The Mechanism of Chromatographic Retention - Solute Stationary Phase Interactions > Experimental Support for the Sorption and Displacement Process > Page 58

It is seen that m-dimethoxy benzene, a solute that eluted at a (k') of 10.5 from a column with the same phase system, also failed to displace any ethyl acetate from the silica gel even when more than 0.5 gram of solute resided on the silica surface. It follows, that the m-dimethoxy benzene also interacted with the surface by a sorption process.

The solute benzyl acetate, however, a more polar solute that eluted at a (k') of 27 from a column with the same phase system, behaved in an entirely different manner. As soon as the solute was added to the system, ethyl acetate began to be displaced from the silica surface in a regular manner. The concentration of ethyl acetate in the mobile phase began to increase, accompanied by an equivalent decrease in the amount of ethyl acetate residing on the silica gel. It is also seen, that the curve relating the benzyl acetate on the silica gel with the amount added, is no longer linear but takes on the characteristic profile of the Langmuir adsorption isotherm at higher concentration levels. This demonstrated that the more polar solute was interacting with the silica surface by a displacement process in contrast to the other three solutes. However, the mode of interaction can differ for different solutes. The example given herewill be unique for the solute/solvent system examined. Other distribution models may exhibit sorption and displacement interactions with very different solute types and at widely different solvent concentrations.

It must be remembered that although changes in solvent composition will change the nature of the surface and consequently, the solute interactions with the surface. The same changes in the mobile phase will also modify the interactions of the solute with the mobile phase and the effect on solute retention can be as great or even greater than the contribution from the stationary phase. Thus, the net effect of changing solvent composition on solute retention will be the accumulative result of stationary phase modification and interactive changes in the mobile phase.