It follows, that the dominant thermodynamic contribution (enthalpy or entropy) to any specific distribution system can be identified from such curves. Such curves are called Vant Hoff curves and an example of two Vant Hoff curves relating log(V') against 1/T for two different types of distribution systems are shown in figure 26.

Figure 26. The Vant Hoff Curves for Two Different Distribution Systems

However, before discussing the contribution of enthalpy and entropy to the distribution coefficient and, thus, retention, the sign convention for the standard entropy, (DS) and the standard enthalpy, (DH) needs to be discussed. When the solute molecule is held by molecular forces in the stationary phase its entropy has been reduced as, in the mobile phase, where the molecular interactions are weaker (and in GC virtually zero), it had a much greater freedom of movement and could behave in a more random manner. It follows that as the entropy (freedom or randomness) of the solute is reduced, the standard entropy change (DS) must be negative. As the molecule is held on the stationary phase by intermolecular forces, energy has been used during the interaction and, thus, the standard enthalpy must also be negative. If (DH) is negative then the first term in equation (22) will be positive in practical systems. Likewise, if (DS) is also negative then the second term in equation (22) will be found to be negative in practical systems. This means that the enthalpy terms and entropy terms oppose one another, the enthalpy term increasing retention and the entropy term reducing retention.

Returning to figure 26, It is seen that distribution system (A) has a large enthalpy value and a low entropy contribution . The large value of means that the distribution is predominantly controlled by molecular forces.

The solute is preferentially distributed in the stationary phase as a result of the interactions of the solute molecules with those of the stationary phase being much greater than the interactive forces between the solute molecules and those of the mobile phase. Because 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".