Study On The Vesicles Formed By Mixed Surfactants As The Novel Pseudostationary Phase In Electrokinetic Chromatography

The vesicles formed from oppositely charged surfactants(catanionic surfactants) as pseudostationary phase in electrokinetic chromatography(EKC) possess of many advantages such as wide elution window and high chemical selectivity. Vesicle, mixed by catanionic surfactants, is a kind of potential and novel pseudostationary phase. In the thesis, effects of surfactants tail-length symmetry and polar head groups on the microstructures of the aggregates in the mixed catanionic surfactants system have been investigated. Two systems with none of agglomeration while mixing cationic and anionic surfactants at any molar ratio have been found.The one, with a bigger vesicle region and larger surface charge density, has been selected as pseudostationary phase in EKC. Chromatographic properties and separation mechanism of the vesicle as pseudostationary phase have also been explored, and this novel vesicle pseudostationary phase was applied for the separation of corticosteroids. The main research contents of the thesis are as follows:1. The influences of surfactants tail-length symmetry and polar head group on aggregates morphologies and the surface charge densities were investigated. Six cationic surfactants including octyltrimethylammonium bromide(C8NM3Br), decyltrimethylammonium bromide(C10NM3Br), dodecyltrimethylammonium bromide(C12NM3Br), octyltriethylammonium bromide(C8NE3Br), decyltriethylammonium bromide(C10NE3Br) and dodecyltriethylammonium bromide(C12NE3Br) were selected to mix with two anionic surfactants(sodium dodecyl benzene sulfonate(SDBS) and sodium dodecyl sulfate(SDS)) at different molar ratio, respectively. The experimental results showed that none precipitate appeared and all of zeta potential values were negative while octyl cationic surfactant mixed with anionic surfactants at any molar ratio. Cationic surfactants tended to be included in the aggregates and wrapped tightly by anionic surfactants. While dodecyl cationic surfactants were mixed with anionic surfactants, turbidity appeared. However, it did not occur at equimolar ratio but at the molar ratio of cationic to anionic surfactant of 6:4 and/or 7:3. The aggregates were negatively charged before the emergence of turbidity, and the aggregates were positively charged after turbidity appeared. In the symmetric system which both surfactants with tail length of twelve carbon atoms, precipitate always appeared at equimolar ratio, and the electrical property of aggregate surface charge depended on the rich surfactants. In addition, it was prone to form vesicles while SDBS acted as anionic surfactant in catanionic surfactants mixtures. Space steric effect of polar head groups had large effects on morphology and stability of aggregates.2. C8NE3Br/SDBS system, which had a large vesicle region and none turbidity phenomena appeared while mixed surfactants at any molar ratio, had been developed as pseudostationary phase in EKC. C8NE3Br/SDBS system at molar ratio of 3:7 had the largest surface charge density, and was introduced as pseudostationary phase. Electrophoretic and chromatographic parameters including elution window, methylene selectivity, polar group selectivity and shape selectivity of the vesicle pseudostationary phase were characterized and compared with SDS micelle system. The results showed that the vesicle pseudostationary phase possessed a wider elution window and a better selectivity. The retention behavior and selectivity differences between the novel vesicle and SDS micelles were evaluated through linear solvation energy relationship(LSER) analysis. Though the cohesiveness and the hydrogen bond acidity had greatest influences on the solutes retention and selectivity in both the vesicle and SDS micelle, the vesicle pseudostationary phase demonstrated a higher hydrophobicity and hydrogen-bond interaction. It indicated that the vesicle pseudostationary phase possessed higher selectivity than the conventional SDS micelle pseudostationary phase.3. C8NE3Br/SDBS mixed systems with different molar ratio have been developed in EKC to separate eight corticosteroids. Zeta potential and size of the mixture of catanionic surfactants with different molar ratio of C8NE3 Br to SDBS were characterized as the total surfactant concentration at 20 mmol·L-1, the ionic strength at 30 mmol·L-1 and p H of the buffer of 9.2. The experimental results showed that zeta potential of all mixtures of catanionic surfactants remained negative. The absolute values of zeta potential were even larger than in only SDBS system as the molar ratio of C8NE3 Br to SDBS less than 4:6, and they decreased as increasing the ratio of cationic surfactants. Moreover, the determination results showed that the size was smallest at ratio of 3:7 and 6:4, instead of at 5:5. TEM testified the formation of vesicles. In addition, the performance of the vesicle as pseudostationary phase by separating eight kinds of corticosteroids with EKC was compared with microemulsion modified with ionic liquid and polymeric micelle. The experimental results proved that the novel vesicle PSP had better separation performances.