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

R. P.J. Ranatunga, Peter W Carr

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Abstract

The goal of this study was to elucidate the roles played by the stationary and mobile phases in retention in reversed-phase liquid chromatography (RPLC) in terms of their individual enthalpic and entropic contribution to the Gibbs free energy of retention. The experimental approach involved measuring standard enthalpies of transfer of alkylbenzenes from typical mobile phases used in RPLC (methanol/water and acetonitrile/water mixtures), as well as 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 examinemore fully the contribution that each phase makes to the overall retention. It was found that the standard enthalpy of retention in RPLC (i.e., solute transfer from the mobile phase to the stationary phase) is favorable, due to the large and favorable stationaryphase contribution, which actually overcomes an unfavorable mobile-phase contribution to the enthalpy of retention. Further, the net free energy of retention is favorable due to the favorable enthalpic contribution to retention, which arises from the net interactions in the stationary phase. Entropic contributions to retention are not controlling. Therefore, to a great extent, retention is due to enthalpically dominated lipophilic interaction of nonpolar solutes with the stationary phase and not from solvophobie processes in the mobile phase. Further, our enthalpy data support a 'partition-like' mechanism of retention rather than an 'adsorption-like' mechanism. These results indicate that the stationary phase plays a very significant role in the overall retention process. Our conclusions are in direct contrast to the solvophobic model that has been used extensively to interpret retention in RPLC.

Original languageEnglish (US)
Pages (from-to)5679-5692
Number of pages14
JournalAnalytical chemistry
Volume72
Issue number22
DOIs
StatePublished - Nov 15 2000

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