Thus, a curve relating ln(V') to 1/T will give a straight line the slope of which will be proportional to the standard enthalpy and the intercept will be related to the standard entropy and, as a consequence, the dominant effects that control the distribution system can be identified from such curves. Such curves are called Vant Hoff curves and an example of two Vant Hoff for two different types of distribution systems are shown in figure 5.

Figure 5. The Vant Hoff Curves for Two Different Distribution Systems.

It is seen that distribution system (A) has a large enthalpy value and a low entropy value that indicates the distribution is predominantly controlled by molecular forces. The solute is preferentially distributed in the stationary phase as a result of solute interactions with the stationary phase being much greater than those with the mobile phase. As the change in enthalpy is the major contribution to the change in free energy,

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

In contrast, for distribution system (B) there is only a small enthalpy change, but a high entropy contribution. Thus, molecular forces do not predominantly control the distribution. The entropy is a measure of the degree of randomness that a solute molecule experiences in a particular phase. The more random and 'more free' the solute molecule is in a particular phase, the greater its entropy. A large negative entropy change means that the solute molecules are more restricted or less random in the stationary phase (B) and this loss of freedom is responsible for the greater solute retention. The change in entropy in system (B) is the major contribution to the change in free energy, so,

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 that, 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.