Causes and remediation of reduced efficiency in micellar liquid chromatography

Broad peaks are obtained when purely aqueous micellar phases are used in micellar liquid chromatography (MLC). The causes of reduced efficiency in MLC are investigated. Slow solute mass-transfer kinetics between micelles, the aqueous phase and the surfactant covered stationary phase are the origins of the efficiency loss. Knox plots show that the reduced efficiency comes from A term increase and, for lipophilic solutes, A and C terms increases. Surfactant adsorption reduces the pore volume and surface area of the stationary phase changing the flow anisotropy (A term). The surfactant adsorbed layer slows down the mass transfer (C term). Three ways for efficiency loss remediation are known: flow-rate reduction, temperature increase and alcohol addition. Alcohols are known to change the micelle structure and to increase the kinetics of micelle formation-destruction. It is shown that the ratio of the alcohol chain length to surfactant alkyl chain length, C-n,C-OH/C-n,C-surf, should be equal or higher than 1/3 to produce the best efficiency enhancements in MLC. Also, the volume of alcohol to be added is not absolute but relative to the surfactant concentration. The alcohol to surfactant concentration ratio should be kept constant. Temperature increases and especially alcohol additions reduce the retention factors. Thermodynamic and kinetics of the micellar exchanges in MLC cannot be dissociated. (C) 1997 Elsevier Science B.V.