Phase behavior of the monoerucin/water system.

Monoerucin is a monoacylglycerol incorporating a cis monounsaturated fatty acid, 22 carbon atoms long, with the double bond at carbon number 13. The original temperature-composition phase diagram for the monoerucin/water system, constructed on the basis of 'consistency' and light microscopy, includes unexpectedly only the lamellar and fluid isotropic phases [Lutton, E.S., 1965. J. Am. Oil Chem. Soc. 42:1068-1070]. In separate studies using X-ray diffraction, the cubic phase [Larsson et al., 1978. J. Sci. Food Agric. 29:909-914] and the inverted hexagonal (HII) phase [Caffrey, M., 1989. Biophys. J. 95:11-21] were shown to be the dominant phases. We have resorted to the traditional isoplethal diffraction method for constructing the equilibrium phase diagram of monoerucin in water with a view to resolving the controversy regarding the thermotropic and lyotropic properties of the system. The study confirms the presence of the pure HII phase that extends from ca. 10 to 22% (w/w) water and from ca. 35 to 125 degrees C, and its coexistence with excess water above ca. 24% (w/w) water in the same temperature range. The low temperature region of the diagram is dominated by the lamellar crystal (Lc) phase, with the existence of the lamellar liquid crystal (L alpha) and cubic-Ia3d phases at intermediate temperatures and hydration levels. Differential scanning calorimetry and polarizing light microscopy were used to confirm the reported equilibrium phase behavior. Full structure characterization of the system and how phase microstructure depends on sample hydration and temperature are reported.