The Mechanism of Chromatographic Retention - Solute Stationary Phase Interactions > Experimental Support for the Sorption and Displacement Process > Page 55

 

Experimental Support for the Sorption and Displacement Process

Scott and Kucera (15) carried out some experiments that demonstrated, sorption and displacement interaction. They dispersed about 10 gram of silica gel in a solvent mixture made up of 0.35%w/v of ethyl acetate in n-heptane. From the adsorption isotherm in figure 21, it is seen that at 0.35w/v of ethyl acetate more than 95% of the first layer of ethyl acetate has been established and very little of the second layer was formed. This concentration was chosen to ensure that, if significant amounts of ethyl acetate were displaced by the solute, it must arise from the first layer on the silica and not the less strongly held second layer. The container was sealed with a serum cap and thermostatted at 25oC. 100 mg aliquots of the solute were added sequentially to the mixture by means of a syringe. After each addition the container was shaken, allowed to come into thermal equilibrium over about 30 minutes and then a 5 ml sample of the solvent taken for GC analysis. Corrections were made for the volume of the samples taken and the ethyl acetate and solute they contained

 

The analysis, gave the amount of ethyl acetate in the solvent and from the amount of ethyl acetate originally added, the amount of ethyl acetate on the silica gel could be calculated. In a similar manner, from the concentration of the solute in the solvent, the amount of solute adsorbed on the stationary phase was determined. The results obtained for the solutes, anisole and nitrobenzene are shown in figure 26. One pair of curves refer to the polar solvent and relate the concentration of ethyl acetate in the solvent (Em) and the concentration of ethyl acetate in the stationary phase (Es), to the total mass of solute added. The other pair of curves refer to the solute and relate the concentration of solute in the solvent (Sm) and the concentration of solute in the stationary phase (Ss) to the total mass of solute added. It should be first noted that the curves relating the concentration of ethyl acetate in the solvent mixture and on the stationary phase, are straight and horizontal.

Thus, although a total of about 1.2 gram of solute was added to the system, and about a third of which resided on the silica, neither anisole nor nitrobenzene displaced any ethyl acetate from the silica gel. It is also seen that the concentration of solute in both phases increased fairly linearly with the solute added, which means that the system was operating over the first part of the solute adsorption isotherm which is sensibly linear. Thus, the solute was interacting with the stationary phase by a sorption on the surface of the ethyl acetate layer.