Individual and Simultaneous Class Separations of Cationic and Anionic Surfactants Using Capillary Electrophoresis with Indirect Photometric Detection

Abstract

Various chromophoric cationic and anionic reagents and combinations thereof are evaluated for individual and simultaneous class separations of nonchromophoric cationic and anionic surfactants using capillary electrophoresis (CE) with indirect photometric detection (IPD). Optimization results indicated a pH of 6-7 with at least 50% methanol is necessary for separations of long-chain cationic (tetrahexylammonium, didodecyl dimethylammonium) or anionic (C 14 -C 18 SO 3 - or C 14 -C 18 SO 4 -) compounds along with medium- to short-chain ionic surfactants of each class. For individual class separations, benzylamine (BA) or 1-ephedrine (EP) are recommended electrolytes for cationic surfactants, and naphthalenemonosulfonate (NMS) provided better detectability than p-toluenesulfonate (PTS) for anionic surfactants. Using an electroosmotic flow driven approach, a combined separation of a five-component tetraalkylammonium cationic and 10-component anionic (C 4 -C 18 SO 3 - ) surfactant mixture is possible in about 45 min using either BA-NMS or BA-PTS electrolytes. The BA-NMS mixture will provide better detectability of anionic surfactants with some loss of sensitivity for cationic surfactants, whereas the reverse is true with the BA-PTS combination. The commercially available pyridinium (PY)-PTS reagent in the absence of methanol could separate a three-component tetralkylammonium cationic and a 10-component anionic (C 1 -C 14 -SO 3 - ) surfactant mixture in less than 7 min.