The Temperature-Composition Phase Diagram and Mesophase Structure Characterization of the Monoolein/Water System

Abstract

The temperature-composition phase diagram of monoolein in water was constructed using X-ray diffraction in the range of $0~^\circ{\rm C}$ to $104~^\circ{\rm C}$ and 0% (w/w) to 47% (w/w) water in the heating direction. The phases identified in this system include the lamellar crystalline (Lc) phase, the lamellar liquid crystalline (Lα) phase, the fluid isotropic (FI) phase, two inverted cubic phases (Q230, Ia3d; Q224, Pn3m), and the inverted hexagonal (HII) phase. The overall features of the monoolein/water phase diagram reported herein match those of existing phase diagrams for this system. There are some important differences, however. Accordingly, 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 phase diagram is based on close to 400 discrete measurements in temperature-composition space recorded as a function of temperature in $5~^\circ{\rm C}$ increments ($3~^\circ{\rm C}$ in the HII phase) and of composition in 2% (w/w) water increments on average. The various mesophases have been characterized structurally as a function of temperature and hydration, and the corresponding thermal and composition expansion coefficients are reported. These and related data show that the average radius of water channels in the fully hydrated bicontinuous cubic Pn3m phase is remarkably sensitive to temperature and to monoacylglycerol chain identity.