Characterization of chemical selectivity in micellar electrokinetic chromatography. VI. Effects of surfactant counter-ion

Linear solvation energy relationships and free energy of transfer data were used to evaluate the influence of the surfactant counter-ion on selectivity in micellar electrokinetic chromatography. It was determined that selectivity differences are dependent on the valency of the counter-ion but not the type of counter-ion. Monovalent surfactants, sodium dodecyl sulfate (SDS) and lithium dodecyl sulfate, have nearly identical selectivity behavior. The divalent surfactants, magnesium didodecyl sulfate and copper didodecyl sulfate also show very similar behavior. However, when the divalent counter-ion species is compared to SDS under similar conditions, significant differences are observed. Most notably, the utilization of divalent counter-ion species of dodecyl sulfate surfactants causes the micelles to become more hydrophobic and a weaker hydrogen bond donating pseudo-stationary phases. It is believed that the divalent counter-ions reduce the electrostatic repulsion between the surfactant head groups and therefore, increase the chain packing of the monomers in the micelle aggregates. This reduces the degree of hydration of the micellar palisade layer leading to a decreased ability of the micelle to participate in polar/polarizable and hydrogen bonding interactions with solute molecules.