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

In reversed-phase micellar liquid chromatography (MLC) organic modifiers ar e usually added to the mobile phase to modify the eluent strength and to in crease the efficiency of the chromatographic peaks. The effect of the modif iers 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 underst anding of the interactions between solute, micelles and bulk liquid in such hybrid eluents. Methanol, acetonitrile and tetrahydrofuran increased the c mc, whereas the other alcohols reduced its value. For butanol and pentanol, which partition into the micelle, the cmc barely changed for alcohol conce ntrations above 4 % and 1.5 %, respectively. This behaviour indicated that the micelle was mainly modified at lower alcohol concentrations by introduc tion of the molecule into the micelle palisade. When the ratio (number of m olecules of alcohol in the micelle)/(number of molecules of surfactant), wa s > 3, new molecules of the alcohol probably dissolved in the micelle core. The dependence of the capacity factors of several solutes on the concentra tion of modifier was similar to that found for the cmc, implying that the e ffects that change the cmc values upon addition of alcohol to a surfactant solution are, at least partially, those that induce reduced retention in ML C - modification of the nature of bulk water and micelle. The change in ret ention observed with SDS mobile phases, when butanol and pentanol are added , are mainly a result of modification of the structure of the micelle.