New three-phase model for micellar liquid chromatography

The amount of sodium dodecyl sulfate (SDS) adsorbed on a reversed-phase octadecylsilica (ODS) stationary phase in micellar liquid chromatographic (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 were largely covered by adsorbed surfactants and the stationary phase in the MLC system could be considered to be the adsorbed surfactant. Accordingly, a new three-phase equilibrium model in which the adsorbed surfactant was assumed to be the stationary phase was proposed to illustrate 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 constant (K3) for direct transfer of the analyte from the surfactant in the mobile phase to the surfactant on the stationary phase could be calculated. The results obtained for K3 using two model analytes showed that the mobile phase and stationary phase portions of the surfactant in MLC had similar but not identical abilities for binding the analyte. The effects and limitations of the use of organic modifiers in MLC are also discussed.