The Development and Characterisation of Microemulsions for the Separation of Pharmaceutical Analytes by HPLC and CE

Microemulsions are unique dispersions of oil in water or water in oil which are stabilised by a surfactant and co-surfactant. The formation of such a mixture was first reported by Hoar and Schulman in the 1940s when they noticed a mixture of hydrocarbon and surfactant turned from milky to clear on the addition of alcohol. The ability of microemulsions to simultaneously solubilise either aqueous or organic components while remaining isotropically clear and thermodynamically stable attracted much attention, particularly in the 1970s for use in tertiary oil recovery. During the early 1990s these surfactant based systems were reported in separation science as mobile phases in HPLC and background electrolytes in CE. Given the capability of microemulsions to dissolve both aqueous and non-aqueous compounds, their use as eluents in HPLC (MELC) and background electrolytes in CE (MEEKC) was further examined. In order to effectively test the ability of microemulsions to separate a diverse range of compounds, they were applied to the simultaneous analysis of oil- and water-soluble vitamins. A novel MELC method was developed for the simultaneous isocratic separation of oil- and water-soluble vitamins. Similarly, a MEEKC method was also developed for separating the same range of vitamins. In addition to separation, all microemulsion compositions examined in the development of the MEEKC method were characterised in terms of droplet size, surface tension, conductivity, and refractive index. When cross correlated with the MEEKC vitamin separation, relationships between choice of microemulsion components, physicochemical measurements and separation were observed. Also, a commonly reported SDS based microemulsion was characterised and applied to the MEEKC separation of pharmaceutical analytes. Initially a pseudoternary phase diagram was constructed to establish a range of possible compositions. Microemulsions across an aqueous dilution line were prepared and physicochemical measurements conducted to establish microemulsion phase type and transition. Results indicated that the measurement techniques employed were capable of describing microemulsion phase type. Furthermore, it was seen that the microemulsion phase type could be correlated with a MEEKC separation.