(427e) Inspection of Solubilization Loci of Functional Series into Micelle Compartments as a Guide to Improve MEKC Selectivity

Micelles can be viewed as entities composed of numerous compartments of distinct hydrophobicities: inner core, palisade, solvation layer, and surface are readily defined in most micellar systems. We have proposed that MEKC selectivity can be rationalized by considering the details of the micellar phase and the manner by which solutes solubilize into the different micelle compartments. By knowing the major locus a solute set occupies into the micelle it is possible to promote changes in that specific locus and therefore improve the solute set resolution.

In our research group, quantitative structure-retention relationships (QSRR) and molecular dynamics experiments with a series of model compounds such as terpenes, phenolic compounds (phenolic acid and flavonoids, anthocyanins included), aromatic amines, and steroids have been used to provide insights on solute partition mechanisms and to define solubilization locus into SDS and LRO micelles.

As a preliminary assessment of solute localization we have proposed the inspection of log k (k being the retention factor in MEKC separations) versus the solute McGown volume plots. We were able to demonstrate for a large number of homologous and functional series that the slope of such plots provides a rough-tuning of solute micelle localization.

Thermodynamic parameters (enthalpy and entropy of partitioning) can be further estimated using vanâ??t Hoff plots (log P versus 1/T) for different surfactant systems. When linear vanâ??t Hoff plots are obtained we have proposed that solutes occupy single micelle compartments, once no variation of phase transfer DH and DS with temperature are observed. A curved vanâ??t Hoff plot suggests that the solute spends its residence time partially between two layers (core/solvation layer, core/palisade or palisade/solvation layer) depending on the kind of surfactant considered. Therefore, linearity of vanâ??Hoff plots in conjunction with the examination of the solute structural features and availability of the oxygen atoms to the solvent guide the assignment of solute location for many solute sets.

Experimentally obtained transfer DH and DS can be correlated with molecular descriptors to provide further insights into solute localization. Descriptors based on combinations of atomic charges and solvent accessible surface areas, solute molecular volumes, among others have been proven the most effective.

Our overall results revealed that the solubilization loci of terpenes is the micelle inner core, steroids occupy the internal interface of the solvation shell and flavonoids, anthocyanins and amines anchor at the surface. Therefore, to enhance the separation of flavonoids, anthocyanins and amines the basicity of surface sulfate in SDS micelles for instance must be changed by introducing spacers, such as co-surfactants and anchored solvents such as pentanol, etc. For terpenes, the inner core must be changed which can be achieved by selecting surfactant tails of different chain lengths. And finally, to optimize separations of steroids, the use of mixed micelles such as SDS/non ionic surfactants might be attempted.