The Thermodynamics of Chromatography - Thermodynamics Basics > Page 1

Thermodynamics Basics

The retention of a solute in a chromatographic system is determined firstly, by the magnitude of the distribution coefficient of the solute between the two phases and secondly, by the amount of stationary phase available to the solute for interaction. This is fully discussed in Plate Theory and Extensions of this series. In addition, the mechanism of distribution has been considered exclusively on the basis of molecular interactions in The Mechanism of Chromatographic Retention . However, the distribution coefficient in chromatography is an equilibrium constant and, consequently, it can be treated rationally by conventional thermodynamics.

It follows, that the distribution coefficient can be expressed in terms of the standard energy of solute exchange between the phases employing the traditional and well established Arrhenious relationship, 

                                    RT ln (K)  =  -DGo                                 (1)

where (R) is the gas constant,
 (T) is the absolute temperature,
and (DGo) is the standard energy.

Now, classical thermodynamics gives another expression for the standard energy which separates it into two parts, the standard enthalpy and the standard entropy.