Micro-structure Of Microemulsions Used In Microemulsion Electrokinetic Capillary Chromatography

Microemulsion electrokinetic capillary chromatography (MEEKC) is an electrodriven separation technique, developed on the basis of micellar electrokinetic chromatography (MEKC). Microemulsion, usually oil droplets dispersed in an aqueous buffer, was applied as separation media in MEEKC to separate solutes based on both their hydrophobicities and electrophoretic mobilities. The microemulsion microstructure including microdroplet size and surface charge density had great effect on MEEKC. The thesis mainly included the following four parts.1. The MEEKC principle, the important parameters and the applications in various fields, especially in analysis of vitamins, pharmaceuticals, natural products and environmental pollutants, were summarized. And also, the researches on the microstructure of microemulsions were introduced. Finally, the determination methods for microstructure were enumerated.2. An electrochemical method to determine the size of microdroplet in microemulsion was established. The diffusion coefficients of microemulsion droplet, using ferrocene as the electrochemical probe in SDS/n-butanol/n-octane oil-in-water microemulsion system, were measured by the cyclic voltammetry, and the particle size of microemulsion droplet was calculated from the diffusion coefficients. Since the composition of microemulsion in MEEKC significantly affects the separation performance, the effects of the composition of microemulsion on the droplet size were investigated. The cyclic voltammetry experiment results showed that increasing amount of surfactant or cosurfanctant would result in a decreased droplet size. But in fact, the particle size of microdroplet would increase with increases of n-butanol content. Cyclic voltammetry had advantage in simple equipment and fast measure speed. But with the n-butanol concentration increasing, ferrocene was existed in both oil core and outer of the droplet, which leads to the inaccurate results. The method was fit for the measurement of microdroplet particle size in oil/surfactant/electrolyte buffer, or small amount of cosurfactant in oil/surfactant/electrolyte buffer microemulsion, but not suitable for microemulsion system in MEEKC owing to a large amount of n-butanol or organic solvents.3. The influences of the composition of microemulsion on the microstructures including the dimension and Zeta potential of microdroplets were measured in details. The average dynamic dimension of microdroplet was measured by DLS, andζpotential was determined to characterize surface charge density of microdroplets. The experiment results showed that increasing amount of surfactant resulted in a decreased microdroplet size but almost invariantζpotential, which would enlarge migration time of microdroplet. With increment of cosurfactant concentration, the microdroplet size had an increasing trend, whereas theζpotential decreased. Thus, observed migration velocity of microdroplet increased, which made the separation window shortened. Neither dimension norζpotential of microdroplet had been changed by varying both the type and the amount of the oil phase. Addition of organic solvent as modifier did not change the microdroplet size, but loweredζpotential. Even if, the migration time of microdroplet still became larger, since EOF slowed down owing to organic solvent in capillary. So, besides increment of surfactant concentration, organic additive could also enlarge the separation window. Increases of cosurfactant concentration were benefit for separation efficiency thanks to the looser structure of swollen microdroplet, and the peaks sharpening might compensate for the resolution and peak capacity owing to a narrow separation window. Except the oil phase, tuning the composition of microemulsion would change the microstructure, eventually could be exploited to optimize the resolution and save analysis time in MEEKC.4. The microstructures between solvent modified micelle in MEKC and microemulsion in MEEKC have been compared, which showed that the particle size and Zeta potential in both micelle and microemulsion were basically the same at 3.3% SDS concentration. Besides, the polarity at the core of micelle and microemulsion, determined with fluorescence, was also similar. The separation of prednisone, hydrocortisone and prednisolone was performed with MEEKC using 2.4% SDS/6.6% 1-butanol/0.5%n-octane/10mmol/L borax buffer, and MEKC using 2.4%SDS/6.6%n-butanol/10mmol/L borax buffer, respectively. The results displayed that elute order, migration time and resolution of the three analytes were almost the same with MEKC or MEEKC. It was concluded that the similar microstructure in solvent modified MEKC and MEEKC leads to similar separation performance. It can be assumed that the swollen micelle as separation carrier will play an important role in electrokinetic chromatography technology.