The ternary phase diagram of lecithin, cholesteryl linolenate and water: phase behavior and structure

The ternary phase diagram of cholesteryl linolenate-egg lecithin-water has been determined by polarizing light microscopy, calorimetry and X-ray diffraction at 23 °C. Hydrated lecithin forms a lamellar liquid-crystalline structure into which small amounts of cholesteryl linolenate are incorporated. The maximum incorporation of cholesterol ester into this lamellar structure varies with the degree of hydration. Increasing the water concentration from 10 to 15% (w/w) increased the limiting molar ratio of cholesteryl linolenate to lecithin in the lamellar phase from 1:50 to 1:22. At intermediate concentrations (15 to 30% water) the cholesteryl linolenate:lecithin ratio remains constant at 1:22. When water is increased to 42.5%, the maximum water content in the lamellar phase, the molar ratio decreased to 1:32. At low water concentrations the cholesterol ester appears to be entirely in the apolar region of the lecithin bilayer, while at higher water concentrations the ester groups of cholesteryl linolenate may be located at the lipid-water interface. At high water concentrations the ester appears to disorder the alkyl chains of the lecithin, giving rise to a thinner lipid layer and an increased surface area per lipid molecule when compared to the lecithin-water system in the absence of cholesteryl linolenate.The lamellar phase is the only phase (except at water concentrations less than 5%) in which all three components mutually interact. All mixtures of the three components having compositions outside the one-phase (lamellar) zone produce additional phases of cholesteryl linolenate or water, or both. Between 23 °C and 60 °C only minor changes in the phase diagram are observed.