Microstructural and Viscoelastic Properties of Liquid Crystal Phases of Soybean Lecithin with Olive and Castor Oils at Low Water Contents

Phase behaviors and microstructural and viscoelastic properties of ternary systems of lecithin/vegetal oils/water at relatively low contents of water are studied. The knowledge about the microstructures and rheological properties of the phases may contribute to improving the development of new biocompatible excipients. Phase determinations are based on a combination of optical microscopy, small-angle X-ray scattering, and rheology measurements. The effect on the phase behavior of the different degrees of saturation of the different fatty acid contents is studied for two biocompatible vegetal oils: olive oil and castor oil. With biocompatible vegetal oils, the lamellar phase is preferred, and the cubic structure coexists in all cases with other liquid crystalline phases but does not appear in a monophasic region. Castor oil can form a pure inverse micellar region. Hexagonal liquid crystalline samples behave like solids. The lamellar liquid crystalline samples behave like a typical weak gel, slightly depending on frequency. For the lamellar phase, high amounts of oil favor a stiffer and more elastic lamellar structure at 37 °C compared to the lamellar bilayer at 25 °C. The increase in temperature may result in a higher degree of structural ordering and the lamellar phase region becomes smaller.