Conclusion

MLC is a mode of HPLC in which solutions of surfactants at a concentration above their critical micellar concentration are employed as mobile phases. Three different equilibria exist for a solute in MLC. It can distribute between the aqueous mobile phase and the micellar mobile pseudophase, between the stationary phase and the micellar pseudophase, and between stationary and aqueous mobile phases. However, for highly hydrophobic compounds, a direct-transfer mechanism from the micellar to the stationary phase has been proposed. Solute retention is related to the concentration of micellized surfactant in the mobile phase through solute-micelle association constants or distribution coefficients that can be calculated as a direct application of MLC techniques.

The great number of interactions that are possible in MLC separations, such as electrostatic, hydropho-bic and esteric, and the modification of the stationary phase by adsorption of monomeric surfactants, make these systems more complicated than conventional RP-HPLC. However, the fact that the amount of the surfactant adsorbed remains constant after equilibrium between mobile and stationary phases allows MLC techniques to achieve rapid micellar and organic modifier gradients.

The control of separation selectivity in MLC can be performed through a great number of parameters;

these include the nature and concentration of the surfactant in the mobile phase, the presence of additives as organic modifiers and salts, and the pH. The fact that the addition of an organic modifier to micellar mobile phases can increase selectivity and reduce analysis time has increased the use of hybrid micellar mobile phases, which also preclude the efficiency loss inherent to MLC as compared to conventional RP-HPLC.

Other applications that can be cited in MLC techniques are directly derived from the special characteristics of micellar solutions. The sensitivity of the detection can be enhanced and biological fluids can be directly injected into the chromatographic systems because of the solubilization of the protein by some surfactants. Finally, the fact that micelles can be considered as chemical models for biomembranes has enabled the application of MLC to hydrophobicity estimation of organic compounds.

Seealso: II/Chromatography: Liquid: Mechanisms: Reversed Phases. Electrophoresis: Micellar Electrokinetic Chromatography. III/Surfactants: Liquid Chromatography.

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