Study of partition models in reversed-phase liquid chromatography based on measured mobile phase solute activity coefficients

Lack of accurate values for solute activity coefficients in aqueous organic solvent mixtures has been an impediment to the development of a detailed model of reversed-phase liquid chromatography (RPLC). In this study we have employed a recently measured set of infinite dilution activity coefficients for the alkylbenzenes (benzene to n-butylbenzene) in mixtures of water with four of the more common organic cosolvents in order to explore the mechanism of the retention process in RPLC. The work indicates that the solvophobic theory of reversed-phase chromatography is essentially correct, that is, most of the free energy of transfer arises from processes taking place in the mobile phase. Analysis of relative solute activity coefficients of two solutes in the bonded phase shows that the stationary phase environment is considerably more polar than that of a bulk long chain alkane. This support the idea that sorbed organic modifier plays a substantial role in establishing the chemistry in the bonded phase domain. The fact that measurements of the activity coefficients of non-polar solutes in methanol-saturated hexadecane are insignificantly different from those in pure hexadecane strongly suggests that the vastly different surface area to volume ratio of bonded and bulk phases is vitally important in bonded-phase RPLC.