Cylindrical-to-Spherical Shape Transformation of Lecithin Reverse Micelles Induced by CO2

The effect of CO2 on the microstructure of L-alpha-phosphatidylcholine (lecithin) reverse micelles was studied. The small-angle X-ray scattering (SAXS) results show that CO2 could induce a cylindrical-to-spherical micellar shape transformation. Fourier transform infrared (FT-IR) and UV-vis techniques were also utilized to investigate intermolecular interactions and micropolarity in the reverse micelles at different CO2 pressures. The reduction of the degree of hydrogen bonding between surfactant headgroups and water with added CO2 was found to be the main reason for the micellar shape transformation. In the absence of CO2, the hydrogen bonding between water and P=O of lecithin forms a linking bridge in the interfacial layer. Therefore, the free movement of the polar head of lecithin is limited and the cylindrical reverse micelles are formed. Upon adding CO2 to the reverse micelles, the hydrogen bonds between lecithin and water in reverse micelles are destroyed, which is favorable to forming spherical micelles. Moreover, the CO2-combined reverse micelles were utilized in the synthesis of silica particles. Rodlike silica nanoparticles were obtained in the absence of CO2, and ellipsoidal and spherical mesoporous silica particles were formed in the presence of CO2. This method of tuning micellar shape has many advantages compared to traditional methods.