Influence of the addition of modifiers on solute-micelle interaction in hybrid micellar liquid chromatography

Abstract

In reversed-phase micellar liquid chromatography (MLC) organic modifiers are usually added to the mobile phase to modify the eluent strength and to increase the efficiency of the chromatographic peaks. The effect of the modifiers methanol, ethanol, 1-propanol, 1-butanol, 1-pentanol, acetonitrile and tetrahydrofuran on the critical micelle concentration (cmc) of the anionic surfactant sodium dodecylsulphate (SDS) has been studied to enable understanding of the interactions between solute, micelles and bulk liquid in such hybrid eluents. Methanol, acetonitrile and tetrahydrofuran increased thecmc, whereas the other alcohols reduced its value. For butanol and pentanol, which partition into the micelle, thecmc barely changed for alcohol concentrations above 4% and 1.5%, respectively. This behaviour indicated that the micelle was mainly modified at lower alcohol concentrations by introduction of the molecule into the micelle palisade. When the ratio (number of molecules of alcohol in the micelle)/(number of molecules of surfactant), was >3, new molecules of the alcohol probably dissolved in the micelle core. The dependence of the capacity factors of several solutes on the concentration of modifier was similar to that found for thecmc, implying that the effects that change thecmc values upon addition of alcohol to a surfactant solution are, at least partially, those that induce reduced retention in MLC—modification of the nature of bulk water and micelle. The change in retention observed with SDS mobile phases, when butanol and pentanol are added, are mainly a result of modification of the structure of the micelle.