Controlling solvent strength and selectivity in micellar liquid chromatography: role of organic modifiers and micelles.

Simultaneous enhancement in elution strength and selectivity which has been previously observed in micellar liquid chromatography (MLC) for a variety of compounds is further investigated. The reasons behind the occurrence of this unique phenomenon are studied, and the influence of micelles and organic solvents on elution strength and selectivity is discussed. A model is developed which explains the dependence of the solvation ability of organic solvents in MLC (represented by the solvent strength parameter, S, of solutes) and the degree of solute interactions with micelles. Whenever the difference in solvent strength parameter values of two solutes in micellar eluents, dS, is positive, maximum selectivity is observed at the weakest eluent strength. When dS less than 0, there exists an inverse relationship between retention and solvent strength parameter so that selectivity monotonically increases with volume fraction of organic solvent in micellar eluents. It is shown that usually there is no direct relationship between the solvent strength parameter in MLC and retention. As a result, selectivity enhancement due to an increase in the concentration of organic modifier (i.e. solvent strength) occurs frequently in MLC. Interestingly, for cases where selectivity decreases with an increase in organic modifier, simultaneous enhancement of selectivity and solvent strength can be observed by increasing micelle concentration. In a sense, the concentrations of organic modifier and micelles complement one another in improving selectivity at higher elution strengths. As a result of this unique phenomenon better separations in shorter analysis times can be observed. The mutual effects of micelles and organic modifier on one another would also require a simultaneous optimization of these two parameters.