Effect of Mixing Oils on the Hexagonal Liquid Crystalline Structures

Structural change in hexagonal liquid crystal (H 1 ) in the water/heptaoxyethylene dodecyl ether (C 12 EO 7 ) system upon addition of mixed oil ( m -xylene + decane) is investigated by phase study and small-angle X-ray scattering (SAXS). The surfactant layer curvature tends to be less positive or negative upon addition of the mixed oil at a high m -xylene/decane ratio, and the H 1 −L α (lamellar liquid crystal) transition takes place. At a low m -xylene/decane ratio, however, the H 1 −I 1 transition occurs with increasing mixed oil content, where I 1 is a discontinuous micellar cubic phase. An excess oil phase directly separates from the H 1 phase at a moderate m -xylene/decane ratio. The change in the effective cross sectional area per surfactant, a S, as a function of oil content was also measured at each mixing fraction of oils by SAXS. m -Xylene molecules tend to penetrate the surfactant palisade layer and widen a S, whereas decane molecules tend to be solubilized in the deep core of surfactant hydrophobic moieties and the a S is not largely expanded. The a S does not jump at each transition of H 1 −I 1 and H 1 −L α . Considering the volume of polyoxyethylene chain (EO chain), the repulsion between the EO chains may increase upon addition of decane, whereas it would decrease upon addition of m -xylene. To balance the repulsion with the attraction due to the interfacial tension between water and the lipophilic part without the big change in a S, the phase transitions take place but the change in the surfactant layer curvature is opposite in both oil systems.