Unexpected retention behavior of supercritical fluid chromatography at the low density near critical region of carbon dioxide

The retention factors of octylbenzene, octadecene, anthracene, and pyrene eluted from columns packed with neat silica and C18-bonded silica by pure carbon dioxide near its critical region increase with increasing temperature along low-density isopycnic lines. This behavior is markedly different from the one observed in nearby regions of the pressure–temperature diagram of CO2, where the retention factors of these compounds decrease with increasing temperature along high-density isopycnic lines. Several possible reasons that could explain this reversal in the behavior of the retention factors are discussed. The most plausible is related to the formation of multilayer adsorption of CO2 on the stationary phases in a density range slightly lower than the critical density.