POLYMORPHISM, MESOMORPHISM, AND METASTABILITY OF MONOELAIDIN IN EXCESS WATER

The polymorphic and metastable phase behavior of monoelaidin dry and i n excess water was studied by using high-sensitivity differential scan ning calorimetry and time-resolved x-ray diffraction in the temperatur e range of 4 degrees C to 60 degrees C. To overcome problems associate d with a pronounced thermal history-dependent phase behavior, simultan eous calorimetry and time-resolved x-ray diffraction measurements were performed on individual samples. Monoelaidin/water samples were prepa red at room temperature and stored at 4 degrees C for up to 1 week bef ore measurement. The initial heating scan from 4 degrees C to 60 degre es C showed complex phase behavior with the sample in the lamellar cry stalline (L(c0)) and cubic (Im3m, Q(229)) phases at low and high tempe ratures, respectively. The L(c0) phase transforms to the lamellar liqu id crystalline (L(alpha)) phase at 38 degrees C. At 45 degrees C, mult iple unresolved lines appeared that coexisted with those from the L(al pha) phase in the low-angle region of the diffraction pattern that hav e been assigned previously to the so-called X phase (Caffrey, 1987, 19 89), With further heating the X phase converts to the Im3m cubic phase . Regardless of previous thermal history, cooling calorimetric scans r evealed a single exotherm at 22 degrees C, which was assigned to an L( alpha) + cubic (Im3m, Q(229))-to-lamellar gel (L(beta)) phase transiti on. The response of the sample to a cooling followed by a reheating or isothermal protocol depended on the length of time the sample was inc ubated al 4 degrees C. A model is proposed that reconciles the complex polymorphic, mesomorphic, and metastability interrelationships observ ed with this lipid/water system. Dry monoelaidin exists in the lamella r crystalline (beta) phase in the 4 degrees C to 45 degrees C range. T he beta phase transforms to a second lamellar crystalline polymorph id entified as beta at 45 degrees C that subsequently melts at 57 degree s C. The beta phase observed with dry monoelaidin is identical to the L(c0) phase formed by monoelaidin that was dispersed in excess water a nd that had not been previously heated.