Dilatometric study of binary mixtures of phosphatidylcholines

Volumes of lipid dispersions as a function of temperature have been measured for two different kinds of binary mixtures of lecithins, (1) DMPC and DSPC and (2) DMPC and DC20PC. Emphasis was placed on DMPC-rich compositions so as to resolve ambiguities regarding solid-phase immiscibility in DMPC-DSPC mixtures. Special attention has been paid to problems of equilibration in the low-temperature phase and to methods of mixing the lipids. We find that there is no solid-solid immiscibility in DMPC-DSPC mixtures, although this system is close to exhibiting such immiscibility, and that DMPC-DC20PC mixtures exhibit pronounced solid immiscibility.