Chemical selectivity in micellar electrokinetic chromatography .2. Rationalization of elution patterns in different surfactant systems

Retention behavior in micellar electrokinetic chromatography (MEKC) is investigated using linear solvation energy relationships (LSERs) for two pseudo-stationary phases, one consisting of cationic micelles of tetradecyltrimethylammonium bromide (C(14)TAB) and the other of an anionic triblock copolymer, poly(methyl methacrylate-ethyl acrylate-methacrylic acid) (Elvacite 2669). It was found that solutes' migration behaviors in these two MEKC systems are mainly influenced by their size (VI 100) and hydrogen bonding acceptor (HBA) strength (beta). However, solutes' hydrogen bonding donor (HBD) strength (alpha) has minor effects on their migration in MEKC. The characteristics of these two systems were compared to three other previously reported anionic micellar systems of sodium dodecyl sulfate (SDS) (anionic hydrocarbon), sodium cholate (SC) (anionic bile salt) and lithium perfluorooctane sulfonate (LiPFOS) (anionic fluorocarbon). It was concluded that hydrogen bonding interactions play a major role in providing different chemical selectivity among these five MEKC systems. Both C(14)TAB micelles and the ionic polymer of Elvacite 2669 provide hydrogen bonding acceptor (HBA) sites for solutes, which is similar to SC micelles. In fact, C(14)TAB is the strongest HBA, while Elvacite 2669 has HBA strength similar to that of SC micelles. On the other hand, the fluorocarbon micelles of LiPFOS are the strongest hydrogen bond donor (HBD) micelles, followed by the weak HBD SDS micelles. In general, cavity formation has little or no effect on chemical selectivity among hydrocarbon surfactant MEKC systems (i.e., SDS, SC and C(14)TAB). Information obtained from the LSER analysis is used to rationalize the elution patterns in MEKC with different types of pseudo-stationary phases.