A study of the thermodynamics and influence of temperature on chiral high-performance liquid chromatographic separations using cellulose tris(3,5-dimethylphenylcarbamate) coated zirconia stationary phases

In chiral HPLC, the separation is based on the differential interaction of a pair of enantiomeric molecules with a chiral selector Temperature will af fect such interactions. Most studies indicate that a decrease in temperatur e increases chromatographic selectivity. This is consistent with an enthalp y-controlled separation, but a more complete characterization of the physic ochemical interactions is required to understand the driving forces for chi ral recognition. In this work we studied the separation of a number of enantiomers on cellul ose iris(3,5-dimethylphenylcarbamate) supported on porous zirconia, over th e temperature range of 0 to 55 degreesC using n-hexano/2-propanol mixtures as the eluent, The differences in the enthalpy (Delta(DeltaH degrees)) and entropy (Delta(DeltaS degrees)) of transfer of the enantiomers from the mob ile to the chiral stationary phase were estimated from van't Hoff plots. Th ese relationships allow the study of the origin of the differences in inter action energies. The most interesting finding is that while most solutes sh ow a negative Delta(DeltaH degrees) difference, the two most easily resolve d enantiomeric pain were separated by on entropy dominated process. Studies of the relationship between the thermodynamics of transfer of these two en tropically controlled separations and the eluent composition showed a subst antial change in the interaction energies of these two solutes with the chi ral polymer when the alcohol was reduced to 2% (v/v). Finally, we show that there is virtually no correlation between Delta(DeltaG degrees) and overal l retention, between Delta(DeltaH degrees) and DeltaH degrees, and little o r no enthalpy-entropy compensation. These findings indicate the extreme dif ficulty in predicting or even correlating chiral selectivity with overall i ntermolecular interactions.