In chiral HPLC, the separation is based on the differential interaction of a pair of enantiomeric molecules with a chiral selector. Temperature will affect such interactions. Most studies indicate that a decrease in temperature increases chromatographic selectivity. This is consistent with an enthalpy-controlled separation, but a more complete characterization of the physicochemical interactions is required to understand the driving forces for chiral recognition. In this work, we studied the separation of a number of enantiomers on cellulose tris(3,5-dimethylphenylcarbamate) supported on porous zirconia, over the temperature range of 0 to 55 °C using n-hexane/2-propanol mixtures as the eluent. The differences in the enthalpy (Δ(ΔH°)) and entropy (Δ(ΔS°)) of transfer of the enantiomers from the mobile to the chiral stationary phase were estimated from van't Hoff plots. These relationships allow the study of the origin of the differences in interaction energies. The most interesting finding is that while most solutes show a negative Δ(ΔH°) difference, the two most easily resolved enantiomeric pairs were separated by an entropy dominated process. Studies of the relationship between the thermodynamics of transfer of these two entropically controlled separations and the eluent composition showed a substantial change in the interaction energies of these two solutes with the chiral polymer when the alcohol was reduced to 2% (v/v). Finally, we show that there is virtually no correlation between Δ(ΔG°) and overall retention, between Δ(ΔH°) and ΔH°, and little or no enthalpy-entropy compensation. These findings indicate the extreme difficulty in predicting or even correlating chiral selectivity with overall intermolecular interactions.
- Cellulose tris(3,5-dimethylphenylcarbamate)
- Chiral separations
- Column liquid chromatography
- Entropically-controlled Separations
- Temperature effects