Fluorinated and dimeric surfactants in micellar electrokinetic chromatography : practical applications and linear solvation energy relationships

Van Biesen, Geert (2008) Fluorinated and dimeric surfactants in micellar electrokinetic chromatography : practical applications and linear solvation energy relationships. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Micellar electrokinetic chromatography (MEKC) is a form of capillary electrophoresis (CE) that uses a surfactant at a concentration higher than its critical micelle concentration for the separation of analytes. The choice of surfactant can be key to a successful analysis. This is first demonstrated via the application of a relatively volatile surfactant (ammonium perfluorooctanoate - APFOA) for the analysis of yV-methylcarbamates by MEKC - electrospray ionization - mass spectrometry (MEKC-ESI-MS), circumventing the well-known problem of signal suppression in MS by non-volatile anionic surfactants. An experimental design type of approach is used to optimize the separation voltage, the nebulizer pressure, and the concentrations of APFOA and isopropanol. -- The type of surfactant can also have a major impact on the quality of the separation, and surfactants (or more generally, pseudostationary phases - PSPs) are often characterized via linear solvation energy relationships (LSERs). This makes it possible to calculate their system constants, which reflect differences in the properties of the micellar and the aqueous phase with respect to cohesiveness, polarizability, dipolarity, and hydrogen bond accepting and donating ability. This methodology was applied to characterize anionic dimeric surfactants based upon the type (hydrophobic, hydrophilic, fluorinated) and length of the spacer connecting the two amphiphilic moieties. Dimeric surfactants that had spacers with one to six ethylene groups, one to four ethoxy groups, and one to three C₂F₄ groups were synthesized. The system constants of dimeric surfactants with hydrophobic spacers do not depend on the length of the spacer. Compared to sodium dodecylsulfate (SDS), the most commonly used PSP in MEKC, they are slightly more cohesive, interact better with polarizable compounds, and are somewhat better hydrogen bond acceptors and worse hydrogen bond donors, while there is no difference in dipolarity. Dimeric surfactants with hydrophilic spacers differ from those with hydrophobic spacers in the sense that there is a modest increase in the hydrogen bond accepting ability and a decrease in the hydrogen bond donating ability as the number of ethoxy groups in the spacer increases, while other system constants are the same. Dimeric surfactants with fluorinated spacers have similar selectivity to dimeric surfactants with hydrophobic spacers, although they are somewhat less polarizable, and less able to act as hydrogen bond acceptors. The results are explained in terms of the interphase model of retention, whereby selectivity differences between surfactants are largely shaped by differences in solvation properties of the interphase region and the bulk solution. Although these dimeric surfactants do not show unusual selectivity compared to the wide variety of PSPs already evaluated for MEKC, it is demonstrated that small selectivity differences can be taken advantage of to fine-tune separations.

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