Multiple effect of surfactants used as additives in background electrolytes in capillary zone electrophoresis: cetyltrimethylammonium bromide as example of model surfactant.

Abstract

Surfactants are frequently used in the preparation of background electrolytes (BGEs) in capillary zone elcetrophoresis (CZE) in order to affect and to optimize both the electroosmotic flow (EOF) and the separation process. Their effects are, however, always multiple, the resulting situation may be very complex and the separation process may even be destroyed. We use the surfactant cetyltrimethylammonium bromide (CTAB) as a model example and bring experimental results and related discussion which elucidate the multiple effect of surfactants in an integrated way. It is shown that even at concentration levels lower than 10(-4) M CTAB strongly reduces the cathodic EOF in bare fused-silica capillaries and converts it into anodic EOF. The magnitude and polarity of the EOF depends not only on the concentration of CTAB but also on the composition of BGEs used. The interactions of CTA cations with the bare capillary wall reduce sorption of cationic analytes and enables their analysis. CTA cations at levels below their critical micelles concentration (CMC) already interact with anionic analytes and reduce their mobilities. This association is strong with highly charged anions and by this, the reversal of the EOF, applying BGEs with highly charged anions is less effective. These interactions are competitive and also depend on the composition of the BGE used. At levels above its CMC, CTAB forms micelles and enables the application of the micellar electrokinetic capillary chromatography (MEKC) mode and the analysis of, e.g., neutral components. Simultaneously, it is shown that the presence of CTAB may increase the number of potentially formed system zones.