Micellar Electrokinetic Capillary Chromatography with Cationic Surfactants

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Abstract
A series of alkyltrimethylammonium chloride and bromide surfactants were evaluated in micellar electrokinetic capillary chromatography (MECC) of urea herbicides, alkylbenzenes and phenylalkylalcohols. The magnitude of the anodal electroosmotic flow obtained with these cationic micellar phases was largely unaffected by the length of the alkyl chain of the surfactant while the migration time of the micelle increased with decreasing the length of the alkyl tail. The net result was an increase in the retention window as the size of the alkyl tail of the surfactant decreased. The breadth of the retention window stayed almost the same when the micelle counterions were changed from chloride to bromide. At constant micellized surfactant concentration, the capacity factors of neutral solutes increased linearly with increasing alkyl chain length of the surfactant, indicating an increase in the hydrophobic phase ratio of the MECC systems. Under this condition, the value of the methylene group selectivity for the homologous solutes was largely unaffected by the length of the surfactant tail. Also, when the micellized surfactant concentration was held constant, the homologous solutes exhibited quasi-homoenergetic retention on the different cationic micellar phases. In addition, when going from a cationic surfactant to an anionic surfactant while keeping the length of the alkyl tail the same, the value of the methylene group selectivity remained unchanged, and the energetic of retention was not affected by the net charge of the micelle. The separation of a mixture of six urea herbicides was best achieved when an MECC system of iow hydrophobic phase ratio and wide retention window, such as dodecy-or decyltromethylammonium chorid (DoTAC or DTAC), was rsed as the miceffar phase. Tetraecy trimenylammomiuma lammonium chloride TTAC micellar phase having medium iiydrophobic character and narrower retention window than DoTAC or DTAC, was slightly less effective in separating the urea herbicide mixture. The overall separation of the urea herbicides could be enhanced by the inclusion of small amounts of octy ltrimethylammonium chloride (OTAC) surfactant into the TTAC micellar phase. This is because the addition of OTAC to the TTAC micellar phase decreased the capacity factors and increased the breadth of the retention window.