Characterization of chemical selectivity in micellar electrokinetic chromatography

This first portion of this work discusses the validity of using Linear Solvation Energy Relationships (LSER's) to characterize pseudo-stationary phases in Micellar Electrokinetic Chromatography (MEKC). Two different models and two different sets of solute descriptor values have been evaluated and compared. It was determined that Abraham, et al.'s revised model and descriptor values are superior to those originally designed by Kamlet, Taft, and co-workers. In addition, statistical analysis shows that all five solute descriptor terms in Abraham, et al.'s model hold unique and relevant information about selectivity in MEKC.; The primary focus of this thesis, however, has been on the evaluation surfactant structural effects on selectivity in MEKC. In particular, the degree to which the surfactant headgroup, counterion, and hydrocarbon chain length influence selectivity in MEKC systems. It has been determined that the headgroup has a substantial effect on the observed selectivity behavior in MEKC. The surfactant counterion and hydrocarbon chain length also influence selectivity but to a lesser extent. Their influence appears to be dependent on other structural properties of the surfactant; most notably the headgroup. Surfactants with larger more "organic" headgroups are more susceptible to selectivity changes when their counterion or chain length is altered. In addition, results suggest that buffer conditions (e.g. surfactant concentration, buffer concentration, and pH) have little effect on selectivity.; LSER analysis also shows that the cavity formation and dispersive interactions of the micellar phase play the most important role in solute retention. However, the selectivity differences between various surfactant systems is entirely explained by the polarity and hydrogen bonding properties of the pseudo-stationary phases. The hydrogen bond accepting ability of the surfactant is directly proportional to the pKa of the headgroup moiety. Comparing the LSER results to with micelle physical properties discussed in the literature, it seems likely that the hydrogen bond donating and polarity characteristics are caused by the waters of hydration near and in the micellar palisade and Stern layers.; Finally, estimation of solute octanol-water partition coefficients (P o/w) via MEKC has been evaluated using the retention factors in an extensive library of pseudo-stationary phases. Similar to the high performance liquid chromatography methods, MEKC suffers from congeneric behavior when a wide range of solutes are used. Using LSER to elucidate the source of this behavior, it was determined that most congeneric behavior is a result of the micellar phase possessing different hydrogen bonding properties than the octanol-water biphasic system. By normalizing the LSER coefficients for the surfactant systems and comparing them to those of octanol-water it is possible to evaluate why systems deviate from a linear model.