Surfactant pseudophases as easy-to-use materials in separation science

A strategy for greening analytical separations is by decreasing the size of the analytical column and the use of benign reagents. Open tubes with 10–75 μm i.d. are commonly employed for this purpose. Open-tubular separations offer the advantages of fast and efficient separations, while only requiring small volumes of reagents, thereby generating minimal waste. Cationic surfactants such as cetyltrimethylammonium bromide (CTAB) are environmentally benign reagents and have been utilised in various open tubular separation modes. Depending on its concentration, CTAB assembles into admicelles or interfacial micelles at the negatively charged capillary wall. In this work, admicelles or interfacial micelles of cationic surfactants CTAB or didodecyldimethylammonium bromide (DDAB) were utilised as green and easy-to-use chromatographic stationary pseudophase for various applications in open-tubular separations. CTAB admicelles were used as stationary pseudophase for the voltage-driven separation of charged analytes, or open-tubular admicellar electrochromatography (OT-AMEC). The separation of charged analytes was explained using the retention (k) equation for OT-AMEC. Mobile phase additives altered k and effective electrophoretic mobility of analytes for selectivity modification. Meanwhile, cyclodextrins as chiral mobile phase additives enabled the separation of racemic compounds in OT-AMEC using dynamic CTAB admicelle coating, and in OT-liquid chromatography using semipermanent DDAB bilayer prepared as a successive multiple ionic layer coating. The retentive capability of CTAB interfacial micelles was exploited for analyte enrichment in capillary electrophoresis. In the technique called pseudophase microextraction (PPME), CTAB acted as a stationary pseudophase analogous to solid stationary phases in solid-phase extraction. PPME enabled increases in detection sensitivity in the order of 102–103, which were comparable to off-line microextraction techniques. The complicated preparation of a solid stationary phase were avoided with the use of CTAB or DDAB as chromatographic stationary pseudophase for achiral or chiral separations in OT-AMEC or OT-LC, or analyte enrichment in PPME. Therefore, CTAB and DDAB demonstrated to be effective, cheap, green, and easy to use in situ self-assembling materials for open tubular separations