Principles and Practice of Chromatography - Factors Affecting the Magnitude of the Distribution Coefficient (K) > Page 19

the distribution, in thermodynamic terms, is said to be "entropically driven".

 

Chiral separations and separations made by size exclusion are examples of entropically driven systems. Chromatographic separations are not exclusively "energetically driven" or "entropically driven". In most cases retention has both "energetic" and "entropic" components which, by careful adjustment, can be made to achieve very difficult and subtle separations.

Thermodynamics show that there are two processes controlling distribution but does not indicate how the distribution can be managed or controlled. To do this, it is necessary to identify how the magnitude of (K) and (Vs) are controlled. In general, (K) is usually determined by the nature and strength of the intermolecular forces between the solute and the two phases. The availability of the stationary phase (the magnitude of (Vs)) is largely determined by the geometry of the stationary phase.

Factors Affecting the Magnitude of the Distribution Coefficient (K)

The magnitude of (K) is determined by the relative affinity of the solute for the two phases. Those solutes interacting more strongly with the stationary phase will exhibit a larger distribution coefficient and will be retained longer in the chromatographic system. Molecular interaction results from intermolecular forces of which there are three basic types.