Study On Spontaneous Formation And Stability Of Vesicle

Vesicles are selfclosed bilayer aggregates and similar to cell membranes. In recent years, there has been increasing interest in vesicles, because of their potential application in biology and medicine as modeling of cell membranes, agents in encapsulation and release of drugs, and microreactor in some special chemical reaction and preparation of nanoparticles. There are two ways prepared vesicles: One, using the exterior energy such as ultrasonic and extrusion; The other, spontaneous formation at right condition, this way has many advantages such as simple operation, stable vesicles and so on. From first reported by Kaler et al. that vesicles may form reversibly from a mixture of cationic and anionic single-chained aliphatic surfactants, scientist have made much study in this aspect.Usually, vesicles formed from mixed surfactants system are very stable, but in some condition, vesicles can aggregate with each other easily. By now, scientists have studied the effect of temperature, addition of salt and the aggregation of vesicles.Today, spontaneous vesicles have been prepared from aqueous mixtures of cationic/anionic surfactants, nonionic surfactant/cosurfactant, zwitterionic/anio- nic surfactants and cationic/cationic surfactants. In this paper, the object of study is cationic/anionic surfactants system. Here, the condition of vesicle formation, the mechanism of vesicle formation and the character of the vesicle were detailed studied.In the first section of the thesis, we dealt with a system composed by two single-tailed surfactants, cationic cetyltrimethylammonium bromide(CTAB) and anionic sodium dodecylbenzenesulfonate(SDBS). Here, the effect of concentration, mixed ratio and prepare procedure on the formation of vesicles were detailed discussed. The results showed that only when CTAB/SDBS≤1/1.6 vesicles can spontaneously form at the concentration of 0.028mol/L; for the system CTAB/SDBS=1/2.4, vesicles spontaneous form when the concentration more than 0.0014mol/L, and vesicles spontaneous form when the concentration more than 0.007mol/L for the system CTAB/SDBS=1/8; Three different preparation procedures were used to prepare samples when CTAB/SDBS=1/2.4 and concentration is 0.028 mol/L, all procedures can spontaneously form vesicles. For the system in which vesicles had formed, for example CTAB/SDBS=1/2.4, when the system was diluted into 0.0018 mol/L, it took on colorless. At the same time, negative staining TEM technique was used to observe the shape of vesicles, the photographs showed spherical vesicles with polydisperse distribution. The sample with molar ratio CTAB/SDBS =1/2.4 and total surfactant concentration 0.028mol/L was chosen to observe in detail, the phenomenon of aggregation and fusion between vesicles explained the diameter of vesicles increased with aging time, at the same time the trend of disorganization of vesicles was also observed. Pack parameter theory of Israelachvil and the model of vesicle formation of Masahiko were used to explain the mechanism of CTAB/SDBS vesicle formation. Besides, NMR and UV technique were utilized and proved that there was no cation-Пinteraction between surfactants CTAB and SDBS.In the second section of the thesis, the formation and stability of vesicles formed from mixture of cationic cetyltrimethylammonium bromide (CTAB) and anionic surfactant were investigated. The three different anionic surfactants were sodium dodecylbenzenesulfonate (SDBS), sodium dodecyl sulfonate (SDS) and sodium dodecyl sulfate (SLS) respectively. The results showed that vesicles all can form in three cationic and anionic (cationanionci) surfactant systems. Although three anionic surfactants have similar molecular structure, when vesicles formed the mixed ratio of catanionic surfactant and concentration were entirely different. Besides, the stability of attained vesicles was also different. The vesicles can form only when CTAB/SDBS≤1/1.6 for CTAB/SDBS system, and the vesicles were very stable even after eight months of preparation. For CTAB/SDS mixed system, the vesicles formed when1/1.6≤CTAB/SDS≤1.4/1 and the vesicles were unstable. They can aggregate and precipitate in several minutes at normal temperature. However, both high temperature and addition of ethanol can extend the time of vesicle stable. For the system of CTAB/SLS, the mixed ratio of vesicles formed spontaneously should be CTAB/SLS≠1. The stability of vesicles was different according to mixed ratio of CTAB/SLS system. The viscidity of vesicle solution increased with time when CTAB/SLS>1, while there was nearly no change after eight months of preparation when CTAB/SLS<1. By comparing, it was thought the structure and property of surfactant have the important effect on the formation of vesicles.However, the experiment results showed that the vesicles formed from cationic/anionic system were often unstable. They can aggregate with each other and form a bigger vesicle by fusing. So, in the last section of experiment, polymerization method was used to enhance the stability of vesicles. The monomer of St and DVB were dissolved in the bilayer of vesicles, then polymerized at 50℃～70℃. The photographs of TEM and AFM showed that the vesicles still keep its integrality after polymerization. The result of Zetaplas showed that the polymerization method prevent the trend of aggregation and fusing between vesicles effectively.