Effects of organic modifiers on retention mechanism and selectivity in micellar electrokinetic capillary chromatography studied by linear solvation energy relationships

The effects of six organic modifiers (urea, methanol, dioxane, tetrahydrofu ran, acetonitrile and 2-propanol) on the retention mechanism and separation selectivity of the bulk buffer in micellar electrokinetic capillary chroma tography (MECC) with sodium dodecyl sulfate (SDS) micelles as pseudo-statio nary phase have been investigated through linear solvation energy relations hips (LSERs). It is found that the retention value in MECC systems with or without organic modifier is primarily dependent on the solvophobic interact ion and the hydrogen bonding interaction with the solute as proton acceptor , while the dipolar interaction and the hydrogen bonding interaction with t he solute as proton donor play minor roles. The effects of the organic modi fiers on the solvophobic, dipolar and hydrogen bonding interactions are eva luated in terms of the relationship between regression coefficient of the L SER equations and the modifier concentration. The variations of the solvoph obic interaction and the dipolar interaction with change of the modifier co ncentration can be approximately explained using the solubility parameter a nd the dipolarity/polarizability parameter of the organic modifier, respect ively. However, the relationships between the hydrogen bond acidity and bas icity of the bulk buffer and the organic modifiers are rather complicated. Those results may be caused from the displacement of organic modifiers to t he water adsorbed on the micellar surface as well as changes in the acidity and basicity of the bulk buffer with the addition of organic modifiers. In addition, it is found that the phase ratio is influenced significantly by the use of organic modifier. (C) 1999 Elsevier Science BN. All rights reser ved.