Chromatographic behaviour in reversed-phase high-performance liquid chromatography with micellar and submicellar mobile phases

Micellar HPLC can be used to separate non-ionic analytes in reversed-phase systems with aqueous mobile phases containing surfactants in concentrations higher than critical micellar concentrations (CMC). Ionic analytes are often separated by ion-pair reversed-phase chromatography in aqueous-organic mobile phases containing lower concentrations of ionic surfactants. The objective of the present work was to compare chromatographic behaviour of non-ionic solutes in micellar mobile phases and in mobile phases containing surfactants in concentrations lower than CMC (submicellar mobile phases). CMC were measured for several cationic and anionic surfactants on an octylsilica column. The Langmuir isotherm was modified to describe the distribution of the surfactants between the stationary and the mobile phases in the submicellar concentration range. Retention behaviour of polar solutes with low retention in reversed-phase systems was measured in aqueous mobile phases containing surfactants in concentrations lower and higher than CMC. According to the form of the dependence of their retention on the concentration of the surfactant in the submicellar and micellar range of mobile phase compositions, the analytes were classified into four different classes. Equations describing the retention in dependence on the concentration of surfactant in submicellar and micellar mobile phases were derived and experimentally verified. The forms of the equations are the same for all analytes, but the meanings of the equation constants depend on the compound class. The selectivity of separation of a number of polar compounds is better in submicellar than in aqueous-organic mobile phases and the efficiency of separation is improved with respect to the micellar HPLC. Further, the sensitivity of non-specific detection, e.g. with refractive index detectors, is significantly better in submicellar than in micellar mobile phases. Submicellar HPLC has been applied to separation of various types of compounds, such as lower alcohols, barbiturates, phenols and nitriles of carboxylic acids used as the intermediates in production of some herbicides.