A study of the enthalpy and entropy contributions of the stationary phase in reversed-phase liquid chromatography

The goal of this study was to elucidate the roles played by the stationary and mobile phases in retention in reversed-phase liquid chromatography (RPL C) in terms of their individual enthalpic and entropic contribution to the Gibbs free energy of retention. The experimental approach involved measurin g standard enthalpies of transfer of alkylbenzenes from typical mobile phas es used in RPLC (methanol/water and acetonitrile/water mixtures), as well a s from n-hexadecane (a simple analogue of the stationary phase) to the gas phase, using high-precision headspace gas chromatography, By combining the measured enthalpies with independently measured free energies of transfer, the entropies of transfer were obtained. This allowed us to examine more fu lly the contribution that each phase makes to the overall retention, It was found that the standard enthalpy of retention in RPLC (i.e., solute transf er from the mobile phase to the stationary phase) is favorable, due to the large and favorable stationary-phase contribution, which actually overcomes an unfavorable mobile-phase contribution to the enthalpy of retention. Fur ther, the net free energy of retention is favorable due to the favorable en thalpic contribution to retention, which arises from the net interactions i n the stationary phase. Entropic conbributions to retention are nob control ling. Therefore, to a great extent, retention is due to enthalpically domin ated lipophilic interaction of nonpolar solutes with the stationary phase a nd not from solvophobic processes in the mobile phase. Further, our enthalp y data support a "partition-like" mechanism of retention rather than an "ad sorption-like" mechanism. These results indicate that the stationary phase plays a very significant role in the overall retention process. Our conclus ions are in direct contrast to the solvophobic model that has been used ext ensively to interpret retention in RPLC.