Effect of a variety of organic additives on retention and efficiency in micellar liquid chromatography

The effect of 21 organic additives (alkanols, alkane diols, dipolar aprotic solvents, alkanes) on the chromatographic behavior (retention, elution str ength, efficiency) of probe solutes of widely differing hydrophobicity, suc h as benzene and 2-ethylanthraquinone, have been examined using a C-18 stat ionary phase and sodium dodecyl sulfate (SDS) micellar mobile phases. The m obile-phase elution strength parallels the octanol-water partition coeffici ents of the additives or their ability to bind to the SDS micellar system, due to the increased solubility in the mobile phase and reduced affinity fo r the additive-modified surfactant-coated stationary phase. The comparison of the elution strength of micellar mobile phases with that of a reference acetonitrile-water system indicates that the elution strength is lower for micellar systems and depends on the nature of the eluted solute. The displa cement of the solute-micelle and solute-stationary phase binding equilibria is quantified for several probe solutes eluted with micellar mobile phases in the presence of 1-propanol, 1-butanol, 1-pentanol, and acetonitrile, A correlation was also observed between the number of theoretical plates and the hydrophobicity of the alcohol additives: the efficiency initially incre ased steeply and reached a plateau, Compared to benzene, a more hydrophobic additive was needed to attain the maximum efficiency for the more hydropho bic 8-ethylanthraquinone analyte. Dipolar aprotic solvents appear to be som e that more effective in enhancing the efficiency than alcohols. The result s are rationalized in terms of the ability of the organic additives to alte r the composition, structure, dynamics, and properties of the micelles and the surfactant-coated stationary phase.