Comparison of the retention of basic compounds in anionic and cationic microemulsion electrokinetic chromatographic systems

Abstract

Retention of ionizable bases in microemulsion electrokinetic chromatography (MEEKC) has been studied using two different systems with anionic and cationic microemulsions. Microemulsion pseudostationary phase is composed of heptane (oil), 1-butanol (cosurfactant) and sodium dodecyl sulfate (SDS, anionic system) or tetradecyltrimethylammonium bromide (TTAB, cationic system) as surfactant. In contrast with micellar electrokinetic chromatography (MEKC) where the retention of neutral compounds is very different in the two micellar pseudostationary phases (SDS and TTAB, respectively); in MEEKC, neutral compounds present very similar retention factor (k) values in SDS and TTAB microemulsion pseudostationary phases. However, the k vs. pH profiles of protonable bases are very different in the two MEEKC systems. In TTAB system, retention increases with pH because of neutralization of the protonated base and partition of the unionized form into the microemulsion. However, a reversed trend is observed in SDS system. Retention decreases with pH because of the formation of an ionic pair between the protonated base and the anionic SDS, much more retained than the unionized base. Thus, it is demonstrated that the two systems behave very similar in the retention of neutral bases, but completely different for retention of protonated bases.