Infinite-dilution partial molar properties of naphthalene and biphenyl in carbon dioxide from supercritical fluid chromatography: Composition effectsin the stationary phase

The temperature and pressure effects on retention of naphthalene and biphen yl in supercritical fluid chromatography are investigated within 35-70 degr ees C and 75-150 bar. The retention factors are used to derive the infinite -dilution partial molar volumes of the two solutes in supercritical carbon dioxide, and the infinite-dilution molar enthalpies of transfer of the two solutes from the stationary phase to supercritical CO2. The partial molar v olumes are converted to quantities characterizing short-range interactions between CO2 and the aromatic hydrocarbon. The resultant partial molar prope rties are corrected for the pressure- and temperature-dependent dissolution of CO2 in the stationary polymer (poly(dimethylsiloxane)). The corrections for composition changes in the stationary phase include a composition deri vative of the solute chemical potential that can only be obtained from a su itable theoretical model. Therefore, it is difficult to give a conclusive a ccount of the performance of the corrections as these reflect the features of the model employed; here, the Scatchard-Hildebrand-Flory-Huggins theory or the Panayiotou-Vera lattice-fluid model are used for the purpose. Howeve r, comparison of the resultant partial molar volumes of naphthalene with li terature data from high-precision, vibrating-tube densitometry indicates th at the composition corrections improve the agreement between chromatographi c and densitometric results.