micellar electrokinetic capillary chromatography (MEKC)

The main separation mechanism is based on solute partitioning between the micellar phase and the solution phase. The technique provides a way to resolve neutral molecules as well as charged molecules by CE. Micelles form in solution when a surfactant is added to water in concentration above its critical micelle concentration (CMC). Even though these anionic micelles are attracted toward the anode, in an uncoated fused silica capillary they still migrate toward the cathode because of electroosmotic flow. However, the micelles move toward the cathode at a slower rate than the bulk of the liquid because of their attraction towards the anode. Neutral molecules partition in and out of the micelles based on the hydrophobicity of each analyte. Consequently the micelles of MEKC are often referred to as pseudo (or moving) stationary phase. A very hydrophilic neutral molecule, eg methanol, will spend almost no time inside the micelle and will therefore migrate essentially at the same rate as the bulk flow and elute earlier. On the other hand, a very hydrophobic neutral molecule, eg Sudan III, will spend nearly all the time inside the micelles and will therefore elute later, together with the micelles. All other solutes with intermediate hydrophobicity will migrate within this migration window. MEKC can be used with ionic substances as well as neutral compounds. A combination of charge/mass ratios, hydrophobicity and charge interactions at the surface of the micelles combine to affect the separation of the analytes.