A linear solvation energy relationship study of the effects of surfactant chain length on the chemical interactions governing retention and selectivity in micellar electrokinetic capillary chromatography using sodium alkyl sulfate elution buffers

We have used linear solvation energy relationships (LSERs) to study the fundamental chemical interactions responsible for solute retention in micellar electrokinetic capillary chromatography (MEKC). We investigated retention in micellar solutions of sodium dodecyl sulfate (SDS), sodium decyl sulfate (SDecS), and sodium octyl sulfate (SOS). The purpose of the study was to elucidate the effect of surfactant chain length on the solute/micelle interactions that ultimately govern retention and selectivity in MEKC. The nature of the solute/micelle interactions were found to be nearly equivalent in all three systems, implying that the chromatographic selectivity in all three systems will be quite similar. Additionally, the LSERs show that solute size and hydrogen bond basicity play the largest roles in determining solute retention and chromatographic selectivity. Finally, from the LSERs and an analysis of the free energy of transfer of methylene units from water to the micellar phase (ΔG(CH₂)°), we conclude that the solutes reside in the polar, hydrated head group region of the micelles, and not in the nonpolar core. Based on the ΔG(CH₂)°values for five different homologous solute series, the effect of the solutes' functional groups on the location and orientation of the solutes inside the micelles is briefly discussed.