The phase diagram of the monoolein/water system: metastability and equilibrium aspects.

Interest in the liquid crystal structure, transport and membrane protein crystallizing properties of the monoolein/water system has grown in the recent past. Monoolein is also an important homolog in a series of monoacylglycerols used to decipher how lipid molecular structure relates to liquid crystal phase behavior--information needed for rational design applications and for understanding the origin of membrane lipid diversity. To make intelligent use of the monoolein/water system, a reliable and detailed temperature-composition phase diagram is needed. The phase diagram of Briggs et al. (J Phys II France 1996;6:723-51) was constructed for this purpose. However, we have established that the liquid crystal phases in the latter below ca. 20 degrees C are metastable. By implementing a sub-zero degree (degrees C) sample incubation prior to data collection in the heating direction, we can reset the system into the lamellar crystal phase which we assume represents equilibrium behavior. We have re-examined the low-temperature part of the phase diagram and characterized structurally the new 'equilibrium' phases by static and time-resolved low- and wide-angle X-ray diffraction and by differential scanning calorimetry. A more complete phase diagram that incorporates the new equilibrium behavior at low temperatures is reported.