Comparison of migration models for acidic solutes in micellar electrokinetic chromatography

The validity of two models that explain the migration of ionisable solutes in micellar electrokinetic chromatography (MEKC), mobility model and retention factor model, has been tested. For this purpose, the mobility (mu) and retention factor (k) of a set of 10 phenolic compounds with different hydrophobicity and pKa values have been determined for several sodium dodecyl sulphate (SDS) concentrations and pH values, and fitted to the models. Results show that in general the retention factor model explains better the retention of ionisable solutes, although for hydrophilic compounds at low SDS concentration, mobility model can give better fits. The different drawbacks pointed out by several authors in relation to both models have been checked, and a deep evaluation of each one has been done. As a result we have observed that, while in the retention factor model the variation of k with pH and [SDS] always follows the same trend, the variation of mu with these variables mainly depends on the value of the binding constant of the neutral form of the solutes to the micelles, KHA(m), which plays a critical role in the fit of the mobility model. Also we provide rules and advices to set up the experimental conditions to apply each model to a particular solute.