| Using packed column supercritical fluid chromatography, experimental retention measurements of n-alkylbenzene probe solutes were performed using pure carbon dioxide, sulfur hexafluoride and mixed carbon dioxide-methanol mobile phases and chemically bonded stationary phases. In one system, carbon dioxide at fixed temperature but over a wide density range, on ethyl-, triethyl-, and octadecylsilane phases were studied. In the others, various mobile phases, densities and temperatures, and octadecylsilane phases were examined.;The general trends and dependencies of solute retention and selectivity on solute size, temperature, mobile phase density and composition are discussed in some detail.;The trends of the mean selectivity with carbon dioxide density were found to be largely independent of the bonded stationary phase employed. This finding implies that the mobile phase primarily influences solute retention shifts. Pure carbon dioxide was found to exhibit greater solvent strengths than sulfur hexafluoride over the same reduced density and temperature ranges, as evident in their respective influence on solute retention, selectivity and enthalpies of transfer. Increasing the mobile phase methanol concentration was found to effect greater decreases in solute retention than does increasing the mean mobile phase density alone. From an assumed linear fit, the methylene group enthalpy of transfer was found to vary by 63(;The segmental interaction energy of a methylene group with carbon dioxide, relative to that with sulfur hexafluoride was found to be 1.05 (;The observed higher retention and selectivities arising from greater extensive solute interaction with octadecylsilane than with ethylsilane phases suggest an absorption-type retention mechanism in the octadecylsilane systems. |
