Surfactant pseudophases for open-tubular liquid chromatography and electrochromatography

Chromatographic science continues to take great interest in column development, particularly in discovering new materials for stationary phases that are efficient, stable, robust and selective. Open-tubular formats for microcolumns are regarded as advantageous due to the improved increased mass transfer, with very good chromatographic performance shown by theoretical studies. Surfactant-based separations by liquid chromatography and electrophoresis have long been established with surfactants above the critical micelle concentration (cmc). New liquid chromatography and electrochromatographic techniques are introduced here that exploit the molecular aggregates of surfactants below and above the cmc as immobile pseudophases suitable for open-tubular capillary columns. Electroosmotic flow measurements were used to characterise surface aggregation of two common ionic surfactants, hexadecyl- or cetyltrimethylammonium bromide (CTAB) and sodium dodecylsulfate (SDS), determining the cmc and the critical surface aggregation concentration (csac) that define admicelle and micellar formation. Admicelles formed between the csac and cmc were employed as soft and immobile pseudophases for the open-tubular admicellar liquid chromatography (OT-AMLC) and admicellar electrochromatography (OT-AMEC) of neutral (alkyl phenyl ketones, pesticides) and ionic (sulfonamides) analytes. Retention behaviour was ascertained using the relative retention times (RRT) showing similar values within the csac-to-cmc range, confirming admicelles were unaltered by pressure and electric field forces. Interfacial and solution micelles formed above the cmc govern the separation behaviour in open-tubular micellar liquid chromatography (OT-MLC) applied to neutral and ionic (food antioxidants) analytes and real sample matrices. Mobile phase conditions (pH, ionic strength, added salts and organic solvents) affected chromatographic behaviour. Analytical figures of merit (linearity, LOD, repeatability) have indicated acceptable employability of these techniques. This thesis introduces a new area of interest in separation science research.