Some thermodynamic aspects of packed column supercritical fluid chromatography

Abstract

Packed column supercritical fluid chromatography (SFC) was employed in the investigation of the effects of density and packing surface silanol groups on the enthalpy and entropy of transfer of the solute between the mobile and stationary phases using CO 2, as the mobile phase and solutes of various polarity. The solutes were aromatics, phenols and amines and the stationary phases were untreated silica gel Partisil-10 thermally treated Partisil-10 and Partisil-10 bonded with octadecylsilane. In the density range 0.3–0.9 g ml −1, the enthalpies decrease as the mobile phase fluid density increases while the entropies remain constant for well coated packings. A moderate pressure drop does not change this trend. The thermodynamic value obtained from a Van 't Hoff plot suggests a strong influence of the retention of polar solutes by surface silanol groups in the low-density region of SFC separation. In addition, a characteristic line of intersection points may be obtained from the extrapolated line of the Van 't Hoff plots. Tested with solutes and packings of various polarity, the characteristic line reveals the nature of the packing materials.