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.

Influence of ester and amide-containing surfactant headgroups on selectivity in micellar electrokinetic chromatography.

The selectivity differences between six anionic surfactants in micellar electrokinetic chromatography (MEKC) are presented and the structural influence of the surfactant head-group is investigated. It was determined that the surfactant structure can have a significant impact on retention and selectivity. Linear solvation energy relationships (LSERs) are used to study the role of solute size, polarity/polarizability, and hydrogen bonding characteristics in determining retention and selectivity. While both the solute size and hydrogen bond accepting ability were found to be the most important factors in solute retention, the hydrogen bonding characteristic of the solutes have the largest influence on selectivity differences between surfactants.

Statistical evaluation of linear solvation energy relationship models used to characterize chemical selectivity in micellar electrokinetic chromatography.

Characterization of retention and selectivity differences between surfactants in micellar electrokinetic chromatography (MEKC) using linear solvation energy relationships (LSERs) has been given a significant amount of attention in the last four years. This report evaluates the validity of using the two LSER models that have been used to fit retention in MEKC in the literature. The results and the fit of the revised model and parameters developed by Abraham and coworkers are compared to the original model developed by Kamlet, Taft, and coworkers. LSERs can generally only be used as a comparative tool to describe the selectivity differences between surfactant systems used in MEKC. With this in mind, it was determined that the results of both models essentially provide the same information about these differences. However, the revised model and parameters have been found to yield a statistically better fit of the MEKC retention data as well as providing more chemically sound LSER coefficients.

Congeneric behavior in estimations of octanol-water partition coefficients by micellar electrokinetic chromatography.

Linear Solvation Energy Relationships (LSERs) are used to explain the congeneric behavior observed when using Micellar Electrokinetic Chromatography (MEKC) to estimate the octanol-water partition coefficient scale of solute hydrophobicity. Such studies provide useful insights about the nature of solute interactions that are responsible for the sources of congeneric relationships between MEKC retention and log Po/w. It was determined that solute dipolarity/polarizability and hydrogen-bonding character play the most important roles in the congeneric behavior observed for many surfactant systems. The individual dipolarity/polarizability and hydrogen-bonding contributions to the free energy of transfer were also investigated.

Characterization of chemical selectivity in micellar electrokinetic chromatography. 4. Effect of surfactant headgroup.

The influence of surfactant headgroups on migration behavior in micellar electrokinetic chromatography is examined. Using linear solvation energy relationships (LSER) and functional group selectivity studies, the effect of six anionic headgroups on chemical selectivity is characterized. The sodium dodecyl surfactants of the sulfate [SO4-], sulfonate [SO3-], carboxylate [CO2-], carbonyl valine [OC(O)NHCH(CH(CH3)2)CO2-], and sulfoacetate [OC(O)CH2SO3-] anions are investigated. Solute size and the hydrogen-bond-donating ability of the micellar phase play the most significant roles in solute retention in all of the surfactants studied. While solute-micelle hydrogen bonding plays a dominant role in the observed selectivity, the dipolarity and polarizability of the micellar phase also have a small influence. The results also suggest that the hydrogen-bond-accepting ability for surfactants is inversely proportional to the proton acidity (pKa) of its headgroup. The observed hydrogen-bond-donating ability and dipolarity of surfactant systems are believed to be a result of the water that resides near the micelle surface.