Principles and Practice of Chromatography - Factors Controlling Retention > The Thermodynamic Explanation of Retention > Page 18

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

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, the distribution is not predominantly controlled by molecular forces. 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 ,