NEW 3-PHASE MODEL FOR MICELLAR LIQUID-CHROMATOGRAPHY

The amount of sodium dodecyl sulfate (SDS) adsorbed on a reversed-phas e octadecylsilica (ODS) stationary phase in micellar liquid chromatogr aphic (MLC) separation has been measured. It was found that the weight percentage of SDS on the stationary phase (16.5%) was similar to the percentage ratio of C/Si (C/Si = 17.1%) of the original ODS stationary phase. This indicated that the octadecyl groups of the ODS packing we re largely covered by adsorbed surfactants and the stationary phase in the MLC system could be considered to be the adsorbed surfactant. Acc ordingly, a new three-phase equilibrium model in which the adsorbed su rfactant was assumed to be the stationary phase was proposed to illust rate the behaviour of analyte partitioning in MLC. The negative impact of the adsorbed surfactant on the efficiency of MLC was discussed in terms of this new model. With the three-phase model, the equilibrium c onstant (K-3) for direct transfer of the analyte from the surfactant i n the mobile phase to the surfactant on the stationary phase could be calculated. The results obtained for K-3 using two model analytes show ed that the mobile phase and stationary phase portions of the surfacta nt in MLC had similar but not identical abilities for binding the anal yte. The effects and limitations of the use of organic modifiers in ML C are also discussed.