The Thermodynamics of Chromatography - Thermodynamics Basics > Page 7

 

 

The large value of  means that the distribution in favor of the stationary phase is dominated by molecular forces.  That is to say, the solute is retained in the stationary phase as a result of molecular interactions and that the forces between the solute molecules with those of the stationary phase are much greater than the forces between the solute molecules and those of the mobile phase. Thus, the change in standard enthalpy is the major contribution to the change in standard energy and it can be said,

 In thermodynamic terms, the distribution is "energy driven".

An example of the type of linear curve that is produced by exclusion chromatography is shown, in an exaggerated form, in figure 2

It is seen that the van't Hoff curve shown in figure 2, is a completely different type to that in figure 1. In this distribution system, there is only a very small enthalpy change  but, in this case, a very high entropy contribution .

 

Figure 2. Exclusion Chromatography Using Entropically Driven Distribution