Thermodynamic and Kinetic Characterization of Porous Graphitic Carbon in Reversed‐Phase Liquid Chromatography

Abstract

The thermodynamics and kinetics of solute transfer on porous graphitic carbon (PGC) are studied for two series of aromatic hydrocarbons, alkylbenzenes and methylbenzenes. The retention behavior is characterized as a function of temperature, pressure, length of alkyl chain, and number and position of methyl substituents. In the thermodynamic studies, the retention factor (k) increases with an increase in the length of alkyl chain or number of methyl substituents. The data indicate that increases in the number of methylene and methyl substituents result in more negative changes in molar enthalpy (ΔHsm). Hence, the transition from mobile to stationary phase is a more energetically favorable, exothermic process with each methylene or methyl group added. The data also show that the change in molar volume (ΔVsm) is close to zero, which is consistent with an adsorption mechanism on PGC. Enthalpy‐entropy compensation is observed for both homologous series, and the compensation temperature suggests that the retention mechanism is distinctly different for the methylene and methyl groups. In the kinetic studies, the rate of solute transfer increases with an increasing number of methylene and methyl groups. The activation enthalpy from the stationary phase to transition state (ΔH‡s) is found to increase with an increasing number of methylene and methyl groups. But the activation enthalpy from the mobile phase to transition state (ΔH‡m) is much smaller than ΔH‡s and, at the same time, is very similar for all the solutes. The activation volumes (ΔV‡s and ΔV‡m) are very close to zero, again owing to the adsorption mechanism on PGC. These results demonstrate that the retention mechanism for PGC, which is based on adsorption, is different from that for octadecylsilica, the most widely used stationary phase in liquid chromatography.