Interaction mechanisms on octadecyl packed columns in subcritical fluid chromatography with CO2–modifier mobile phases

An experimental design was carried out for describing the interaction mechanisms between solutes and octadecyl bonded silicas in subcritical fluid chromatography (SubFC), with CO2–methanol and CO2–acetonitrile mobile phases. The effects of modifier amount, temperature and outlet pressure were studied. The homologous series of alkylbenzenes was mainly used as probe, and results were in part assessed with other series. Curves between the methylene selectivity (αCH2) and the alkyl chain carbon number (Cn) were plotted, because changes of slope or discontinuity in these curves are yielded by interaction mechanism modifications. Moreover, the linearity of the Van ‘t Hoff curves with CO2–acetonitrile mobile phases has enabled one to calculate the transfer enthalpy (ΔH) for each homologue. The curves log k=f(−ΔH) allow a discrimination of the retention behaviors between the short and the long homologues for CO2–acetonitrile mobile phases. Depending on the analytical conditions, different oriented partition mechanisms occur for the long homologues, when the short ones seem to be fully embedded into the grafted chains near the silica surface. With methanol–CO2 mobile phases the discrimination between the homologues disappears and the methylene selectivity curves correspond to a bulk partition mechanism. The differences in the interaction mechanisms following the modifier nature are related to the adsorption the mobile phase onto the stationary phase, because the amount of adsorbed mobile phase modifies the bonded chain mobility. With methanol, an important adsorption of the mobile phase occurs, when this adsorption is reduced with acetonitrile. In this latter case, an anisotropy in the stationary phase mobility can explain the observed difference in the interaction mechanisms of homologues. Finally, effects of stationary phase chain length (from C18 to C22) and bonding density (from 2.5 to 3.4 μmol m−2) were also reported.