Phase behavior of poly(oxyethylene) cholesteryl ether/novel alkanolamide/water systems

The phase behavior and microstructure of mixed nonionic surfactant systems containing poly(oxyethylene) cholesteryl ether (ChEOn, n=15 and 10), a new alkanolamide-type foam booster, dodecanoyl N -methylethanolamide (NMEA-12), and water, were investigated at 25 degrees C by means of visual observation and small-angle X-ray scattering. In the ChEO(15)/water binary system, aqueous micellar (W(m)), discontinuous cubic liquid crystal (I(1)), hexagonal (H(1)), rectangular ribbon (R(1)), lamellar (L(alpha)), and solid (S) phases are successively formed with increasing surfactant concentration. Although the R(1) phase is an intermediate phase formed in a very narrow composition range in conventional surfactant systems, its domain is unusually wider than that of H(1), which may be attributed to the packing constraint caused by the bulky cholesteric group in the lipophilic core of the aggregate. Upon addition of lipophilic NMEA-12 to the ChEO(15)/water binary system, the interfacial curvature of the aggregates decreases, and the micellar or liquid crystal phases formed in the binary system transform to the reverse micellar (O(m)) phase via the L(alpha) phase existing over a wide concentration range. The SAXS results establish an epitaxial relationship between the (11) plane of the R(1) phase and the (10) plane of the L(alpha) phase. The ChEO(10)/NMEA-12/water system shows a phase diagram of similar general appearance, except that the W(m) to R(1) phase transformation occurs via an optically anisotropic liquid crystal phase of unknown structure and the R(1) to L(alpha) phase transition occurs through a narrow intermediate defected lamellar (L(alpha)(H)) phase. The variation in the aggregate size and shape and the unit cell of the R(1) phase formed in ChEOn/NMEA-12/water systems is also discussed.