Electrokinetic chromatographic characterization of novel pseudo-phases based on N-alkyl-N-methylpyrrolidinium ionic liquid type surfactants.

MEKC and linear solvation free energy relationships (LSFERs) have been used to characterize the solute distribution between water and self-assemblies formed from ionic liquid type mono-chain cationic surfactants containing a cyclic pyrrolidinium head group. Several features of the solvation environment afforded by these micellar solutions were found to be quite different from that of CTAB, a structurally analogous cationic surfactant with a conventional, acyclic quaternary ammonium head group. None of the LSFER coefficients were found to vary in any systematic way with increasing alkyl chain length for these unique surfactants. Overall, however, these surfactants display different behavior than do all known cationic detergents such as CTAB. In chemical terms, pseudo-phases formed by these N-alkyl-N-methylpyrrolidinium bromides interact more strongly with polar compounds and less strongly with compounds having nonbonding or pi-electrons and are more cohesive compared to the well-studied CTAB.

Cationic and perfluorinated polymeric pseudostationary phases for electrokinetic chromatography.

Separation selectivity in electrokinetic chromatography (EKC) is directly affected by the chemistry and solvent characteristics of the pseudostationary phase (PSP). The chemical selectivity of micellar PSPs has been previously demonstrated to vary significantly between anionic and cationic surfactants as well as between hydrocarbon and fluorocarbon surfactants. Polymeric PSPs have also been demonstrated to provide unique selectivity. In the current study, four cationic polymeric pseudo-stationary phases, two of which have perfluorinated pendant groups, are introduced and characterized as PSPs in EKC. Their performance and selectivity is compared to conventional micellar PSPs with similar structure. The solvation characteristics and selectivity of the four polymers most closely resemble those observed for cationic micelles. The polymers are all more cohesive and more polar than their hydrocarbon micellar counterparts. The fluorocarbon PSPs did show preferential interaction with fluorocarbon solutes, were somewhat more cohesive, and were stronger hydrogen bond donors. However, the presence of fluorocarbon moieties did not have as dramatic an effect on selectivity as was observed and published previously for fluorocarbon micelles. This may result from the selectivity being dominated by the presence of the cationic head groups or from the fluorocarbon character of the pendant groups being moderated by the presence of hydrocarbon functionality on the polymer backbones.

Performance and selectivity of polymeric pseudostationary phases for the electrokinetic separation of amino acid derivatives and peptides.

Two polymeric pseudostationary phases, one an acrylamide polymer and the second a siloxane polymer, have been investigated for the separation of naphthalene-2,3-dicarboxaldehyde (NDA)-derivatized amino acids and small peptides. The dervatized amino acids were detected by UV absorbance and laser-induced fluorescence (LIF) detection. The polymers provided very high efficiency and good selectivity for the separation of the amino acids. The separation selectivity using the polymers was significantly different from that of SDS micelles, and there were subtle differences in selectivities between the polymers. Although very good detection limits were obtained with LIF detection, a significant background signal was observed when the polymers were not washed to remove fluorescent impurities. The polymers did not separate the peptides very well. It is postulated that the fixed covalent structure of the polymers prevents them from interacting strongly or efficiently with the peptides, which are large in relation to the analytes typically separated by electrokinetic chromatography using polymers.

Conformational effects on the performance and selectivity of a polymeric pseudostationary phase in electrokinetic chromatography.

The effect of the conformation of a polymeric pseudostationary phase on performance and selectivity in electrokinetic chromatography was studied using an amphiphilic pH-responsive polymer that forms compact intramolecular aggregates (unimer micelles) at low pH and a more open conformation at high pH. The change in conformation was found to affect the electrophoretic mobility, retention, selectivity, and separation efficiency. The low-pH conformer has higher electrophoretic mobility and greater affinity for most solutes. The unimer micelle conformation was also found to provide a solvation environment more like that of micelles and other amphiphilic self-associative polymers studied previously. It was not possible to fully characterize the effect of conformation on efficiency, but very hydrophobic solutes with long alkyl chains appeared to migrate with better efficiency when the unimer micelle conformation was employed. The results imply that polymers with a carefully optimized lipophilic-hydrophilic balance that allow self-association will perform better as pseudostationary phases. In addition, the results show that electrokinetic chromatography is a useful method for determining the changes in solvation environment provided by stimuli-responsive polymers with changes in the conditions.