Morin N, Guillaume Y C, Peyrin E, Rouland J C
Laboratoire de Chimie Physique et Minérale, Faculté de Médecine et de Pharmacie, Besançon, France.
Anal Chem. 1998 Jul 15;70(14):2819-26. doi: 10.1021/ac980194p.
The high-performance liquid chromatography retention mechanism of a series of six imidazole derivatives was investigated over a wide range of mobile-phase compositions, pH, and column temperatures using a beta-cyclodextrin (beta-CD)-bonded chiral stationary phase. Thermodynamic constants for the transfer of a solute from the mobile to the beta-CD stationary phase were determined. Different van't Hoff plot shapes were observed with mobile-phase pH values, indicating a change in the retention mechanism. Enthalpy-entropy compensation revealed that the solute retention mechanism was independent of the compound molecular structure, the same at pH 7 and 7.5, but changed at pH 6.5, 8, and 8.5. Differential scanning calorimetry and thermogravimetric analysis were used to show different thermal features for the beta-CD stationary phase at pH 6.5, 8, and 8.5 and at pH 7 and 7.5. A new theory was presented to explain the beta-CD cavity structure balance between an ordered and disordered state. Variations of column temperature and mobile-phase pH tend to cause this phase transition between these two states, explaining the thermodynamic constant variations with pH and temperature.
