Small-angle X-ray scattering technique has been used to study multilayer lipid membranes of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and the 3:1-mixture DPPC/1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) in excess water. The temperature dependences of the repetition period for lipid bilayers in the temperature range 20–55 ◦C are obtained. A comparative analysis of the scattering curves obtained for multilayer membranes showed that, below a temperature of 40 ◦C, there emerges an additional ordering with a repetition period of 66 Å in the lipid mixture, which we associate with the lipid phase separation. A disappearance of the so-called ripple (wave-like) phase of DPPC lipid in the mixture is also observed.

The results of a study of the temperature and pressure dependence of isothermal compressibility of dimyristoylphosphatidylcholine (DMPC) are presented. The data were obtained using a small-angle X-ray scattering instrument in the Institute of Synthetic Polymer Materials (Moscow, Russia) and a device designed in the Laboratory of Neutron Physics, Joint Institute for Nuclear Research (Dubna, Russia), for studying the liquid systems by a volumetric P-V-T method. The minute baric investigation of the region of the main phase transition (transition from the gel phase into the liquid crystalline phase) was carried out. The points of the main phase transition were determined, and the phase diagram of DMPC was plotted over the pressure range of 1–800 bar at temperature from 24 to 35°C.