Lyotropic and Thermotropic Phase Behavior of Hydrated Monoacylglycerols: Structure Characterization of Monovaccenin (C18:1c11). Hong Qiu & Martin Caffrey, The Ohio State University, Columbus, OH 43210

The temperature-composition phase diagram of monovaccenin (a C18:1c11 monoacylglycerol) in water was constructed using small-angle x-ray scattering, in the temperature range of ca. 0 deg.C to 110 deg.C and composition range of ca. 0 % to 60 %(w/w) water in the heating direction. Since the long term objective of this research effort is to establish the relationship between lipid molecular structure and lyotropic/thermotropic mesophase propensity by comparing phase diagrams of a homologous series of lipids, every effort has been taken to ensure that the current phase diagram represents equilibrium behavior and that the assorted phase boundaries have been determined accurately. The interpreted equilibrium phase diagram and the "nonequilibrium phase diagram" of the system are based on several hundred discrete x-ray diffraction measurements in temperature-composition space recorded as a function of temperature in 5 deg.C increments and of composition in 4 %(w/w) water increments on average. The phases identified in this system include the lamellar crystalline (L_c) phase, the lamellar liquid crystalline (L_a) phase, the fluid isotropic (FI) phase, two inverted cubic phases (Q²³⁰, Ia3d; Q²²⁴, Pn3m), and the inverted hexagonal (H_II) phase. The monovaccenin/water phase diagram reported herein is remarkably similar to that of the monoolein (a C18:1c9 monoacylglycerol)/water system. However, a comparison of these systems shows that there are important differences in transition temperatures and phase boundary positions, because of differences in the effective molecular length and shape. We have found that these two structural features are important elements in the relationship between molecular structure and lyotropic/thermotropic mesophase propensity. The sensitive response to temperature and lipid identity of the average water channel radius in the fully-hydrated, bicontinuous cubic Pn3m phase makes monoacylglycerol systems important potential candidates for controlled drug release.