Retention and Partition Behaviors of Solutes in a Surfactant-Based Mobile Phase at Concentrations Approaching the Critical Micelle Concentration in Liquid Chromatography

Abstract

Previously developed three-phase retention model has been extensively used to understand the partition behaviors of solutes in micellar liquid chromatography (MLC). The model has been used to explain the linear plots of the reciprocal of the solute retention factor versus the micelle concentration in the mobile phase. Because a phase equilibrium model of micellization is assumed, the retention model is generally applied to systems at surfactant concentrations above the critical micelle concentration (CMC). This study investigated the retention behaviors of five solutes, namely acetone, benzaldehyde, acetophenone, benzene, and benzyl alcohol, as a function of the concentration of dodecyltrimethylammonium bromide (DTAB) in water on a C18 sorbent. Distinct retention behaviors were observed for DTAB concentrations below and above the CMC, which have not been adequately explained by the retention models described in the literature. Thus, a new MLC retention model based on a mass action model of micellization was developed herein. Retention and partition behaviors were found to be determined by three key dimensionless groups, representing the partition coefficients of the solutes between the DTAB-uncomplexed aqueous phase and the stationary, DTAB-complexed aqueous, and micellar phases. Thermodynamic equilibrium constants were estimated from fitting the model to the retention factor data. The new model showed a good fit to the HPLC data below and above the CMC and was reliable for describing the transition of distinct retention behaviors near the CMC.