INVESTIGADORES
CASTELLS Cecilia Beatriz
artículos
Título:
Influence of temperature on chiral high-performance liquid chromatographic separations on cellulose 3,5-dimethylphenylcarbamate coated zirconia
Autor/es:
C. B. CASTELLS; CARR, P. W.
Revista:
CHROMATOGRAPHIA
Editorial:
VIEWEG
Referencias:
Lugar: ALEMANIA; Año: 2000 vol. 52 p. 535 - 542
ISSN:
0009-5893
Resumen:
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 (D(DH°)) and
entropy (D(DS°)) 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 D(DH°)
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 D(DG°) and
overall retention, between D(DH°) and DH°, and
little or no enthalpy-entropy compensation. These findings indicate the extreme
difficulty in predicting or even correlating chiral selectivity with overall
intermolecular interactions.