Bonded Phases - Solvent Solute Interactions with the Surface of a Reverse Phase

Solvent/Solute Interactions with the Surface of a Reverse Phase

Solvents and solutes interact somewhat differently with a reverse phases. Under chromatographic conditions the solvent is in continuous contact with the stationary phase and is in permanent equilibrium with it (unless gradient programming is being employed) whereas the other, the solute, interacts transiently with the reverse phase as it migrates down the column. Consequently, the interaction of solvent and solute with the reversed phase will be considered separately starting with the interactions of the solvent.

Solvent Interactions with the Surface

The nature of the interaction between solvent molecules and a reverse phase surface are basically similar to the complementary interactions of solvent molecules with a silica gel surface. A layer of solvent molecules is assembled on the stationary phase surface by absorption, the primary difference being that the interaction between the solvent molecules and the reverse phase will be dispersive in nature in contrast to those interactions that occur with the silica gel which for the main part are polar. The adsorption isotherm exhibited by the reverse phase is described by the Langmuir equation but, due to the interactions with the reverse phase being exclusively dispersive and are not a mixture of dispersive and polar interactions as in the case of silica gel, a careful study of the adsorption isotherms on a reverse phase can afford a more precise understanding of the surface character than is possible for silica gel. The Langmuir adsorption isotherm equation has been developed for both mono-layer adsorption and bi-layer adsorption on the surface of silica gel. However, as the adsorption data will be processed in a different way when the theory is applied to reverse phases, the Langmuir equation will be derived in a more pertinent manner. The derivation will be that described by Scott and Simpson (29) specifically for the examination of reverse phases.

The Derivation of the Langmuir Adsorption Isotherm for Reverse Phases

This derivation provides equations that will permit the inclusion of the pertinent chromatographic properties of an LC system in the expression that describes the retention volume of the adsorbed solvent. The expressions will contain the distribution coefficient of the solvent between pure water and the reverse phase, the chromatographically effective surface area of the reverse phase and the solvent concentration. As a consequence, it will be demonstrated how the adsorption isotherm of the solvent (or moderator) can be determined from chromatographic measurements.

Consider 1 cm2 of surface carrying an adsorbed layer of solvent at a concentration (Cs) on the surface in contact with a liquid containing (Cm) g of solvent per cm-3 of the aqueous solvent mixture. Assume the molecular weight of the solvent is (M) and the area covered by the solvent molecules when adsorbed on the surface is (S).

Thus, assuming a mono-layer of solvent is formed on the surface, the area of exposed surface (y), is given by,

.................................. (2)

where, (N) is Avogadro's Number

The number of molecules (N1), leaving the surface will be proportional to the concentration of adsorbed molecules and a constant (b) .

N1 = bCS

The constant (b) is that fraction of the adsorbed molecules that acquire sufficient kinetic energy to overcome the molecular forces holding the molecules to the surface and consequently are able to leave it.