| As a non-viral vector in the gene transfection, of which the transfer process is closely associated with cell membrane, Gemini surfactant has become a major focus of research. In this work, the interaction between cationic Gemini surfactant (18-s-18, s=3,4,6,8,10 and 12) and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) was investigated by means of Langmuir-Blodgett (LB) technology, atom force microscope (AFM) and transform infrared spectroscopy (FT-IR) spectrum.The interfacial mixed properties of 18-s-18 and DPPC show that when s<6, the Gibbs free energy of mixing and interaction parameter are less than 0, which implies that the mixed films of these two components are more stable than these of one pure component and there appears the attractive interaction between 18-s-18 and DPPC. On the contrary, when s≥6, the Gibbs free energy of mixing and interaction parameter are more than zero in some certain molar fractions, which indicate that the repulsive interaction between 18-s-18 and DPPC increases under these conditions and it becomes more and more strong as the spacer groups rises. The latter means that the mixed film is less stable compared with that of one-component when s>6 in some certain molar fractions and the instability is also increasing with the spacers.The morphologies of the 18-s-18 and DPPC mixed film are significantly varied at the different surface pressures and spacer groups. At the lower surface pressures, the thread and/or sheet structures are formed at the air/water interface, while as the surface pressures rise, these thread and sheet structures have a tendency to aggregate and form the tree-like and/or bigger sheet structures. In addition, the different spacer groups would also lead to the transition of the morphology of the mixed film from the fibril to the sheet structure. These results indicate that the spacer groups and the surface pressure could play an important role in determining the properties of the mixed films.The results of FT-IR spectra are assigned as follows:the absorption bands at 2918 cm-1 and 2850 cm-1 are attributed to the asymmetric and symmetric stretching vibrations of-CH2-of alkyl chains in amphiphiles, respectively, the single absorption band at 1469 cm-1 to the shearing vibration of-CH2-, the band at 1738 cm-1 to the carbonyl absorption peak of the DPPC molecules, the bands near 1235 cm-1 to asymmetric stretching vibration absorption peak of PO2-, which shift from 1233 to 1239 cm-1 as the increasing molar fraction of DPPC. These characteristic bands and the shift of the asymmetric stretching vibration absorption peak of PO2- indicate that i) the alkyl chains are highly ordered (trans-zigzag conformations); ii) as a whole/there is the strong interaction between the PO2- of DPPC and the cationic head group of Gemini surfactant. |
