PHASE-EQUILIBRIA, PARTIAL MOLAR ENTHALPIES, AND PARTIAL MOLAR VOLUMES DETERMINED BY SUPERCRITICAL FLUID CHROMATOGRAPHY

A variety of types of thermodynamic properties have been determined at infinite dilution by supercritical fluid chromatography. A key challenge is to identify clearly the retention mechanism. An experimental technique is presented for the measurement of retention due to absorption into a bulk C-18 liquid (stationary) phase, independently of the adsorption on the support. The important effect of the swelling of the liquid phase by the fluid phase is included. Distribution coefficients are presented for naphthalene and phenanthrene between CO2 and the C-18 liquid phase, and used to determine Henry's constants in the liquid phase and solute partial molar volumes and enthalpies in the fluid phase. In the highly compressible region of CO2 at 35-degrees-C, solute partial molar enthalpies have been found to reach negative values of hundreds of kJ/mol, indicating strongly exothermic solute-solvent clustering.