The Thermodynamics of Chromatography - The Distribution of Standard Energy Between Different Types of Molecular Interactions > Page 46

  

 

This procedure of allotting different parts of the standard energy to different interactions has been employed to determine the so-called 'binding' constants between solute and stationary phase and also to study 'complexation constants'.

Binding constants and complexation constants have been introduced to distribution system theory, largely in an attempt to account for the wide range of strengths associated with polar interactions between molecules. Some schools of thought consider that polar interactions need to be treated in a significantly different mannerto dispersive interactions. As a consequence, alternative terms have been introduced to more clearly describe the different polar interaction strengths that can occur. In addition, other concepts have been introduced, such as the portion of the molecule that is negatively charged due to an asymmetric accumulation of electrons has been given the term electron donor site; similarly, the area of electron depletion in the molecules (that electrically balances the site of electron accumulation) has been given the term electron acceptor site.

However, to recover the fundemenatal simplicity of the interactive process, it should be understood that basically, there can be negatively and positively charged sites on a molecule and these localized charges cause molecular interaction. Again, because polar interactions have such a range of strengths, certain ranges of polarity have been assumed to be due to specific types of electrical interaction and have been given the terms (10) such as nonbonding lone pair, bonding p orbital, vacant orbital, antibonding p orbital, antibonding s orbital, etc., all of which are based on the presently accepted electron configuration of the molecule concerned. In practice however, whatever the rationale, the strength of the interaction and, thus, its effect on solute retention, is directly related to the intensity of the interacting charges.Fromthepoint of viewofseparation technology, whether the introduction of these alternative terms helps to explain the separation process is a moot point, as, even in theory, the individual effects can be extremely difficult to completely isolate from one another.