Microemulsion HPLC

502 What is MELC? Microemulsions are stable, isotropically clear solutions consisting of an oil (such as octane) and water stabilized by a surfactant and co-surfactant. To form such a mixture the interfacial tension between the oil and water has to be decreased by the addition of both a surfactant and a cosurfactant. Sodium dodecyl sulphate (SDS) is a common and widely used surfactant.1,2 Medium chain alcohols, such as butanol or pentanol, are commonly used co-surfactants. Microemulsions contain nanometre-sized surfactant coated droplets of oil suspended in water — these are referred to as oil-in-water (o/w) microemulsions. Figure 1 shows a typical o/w microemulsion droplet structure. Water-in-oil (w/o) microemulsions are also possible where surfactant-coated droplets of water are suspended in oil. In Microemulsion liquid chromatography (MELC)1–4 a microemulsion mobile phase is used with conventional reversed-phase (RP) columns. Generally o/w microemulsions are used in MELC. MELC is an extension of micellar liquid chromatography (MLC). In MLC a surfactant is added in excess of the critical micelle concentration (CMC) with the result that the mobile phase contains a large amount of micelles. The micelles affect Microemulsion HPLC