The chiral surfactant dodecoxycarbonylvaline (DDCV) has proven to be an effective pseudostationary phase for the separation of many enantiomeric pharmaceutical compounds. In this study the elution range and the prediction of octanol-water partitioning for the DDCV micellar system was examined. Through incorporation of DDCV in mixed micelles and unilamellar vesicles, enhancement of the elution range was observed. The mixed micelles contained a second anionic surfactant, sodium dodecyl sulfate (SDS), while the vesicles were composed of DDCV and the cationic surfactant cetyltrimethylammonium bromide (CTAB). Enantioselectivity, as well as other chromatographic and electrophoretic parameters, were compared between the mixed micelles, vesicles, and DDCV micelles. The hydrophobicity of the DDCV system was also evaluated as a predictor of n-octanol-water partition coefficients for 15 beta amino alcohols. The correlation between the logarithm of the retention factor (log k) and log P(ow) for seven hydrophobic beta-blockers and eight beta-agonists were r2 = 0.964 and r2 = 0.814, respectively.
Using the van't Hoff relationship, the enthalpy and entropy of transfer of 12 pairs of enantiomeric pharmaceutical compounds, including -agonists, -blockers, and other -amino alcohols, were measured from the aqueous phase to Ž q q q . three different counterion Li , Na , and K chiral micellar phases. Selectivities, distribution coefficients, and Gibbs free energies were also determined. Whereas the transfer of solute from the aqueous phase to the chiral micellar phase was Ž . nearly always favorable from an enthalpic viewpoint y23 -⌬ HЊ -1 kJrmol , it Ž . was not always so from an entropic perspective y25 -⌬ SЊ -57 Jrmol K . Dramatic differences in ⌬ HЊ and ⌬ SЊ were often observed between enantiomers. The lithium counterion system gave the most favorable entropic contribution Ž . positive ⌬ S to the free energy of solute transfer, while the potassium counterion Ž . system gave the most favorable enthalpic contribution negative ⌬ HЊ . The enthalpy and entropy changes for the solutes follow a linear free-energy relationship for all counterion systems, indicating a similar enantiomeric mechanism of separation. Experimentally determined thermodynamic parameters were related to changes in the polarity of the N-dodecoxycarbonylvaline micelles as a function of counterion to better understand the interactions of enantiomers with these micelles. In contrast to the large changes in ⌬ HЊ and ⌬ SЊ observed for individual solutes as the counterion was changed, ⌬GЊ and enantioselectivity remained similar.
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