Phase equilibria and structure of dry and hydrated egg lecithin.

The behavior of purified egg lecithin in water has been investigated in relation to the quantity of water present and the temperature. The complete binary phase diagram of egg lecithin-water is presented as well as X-ray diffraction data on selected mixtures. Dry egg lecithin is present in at least partially crystalline form until about 40 degrees C. Above this temperature it forms a "wax-like" phase up to about 88 degrees C. From 88 to 109 degrees C it forms a viscous isotropic phase which gives face-centered cubic spacings by X-ray analysis. Above 110 degrees C its texture is "neat" and the structure is assumed to be lamellar until its final melting point at 231 degrees C. Hydrated lecithin forms (except for a small zone of cubic phase at low water concentrations and high temperature) a lamellar liquid crystalline phase. This phase contains up to 45% water at 20 degrees C. Mixtures containing more water separate into two phases, the lamellar liquid crystalline phase and water. In the melting curve of hydrated lecithin a eutectic is noted at about 16% water and the cubic phase seen when less water is present disappears at this composition of the mixture. These facts, along with previous vapor pressure measurements, suggest that there is a structural change at about 16% water. X-ray diffraction studies of lecithin at 24 degrees C and calculations from these data suggest that the reason for this may be the presence of a "free water layer" when more than 16% water is present.