TEMPERATURE-DEPENDENT AND PRESSURE-DEPENDENT PHASE-BEHAVIOR OF MONOACYLGLYCERIDES MONOOLEIN AND MONOELAIDIN

We used x-ray and neutron diffraction to study the temperature- and pr essure-dependent structure and phase behavior of the monoacylglyceride s 1-monoelaidin (ME) and 1-monoolein (MO) in excess water. The monoacy lglycerides were chosen for investigation of their phase behavior beca use they exhibit mesomorphic phases with one-, two-, and three-dimensi onal periodicity, such as lamellar, an inverted hexagonal and bicontin uous cubic phases, in a rather easily accessible temperature and press ure range. We studied the structure, stability, and transformations of the different phases over a wide temperature and pressure range; expl ored the epitaxial relations that exist between different phases, and established a relationship between the chemical structure of the lipid molecules and their phase behavior. For both systems, a temperature-p ressure phase diagram has been determined in the temperature range fro m 0 to 100 degrees C at pressures from ambient up to 1400 bar, and dra stic differences in phase behavior are found for the two systems. in M O-water dispersions, the cubic phase Pn3m extends over a large phase f ield in the T,p-plane. At temperatures above 95 degrees C, the inverte d hexagonal phase is found. In the lower temperature region, a crystal line lamellar phase is induced at higher pressures. The phases found i n ME-water include the lamellar crystalline L(c) phase, the L(beta) ge l phase, the L(alpha) liquid-crystalline phase, and two cubic phases b elonging to the crystallographic space groups lm3m and Pn3m. In additi on, the existence of metastable phases has been exploited. Between coe xisting metastable cubic structures, a metric relationship has been fo und that is predicted theoretically on the basis of the curvature elas tic energy approximation only.