Interactions on the Surface of Bonded Phases

Solute interactions with the stationary phase in LC, differ from those in gas chromatography (GC) due to the preponderance of solvent that is concurrently interacting with the surface. In fact, in LC, the solute and the solvent interact in exactly the same manner with the stationary phase, but the concentration of the solvent is many orders of magnitude greater than that of the solute. Consequently, an equilibrium arises between the solvent and the stationary phase which is perturbed by the passing presence of the solute. It has been well established that solvents are adsorbed on the surface of a bonded phase as mono-or binary layers according to the Langmuir adsorption isotherm. The equation for the Langmuir adsorp

Figure 4 Alternative forms of solute interaction with the surface of a reversed phase. (A) Surface incompletely and (B) surface completely covered with solvent molecules. Open circles, alcohol; filled circles, solute.

tion isotherm of a monolayer is as follows:

b + ac where a is the fraction of the surface covered by the solute, c is the concentration of the solute in the mobile phase, and a and b are constants.

At high concentrations a becomes unity and the surface is completely covered with the more strongly adsorbed solvent. An example of the monolayer adsorption isotherms of some aliphatic alcohols from their aqueous solutions is shown in Figure 2.

It is seen that the most polar alcohol, methanol, does not saturate the surface until the methanol concentration of the mobile phase is in excess of 30%. In contrast, the surface is completely covered with the more dispersive alcohol, butanol, when its concentration is less than 5%.

In reversed-phase chromatography, aqueous solutions of organic solvents such as methanol and acetonitrile are usually employed as the mobile phase. Consequently the solute molecules will also exhibit Langmuir-type adsorption isotherms when interacting with the surface of a phase. An example of the adsorption isotherm of acetophenone between a mobile phase containing 40.4% v/v acetonitrile (the concentration at which the reversed phase is completely wetted by the solvent) and the phase surface covered with adsorbed solvent is shown in Figure 3.

Only the initial approximately linear part of the isotherm is shown, indicating that the adsorption process is similar to that exhibited by the mobilephase solvent. It is clear that the solute can interact with the reversed phase in a number of ways depending on the equilibrium conditions that exist between the solvent in the mobile phase and the surface. The alternative forms of interaction are depicted in Figure 4.

In the first case, where the concentration of the solvent is insufficient to cover the entire surface, the solute can interact directly with the uncovered surface, interact with the solvent molecules covering the surface or displace a solvent molecule and interact directly with the surface. All these alternatives will occur during the passage of the solute through the column. In the second case, where the concentration of the solvent is sufficient to ensure that the surface is covered with the solvent molecules, the solute can only interact with the solvent molecules covering the surface, or displace a solvent molecule and interact directly with the surface. The latter can only occur if the interaction energy between the solute and bonded moiety is greater than that between the solute and the adsorbed solvent.

It is seen that the interaction of the solute with the reversed phase can be quite complex and will depend not only on the nature of the solute itself, but on both the nature and concentration of the solvent in the mobile phase. It is clear that a ternary solvent system would be even more complex, and if the solvent composition is changed during development (as in gradient elution) the distribution system at any particular concentration, although possible to calculate from adsorption isotherm data, can be a very complicated procedure indeed.

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