Segregation and phase transition in mixed lipid films

Energy dispersion X-ray diffraction (EDXD) was applied to investigate the structure of partly dehydrated mixed films formed by the phospholipid dimyristoyl phosphatidylcoline (DMPC) and any of the three diastereomers of the dicationic gemini surfactant (2S,3S)-2,3-dimethoxy-1,4-bis(N-hexadecyl-N,N-dimethylammonium) butane dibromide. As the surfactant to lipid molar ratio (R-S/L) increases, the gemini monotonically solubilizes the lipid bilayer promoting the formation of a cubic phase of space group Pm3n segregating from the residual lamellar phase of the lipid. Finally, at R-S/L = 1, the phase transition is complete. The mixed film at the highest surfactant to lipid molar ratio (R-S/L = 2.3) was hydrated by a vapor saturated atmosphere. At full hydration, a cubic to lamellar phase transition occurs. Coarse grain dynamic investigations, carried out as a function of both the surfactant to lipid molar ratio and the number of water molecules for amphiphile unit, allowed us to elucidate the structure of the emerging cubic phase and the hydration-induced structural pathway of the cubic to lamellar phase transition observed by EDXD.