Mono Layer Adsorption

A solvent can be adsorbed from a solvent mixture on the surface of silica gel according to the Langmuir equation for monolayer adsorption. Examples of mono-layer adsorption isotherms obtained for benzene, chloroform and butyl chloride are shown in Figure 1. It is seen that the surface becomes completely covered, and its interactive characteristics severely modified at relatively low concentrations of the second solvent in the mobile phase ( > 10% v/v). If the second solvent is more polar, a quite different type of adsorption isotherm applies. In this case, bi-layer adsorption can take place as shown by the isotherms in Figure 2. Bi-layer adsorption is not uncommon and the bi-layer adsorption isotherm equation can be derived by a simple extension of the procedure used to derive the Langmuir adsorption isotherm. It should be noted that, due to the strong polarity of the hydroxyl groups on the silica, the initial adsorption of the ethyl acetate on the silica surface is extremely rapid.

The individual isotherms for the two adsorbed layers of ethyl acetate are included in Figure 2. The

Figure 1 Langmuir adsorption isotherms for benzene, butyl chloride and chloroform.

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Concentration of ethyl acetate in the solvent (% w/v)

Figure 2 The individual and combined adsorption isotherms for ethyl acetate on silica gel.

Concentration of ethyl acetate in the solvent (% w/v)

Figure 2 The individual and combined adsorption isotherms for ethyl acetate on silica gel.

two curves, although similar in form, are quite different in magnitude. The first layer which is very strongly held is complete when the concentration of ethyl acetate is only about 1% w/w. At concentrations in excess of 1% w/w the second layer has only just begun to be formed. The formation of the second layer is much slower and the interactions between the solvent molecules with those already adsorbed on the surface are much weaker. Assuming that the total area covered by the first layer will be the same as the area covered by the second layer, then only about one third of the layer is complete at a concentration of about 4% w/v. This is in striking contrast to the formation of the first layer which is virtually complete at an ethyl acetate concentration of 1% w/v.

From the point of view of solute interaction with the surface, it is seen that it can now be very complex indeed. In contrast to the less polar or dispersive solvents, the character of the interactive surface will be modified dramatically as the concentration of the polar solvent ranges from 0 to 1% w/v. However, above 1% w/v, the surface will be modified more subtly, allowing a more controlled adjustment of the interactive nature of the surface.

Multi-layer adsorption is also feasible, for example the second layer of ethyl acetate might have an absorbed layer of the dispersive solvent «-heptane on it. However, any subsequent solvent layers that may be generated will be situated further, and further, from the silica surface and are likely to be very weakly held and sparse in nature. Under such circumstances their presence, if, in fact, real, may have little impact on solute retention.

Interactive Mechanisms on the Surface of Silica Gel in Normal-Phase Liquid Chromatography

It is clear that the interaction of the solute molecules with the stationary phase surface can be quite complex and also change with the composition of the mobile phase. There are two ways a solute can interact with a stationary phase surface. The solute molecule can interact with the adsorbed solvent layer and rest on the top of it. This is called 'sorption interaction' and occurs when the molecular forces between the solute and the stationary phase are relatively weak compared with the forces between the solvent molecules and the stationary phase. The second type is where the solute molecules displace the solvent molecules from the surface and interact directly with the stationary phase itself. This is called 'displacement interaction' and occurs when the interactive forces between the solute molecules and the stationary phase surface are much stronger than those between the solvent molecules and the stationary phase surface. An example of sorption interaction is shown in Figure 3(A). Its linear relationship is clearly demonstrated and it is seen that the distribution coefficient (which controls retention) can be adjusted to any selected value by choosing the appropriate mixture of the two solvents.

Reiterating the equation proposed by Purnell, for two solvents (A) and (B) in GC (see Figure 4):

Figure 3 Different forms of molecular interaction with the silica gel surface.

The results of Katz et al. can be algebraically expressed in a similar form for LC:

Figure 4 Graph of corrected retention volume against volume fraction of stationary phase.

phase, or stationary phase surface area, gives the corrected retention volume, i.e.:

For chromatography purposes the product of the distribution coefficient and the volume of stationary

In the experiments of Katz et al. (see Figure 5), that validated the relationship given in eqn [4], the distribution coefficients (K) were referred to the solvent phase (mobile phase) whereas in LC it is the mobile phase composition that is changed and the distribution coefficients (K") are referred to the stationary phase. Thus:

Substituting for the corrected retention volumes in eqn [3] for inverse phase system:

Figure 4 Graph of corrected retention volume against volume fraction of stationary phase.

Figure 5 Graphs relating distribution coefficient to solute retention for n-pentanol.

Simplifying:

In practice a more convenient way of expressing solute retention in terms of solvent concentration for a binary solvent mixture as the mobile phase is to use the inverse of eqn [5], i.e.:

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