Chapter Chiral Microemulsion Electrokinetic Chromatography
Microemulsion electrokinetic chromatography (MEEKC) is the newest and least published of the electrokinetic chromatography (EKC) options, particularly in terms of chiral separations. Microemulsions are aggregates in the order of 1-5 nm that are created by combining and sonicating a surfactant, an oil, and usually a cosurfactant for stability. They are optically transparent and thermodynamically stable when the
components are present in the proper ratio. In comparison to other surfactant pseudostationary phases (PSPs), microemulsions are less rigid , can solubilize more hydrophobic compounds, have a more tunable elution range (migration window) , and offer a wider variety of parameters to optimize. An important difference compared to other EKC modes is sample diluent. In MEEKC, it is best to dissolve the sample in the microemulsion solution instead of just the background electrolyte to obtain reproducible separations and satisfactory peak shape. The two main classes of microemulsions are water-in-oil and oil-in-water. Water-in-oil (w/o) systems contain a water core surrounded by a surfactant in an oil bulk phase and are not commonly used in EKC. Oil-in-water (o/w) microemulsions consist of an oil core (hydrocarbon or other water immiscible liquid) encased by a surfactant/cosurfactant (typically a short chain alcohol) outer shell; general representations of a MEEKC separation and an o/w aggregate are given in Figures 9.1 and 9.2, respectively. As shown in Figure 9.3, o/w microemulsions can be converted to w/o systems by increasing the oil concentration. When the water and oil are present in near equal amounts, a bicontinuous type of phase exists where a honeycomb-like structure is found instead of discrete droplets.