The Thermodynamics of Chromatography - The Distribution of Standard Energy Between Different Types of Molecular Interactions > Complexation > Page 47

  

 

Complexation

According to the theory of complexation (as applied to chromatographic retention), when polar interaction occurs between a solute molecule and a molecule of stationary phase, an associate (complex) is assumed to occur which (it is also assumed) actually removes the solute from the migration process. For obvious reasons, this type of interaction has been termed complexation. To meet the requirements of solute removal from the elution process, in GC the complex is assumed to have zero vapor pressure (this is also a quastioable assumption, but may be close to reality, if the stationary phase molecule is large enough). Under such circumstances it will remain stationary in the column until (as a result of the equilibrium kinetics) 'decomplexation' takes place and the individual components are again generated. The uncomplexed solute molecule can now continue to migrate along the column until complexing again occurs. In the authors' opinion, the difference between complexing and strong interaction is by no means clear and, in any event, the net effect of both will be the same, i.e., strong solute retention.

This is one approach to the explanation of retention by polar interactions, but, not surprisingly, the subject, at this time, remains controversial. Doubtless, if interaction is strong and the two molecules remain interacting for a finite time then such a concept as complexation would then describe the type of interaction (e.g., olefin retention on silver nitrate doped stationary phases in GC). However, if dispersive interactions (electrical interactions between randomly generated dipoles that can also have a wide range of interactive energies) can be explained without the need to invoke the concept of complexation, it is not clear why polar interactions (electrical interaction between permanent or induced dipoles) need to be considered differently.