The Thermodynamics of Chromatography - Thermodynamics Basics > Page 8

  

 

This distribution system is not dominated by molecular forces. The relatively large entropy change is a measure of the loss of randomness or freedom that happens when the solute molecule transfers from one phase to the other. The more random and 'more free' the solute molecule is in a particular environment, the greater its entropy in that environment. The large entropy change shown in system (B) (figure 2), indicates that the solute molecules are more constrained in the stationary phase (e.g., confined in the pores of the exclusion media) than they were in the mobile phase. This restriction is responsible for the greater distribution of the solute in the stationary phase and its greater retention. Because the change in entropy is the major contribution to the change in standard energy,

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

Chiral separations and separations that are dominated by size exclusion are examples of entropically driven separations.

It is important to understand that chromatographic separations can not be exclusively "energetically driven" or "entropically driven"; both components will always be present to a greater or lesser extent. It is by the careful adjustment of both the "energetic" and "entropic" components of a distribution that very difficult and subtle separations can be accomplished.