Phase diagrams of pseudo-binary phospholipid systems. I. Influence of the chain length differences on the miscibility properties of cephaline/cephaline/water systems.

The miscibility properties of homologous cephalines (PEs) were studied by means of differential scanning calorimetry (DSC). The phase diagrams of 5 pseudo-binary cephaline/cephaline/water systems (50% water) are discussed. In the high temperature L alpha-phase, all the homologous cephalines of fatty acid chain length from C = 12 to C = 18 were completely miscible. On the other hand in the low temperature L beta-phase, a miscibility gap occurred in dependence on the differences of the acyl chain lengths. Further, a distinct succession of the phase diagram types was observed according to increasing chain length differences of the PEs: complete miscibility (systems di-(C12:O)-PE/di-(C14:O)-PE/H2O; di-(C14:O)-PE/di-C(16:O)-PE/H2O)----peritectic mixing behaviour (systems di-(C12:O)-PE/di-(C16:O)-PE/H2O; di-(C14:O)-PE/di-(C18:O)-PE/H2O)----eutectic mixing behaviour (system di-(C12:O)-PE/di-(C18:O)-PE/H2O). The change in the type of phase diagram from azeotropic to semi-azeotropic and from semi-azeotropic to eutectic is interpreted by means of the Landau theory.