The Thermodynamics of Chromatography - Other Thermodynamic Methods that are Used for Studying Chromatographic Systems > Page 60

                            

 

Taking the logarithms,  

                              

 

 

Introducing the functions for standard enthalpy and standard entropy,

                 

 

     Thus                    (35)

Equation (35) is an expression for the temperature at which the separation ratio of the two solutes will be independent of the solvent composition. The temperature is determined by the relative values of the standard enthalpies of the two solutes between each solvent and the stationary phase, together with their standard entropies between each solvent and the stationary phase. If the separation ratio is very large, there will be a considerable difference between the respective standard enthalpies and entropies of the two solutes. As a consequence, the temperature at which the separation ratio becomes independent of solvent composition may well be outside the practical chromatography range. However, if the solutes are similar in nature and are eluted with relatively small separation ratios (for example in the separation of enantiomers) then the standard enthalpies and entropies will be comparable, and the temperature/solvent-composition independence is likely be in a range that can be experimentally observed.