Effect of cosurfactant on separation selectivity in solvent-modified MEKC: the diclofenac case

Solvent-modified Micellar ElectroKinetic Chromatography (MEKC) shows performances in selectivity tuning and separation efficiency similar to MicroEmulsion Electrokinetic Chromatography (MEEKC), with the advantage of much greater composition flexibility of adding modifiers to the background electrolyte. In our previous study, a MEEKC method with the addition of methyl- β-cyclodextrin was developed for the simultaneous assay of diclofenac and its impurities, involving both neutral and charged compounds as analytes. In the scouting phase for selecting a suitable operative mode, the presence of the cosurfactant n-butanol, both in MEEKC and in MEKC, was found to be compulsory in order to achieve the separation, demonstrating that cosurfactant plays an active role in controlling the partition and migration of the analytes. Based on these results, this study presents a comprehensive investigation on the effect of the cosurfactant on separation selectivity in SDS-based MEKC, performed by Molecular Dynamics (MD) and NMR, in order to contribute to the understanding of the involved intermolecular interactions. MD constitutes an essential tool for exploring the mechanism of molecular recognition and NMR should be used to confirm the experimental and simulation results. From the retention behavior of the solutes, information with respect to the physical and chemical properties of the analytes, such as the extent of solute association with micelles, was obtained. Capacity factors and effective mobilities of the solutes were calculated and compared with the potential and the gain energy of inclusion complexes between analytes with SDS and n-butanol obtained by MD. Nuclear Overhauser effect spectroscopy NMR (NOESY) experiments were carried out to confirm the mechanism of separation.