Revisionist look at solvophobic driving forces in reversed-phase liquid chromatography

Abstract

Based on the use of alkylbenzenes as test solutes, most of the free energy of retention in reversed-phase liquid chromatography (RPLC) is shown to arise from net attractive (exoergic) processes in the stationary phase, and not from net repulsive (endoergic) processes in the mobile phase. The classical view of the “passive” role of bonded phase ligands is challenged. However, it is also shown that variations in retention upon changing the mobile phase are dominated by alterations in the net processes in the mobile phase. Furthermore, it is shown that the free energy of transfer of a methylene group from the mobile phase to a bonded reversed phase over a wide range in mobile phase composition, is similar but not equal, to the free energy of transfer of a methylene group from the same mobile phase to pure bulk hexadecane. This observation is in accord with the partition model view of the mechanism of RPLC. Finally, by comparison of measured and computed activity coefficients, the regular solution theory is shown to be a grossly inadequate model of interactions in water and hydro—organic mixtures. It should not be used to model retention in aqueous mobile phases.