Exploring the intermolecular interactions acting in solvent-modifiedMEKC by Molecular Dynamics and NMR: The effect of n-butanol onthe separation of diclofenac and its impurities

tAn integrated approach involving CE experiments, Molecular Dynamics (MD) simulations and two-dimensional NOE spectroscopy (2D-NOESY) experiments was employed to elucidate the intermolecularinteractions and the separation mechanisms involved in a solvent-modified MEKC method for the simul-taneous determination of diclofenac sodium and its impurities. The CE findings indicated that the additionof n-butanol (nBuOH) to the SDS micellar solution played a primary role for controlling the partitioninginto the mixed micelles and the migration of the analytes and that the presence of nBuOH as cosurfactantwas compulsory for achieving the complete separation of the compounds. The different capacity factorsof the analytes were calculated and a change in solute association with the mixed micelle when changingthe SDS/nBuOH molar ratio was highlighted. The optimal SDS/nBuOH molar ratio for the electrophoreticseparation was 1:8. On the other hand, both MD simulations and NMR experiments indicated that themost favorable molar ratio for the formation of mixed SDS/nBuOH micelles was 1:2. These results sug-gested that probably there is an excess of nBuOH in the background electrolyte, both as free moleculesand in form of aggregates, which is able to interact with the analytes, and thus may compete with mixedmicelles for the considered compounds. The calculated values of gain in potential energy of the ana-lytes when included in mixed micelles were in agreement with the observed migration order of thecompounds. The role of methyl--cyclodextrin (MCyD) in the background electrolyte was also inves-tigated, since the addition of this CyD to the solvent-modified MEKC system was found to be useful toreduce the analysis time. MD simulations and 2D-NOESY spectra highlighted the formation of inclusioncomplexes with MCyD not only with the analytes, but also with SDS. MCyD may lower the availabilityof both SDS and nBuOH for forming micelles and mostly may compete with the mixed micelle as a secondpseudostationary phase.