A Study On The Mixture System Of Poly (D, L-lactide-co-glycolide) And Amphiphilic Molecules At Air/water Interface

Poly(D,L-lactide-co-glycolide) (PLG) have recently received much attention due to their biodegradable, biocompatible and nontoxic features, and they have been applied to drug release and DNA delivery systems. However during microparticle preparation, the DNA was exposed to conditions that could cause denaturation and degradation, such as organic/aqueous interface, localized high temperature, and freeze drying. In addition, the use of very high doses of DNA is less favorable from a process economics standpoint. Therefore people have tried to solve the released efficiency in the DNA delivery system by all kinds of ways. Among these a well develop way was that using cationic amphiphile modify the microparticle of PLG in order to obtain the cationic microparticles. In this way DNA could be absorbed to the surface of the microparticle by the electrostatic interaction. After preparation and characterization, a significant improvement was achieved comparing the naked DNA. However the application of this system was still based on the experience, so understanding the interaction of the PLG with amphiphile was very important. Because of the complexity of the mixture of polymer with surfactant, we researched the system on the simplest two-dimensional interface, that is, air/water interface. We synthesized a series of amphiphilic surfactants with azobenzene, which included cationic surfactants and nonionic surfactant. Firstly we researched the properties of the amphiphilic surfactants and PLG at air/water interface respectively by π-A isotherms, BAM and AFM. We knew that amphiphilic surfactants could exist at air/water interface stably and the aggregates of the surfactants dispersed evenly. While PLG could aggregate so easily that there were some dot aggregates on very low surface pressure at air/water interface. And there was an obvious phase transition at air/water interface. In order to understand the interaction between amphiphiles and PLG, we obtained a series of π-A isotherms which has different molar ratio of the amphiphiles. And the miscibility of the PLG with amphiphiles could be obtained by Molar fraction-Area on different surface pressure. The results showed that the miscibility of PLG with cationic amphilphile was very well while the phase separation between PLG and nonionic amphiphle occurred at air/water interface. And the conclusion was also proved by the images of BAM and AFM. The miscibility of cationic amphiphiles with PLG was related to the polarity of the amphiphiles. We put the mixture of PLG with amphiphiles on DNA aqueous subphase, the results of the π-A isotherms, images of BAM and AFM showed that the mixture of cationic surfactants with PLG could complex with DNA evenly at air/water interface while the mixture of nonionic surfactant with PLG could not complex with DNA. We used the characteristic of the pyridine group which could be protonized by adjust the pH of the subpahse understanding the relation between nonionic surfactant and cationic surfactant. From the π-A isotherms and BAM images, it has been proved that the nonionic surfactant could be protonized by reducing the pH of the subphase and the extent of the protonize was increased with decreasing pH value. It was interesting that when the mixture of PLG with the amphiphile was spread onto the aqueous subphase with different pH value, the π-A isothermsof the mixed system showed that the phase transitions of PLG and C12AzoC6OPy could still keep respectively for they had been separated absolutely at the air/water interface. And it was amusing that dendritic crystals arose under low surface pressure when the pH of subphase was just 2.6. Accompanying the surface pressure up to the plateau, two different phases, the circular structure could be observed obviously. We also detect the phase separation of LB films of the mixture with different pH of subphase by atomic force microscopy (AFM), the results were also in good agreement with those of BAM and it can prove that the phase separation between PLG and C12AzoC6OPy be in the up and down way.