The Structure of Gemini Surfactant Self-Assemblies Investigated by Energy Dispersive X-ray Diffraction

Role of the spacer stereochemistry on the structure of solid-supported gemini surfactants aggregates

Energy dispersive X-ray diffraction was applied to investigate the role of the spacer stereochemistry on the structure of the solid supported aggregates of three stereoisomeric cationic gemini surfactants, 2,3-dimethoxy-1,4-bis-(N-hexadecyl-N,N-dimethylammonio)butane dibromide. Solid-supported Gemini surfactant aggregates self-assemble into highly interdigitated multibilayer stacks. Structural properties, such as the bilayer thickness, the headgroup size, the thickness of the hydrophobic core, and the size of the interbilayer water region, were derived from electron density profiles. Results show that the stereochemistry of the spacer controls the structural properties of the solid-supported interfacial aggregates.

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 Pmn 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.