Figure 4.56
Separation in MEKC. S = solute.or 'eluting' with the same velocity as the micelles. Hydrophilic neutral solutes that do not interact with
the micelles have the shortest retention times, eluting with the EOF. Selectivity can be varied widely in
MEKC by changing the surfactant and its concentration provided a minimum molar concentration
known as the critical micelle concentration (CMC) is always exceeded. Examples of commonly used
surfactants and their CMC values are given in Table 4.25. Micelle formation and micelle-solute
interactions can be dramatically affected by adding electrolytes and organic solvents such as methanol
or acetonitrile, and changing the buffer composition, pH and temperature. As in CZE these factors also
influence the EOF by affecting the charge on the capillary wall.
MEKC is being applied in many areas where neutral solutes are to be separated. These include
compounds of environmental interest, pharmaceuticals, drugs of abuse and nucleic acids. The high
efficiency coupled with chromatographic partitioning afforded by the micellar pseudo-stationary phase
gives it great versatility including chiral recognition through the use of chiral surfactants and chiral
additives.
Table 4.25 Surfactants used in micellar electrokinetic capillary chromatography
Surfactanta Type CMC in water (M) Aggregation no.
SDS anionic 8.1 × 10 –^362
CTAB cationic 9.2 × 10 – 4 61Brij- 35 nonionic 1.0 × 10 –^440
sulphobetaine zwitterionic 3.3 × 10 –^355
a SDS, sodium dodecyl sulphate; CTAB, cetyltrimethylammonium bromide; Brij-35,
polyoxyethylene- 23 - lauryl ether; sulphobetaine, N-dodecyl-N, N-dimethyl-ammonium-
3 - propane- 1 - sulphonic acid.