Modified electroosmotic flow by cationic surfactant additives in capillary electrophoresis: Evaluation of electrolyte systems for anion analysis

Abstract

A detailed experimental study of the variables affecting flow reversal was conducted in electrolyte systems commonly used for the determination of small anions. The electroosmotic flow mobilities of chromate (5 mM, pH 8) and 3,5-dinitrobenzoate solutions (concentration range 1.25–10 mM and pH 4–9) were determined by monitoring the migration time of pure water with indirect UV detection at 254 nm. The addition of the cationic surfactants cetyltrimethylammonium bromide, tetradecyltrimethylammonium bromide and lauryltrimethylammonium bromide, computed as fractions of the standard critical micelle concentration (std CMC), was investigated. The results suggested that flow reversal follows a single transition adsorption mechanism, in which the capillary inner surface is occupied by a layer of hemi-micelles, i.e. two dimensional aggregates formed by lateral interaction of surfactant tails. Once flow reversal is achieved, with surfactant concentration approximately above 10% std CMC, the nature of the electrolyte anion as well as the pH of the solution does not exert any significant effect on the magnitude of the electroosmotic flow. However, at this condition, the electroosmotic flow mobility varies linearly with the logarithm of the anion concentration.