Chromatographic Efficiency in Micellar Liquid Chromatography: Should it Be Still a Topic of Concern?

Micellar liquid chromatography (MLC) was first proposed as an attractive alternative to avoid the use of organic solvents. It was soon apparent that pure micellar solutions yield poor efficiencies. This problem was remediated by the addition of a small amount of an organic solvent. However, the general opinion of the poor peak shape has prevailed as a handicap for MLC, in spite of the fact that the hybrid mode often offers similar or even improved efficiencies (for basic compounds) relative to that attained in the hydro-organic mode. Only the efficiencies for apolar non-ionizable compounds are still clearly inferior. This work describes the type of interactions and polarity changes with organic solvent and surfactant adsorption, and the stationary phase architecture and wetting with micellar mobile phases, compared to hydro-organic mobile phases. The predominant term influencing band broadening in MLC appears to be stationary phase mass transfer. Organic solvents produce a thinner surfactant layer on the column, which permits better solute diffusion. Anionic surfactants suppress the silanol effect for basic compounds due to the protecting coating of the stationary phase by the surfactant, which seems to be more effective than the direct electrostatic interaction of amines with free silanols.