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Surface excess
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Here we will more closely investigate the relation between the retention factor and the corresponding adsorption of the solute to the stationary phase surface.

 

The following questions will be discussed:

- Is the retention factor of a solute proportional to its adsorption constant to the stationary phase surface?

OR

- Is the retention factor of a solute proportional to its surface excess on the stationary phase surface?

 

SURFACE EXCESS

 

In the thermodynamics of adsorption to surfaces the concept of surface excess is used. The surface excess is the extra amount per unit area of the solute that is present at or near the surface, when the surface is equilibrated with the mobile phase containing the solute.

 

The concept is illustrated in the following figure.

 

Mathematically the surface excess can be written as follows:

 

G = ( na - ni )/ As

 

where 

 

na = the amount of the solute at or close to the surface.

 

ni = the amount of the solute in the presence of a physically and chemically inert surface

 

As = surface area 

 

The answer to the questions above is that the retention factor is not proportional to the adsorption constant nor to the surface excess.

 

The reason is that the retention factor depends on how the column void volume is determined. However, the situation is not so bad. If the procedures that are used to determine the column void volume give a reasonably correct value AND for retention factors not too small, i.e. larger than 2,  the simple textbook relation usually is an acceptable approximation.

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