The Thermodynamics of Chromatography - Thermodynamics Basics > Page 2

Thus,                            DGo = DHo - TDSo                                 (2)    

where (DHo) is the standard enthalpy,
    and (DSo) is the standard entropy.

The standard enthalpy and standard entropy represent two distinctly different portions of the energy associated with distribution and are related to quite different parts of the distribution processes.


The enthalpy term represents the energy involved when the solute molecules break their interactions with the mobile phase and interact with, and enter, the stationary phase. These interactions result from intermolecular forces that are electrical in nature (see book 7 for details) and are accompanied by the absorption or evolution of heat.


However, when the solute interacts with the stationary phase, because the interactive forces between the solute and the stationary phase molecules are stronger than those between the solute molecules and the mobile phase, the solute molecules are held more tightly and, consequently, are more restricted. This motion restriction, reduced freedom of movement or loss of randomness  is measured as the entropy change. The nature of the entropy change can be illustrated in GC, when the distribution of a solute between a gas and a liquid is considered. In the gas, the solute molecules have high velocities and can travel in any direction. However, when in the liquid phase, they are held strongly by interacting molecular forces to the molecules of stationary phase and can no longer travel through the phase at high velocities or with the same directional freedom of movement.