophilic head of the surfactant keeps constant with increase in the oil weight fraction at a fixed cationic surfactant weight fraction as the interlayer spacing of the Pm 3 n cubic phase greatly increases. However, it decreases with an increase in the cationic surfactant weight fraction at fixed oil weight fraction as the interlayer spacing of the Pm 3 n cubic phase slowly increases. So the solubilization for oil can be increased by adjusting the mean curvature of the surfactant interface layer with the addition of mixed surfactants and long-chain oil.