Migration behavior and selectivity of beta-blockers in micellar electrokinetic chromatography - Influence of micelle concentration of cationic surfactants

The influence of micelle concentration of cationic surfactants on the migration behavior and selectivity of ten beta-adrenergic blocking agents in micellar electrokinetic chromatography (MEKC) were systematically investigated at pH 6.5 and 7.0. Tetradecyl- and hexadecyltrimethylammonium bromides (TTAB and CTAB) were selected as cationic surfactants. The results indicate that, in addition to buffer pH, micelle concentration is an important separation parameter that influences the migration and selectivity of beta-blockers in MEKC. The migration behavior and selectivity of labetalol and propranolol are most markedly affected. The resolution of peaks between atenolol, metoprolol and levobunolol greatly enhances on increasing the micelle concentration. In contrast, the peaks between acebutolol and nadolol and those between timolol and atenolol become unresolvable at concentrations near 30 mM at pH 7.0. Complete separation of these beta-blockers was achieved either with CTAB and TTAB at a concentration in the range 15-20 mM and 12-15 mM, respectively, at pH 7.0 or with CTAB at a concentration in the range 27-30 mM at pH 6.5. Moreover, partition coefficients of beta-blockers between the aqueous and micellar phases at pH 7.0 were evaluated. The plot of the logarithm of migration factor (log k') versus the logarithm of octanol-water partition coefficient (log P-ow) reveals that, the migration of beta-blockers possessing small hydrogen bond strength depends on the extent of micellar solubilization based on hydrophobic interactions, whereas the migration and selectivity of beta-blockers with hydrogen bond donor characteristics are influenced considerably by hydrogen bonding interactions, in addition to hydrophobic interactions, in MEKC. (C) 1997 Elsevier Science B.V.