Chemical selectivity in micellar electrokinetic chromatography: characterization of solute-micelle interactions for...

The influence of surfactant type on migration behavior and chemical selectivity in micellar electrokinetic chromatography (MEKC) is investigated through linear solvation energy relationships (LSER) and functional group selectivities. In LSER modeling, solutes' capacity factors are correlated with their structural descriptors such as size, dipolarity, and hydrogen-bonding abilities. Using the LSER methodology, useful information about the nature of solute interactions with different types of surfactant aggregates can be obtained since capacity factor in MEKC is directly related to solute distribution between the bulk aqueous solvent and micelles. High correlations were observed for different LSER models of migration behavior in MEKC for a group of 60 uncharged aromatic compounds of non-hydrogen bonding (NHB), hydrogen-bonding acceptor (HBA) bases, and hydrogen-bonding donor (HBD) acids. In two anionic, hydrocarbon micellar systems of sodium dodecyl sulfate (SDS) and sodium cholate (SC), retention is primarily influenced by the size of molecules and their hydrogen bond accepting basicity. Their dipolarity/polarizability and hydrogen bond donating acidity play minor roles. Capacity factors of solutes in SDS and SC systems increase with their size and decrease for stronger hydrogen bond acceptor bases. These results are similar to those observed for other systems where hydrophobic interactions play a major role, e.g., solute distribution in the 1-octanol-water solvent system or retention in reversed phase LC. In MEKC with an anionic fluorocarbon surfactant, lithium perfluorooctanesulfonate (LiPFOS), however, size and solute HBD acidity are the two predominant factors. The LSER results indicate that compounds find the SDS micellar environments slightly less cohesive (i.e., more apolar) than the SC micelles, while the LiPFOS micelles are the most cohesive among the three surfactant aggregates and 1-octanol provides the least cohesive environment. The fluorocarbon micelles of LiPFOS, on the other hand, are the strongest hydrogen bond donor acids, followed by SDS, SC, and 1-octanol, respectively. The SC micelles have the most hydrogen bond acceptor basic characteristics, followed by 1-octanol, SDS, and LiPFOS micelles. It can be concluded that selectivity differences between these surfactant types in MEKC is primarily due to hydrogen-bonding interactions rather than the dipolar interactions. Comparing the perfluorinated and the hydrocarbon surfactants, even solute size can play a role in selective migration patterns. In addition, information from polar and hydrophobic group selectivities confirm the LSER conclusions about the underlying interactions that control migration behavior and chemical selectivity in MEKC.(ABSTRACT TRUNCATED AT 400 WORDS)