The temperature-composition phase diagram of monomyristolein in water: equilibrium and metastability aspects

The temperature-composition phase diagram of monomyristolein in water was constructed using x-ray diffraction. Low- and wide-angle diffraction patterns were collected from samples of fixed hydration as a function of temperature in the heating direction on x-ray-sensitive film and/or image plates. The phases identified in the system include the lamellar crystalline phase, the lamellar liquid crystalline phase, the fluid isotropic phase, and two inverted cubic phases. Particular attention has been devoted to the issues of phase equilibrium and phase boundary verification. Cubic phase undercooling was examined by adjusting the temperature of several samples in the cubic phase to a value where the lamellar liquid crystalline phase represents equilibrium behavior. Cooling-induced structure and phase changes were monitored continuously over a 30-min period by recording low-angle diffraction patterns from the samples using a streak camera. The cubic-to-lamellar transition rate decreased with increasing sample hydration. Additionally, the transition proceeded more rapidly at an incubation temperature of 25 degrees C compared to that at 0 degrees C. A mechanism is proposed that accounts for the hydration and temperature sensitivity of the phase transition under nonequilibrium conditions.