The Thermodynamics of Chromatography - The Distribution of Standard Energy Between Different Types of Molecular Interactions > Page 45

The Distribution of Standard Energy Between Different Types of Molecular Interactions

In contrast to distributing the standard energy between different groups in a solute molecule, the standard energy can also be dispensed between the different types of forces involved in the solute/phase-phase distribution. This approach has been elegantly developed by Martire et al. (8). In a simplified form, the standard energy can be divided into portions that result from the different types of interaction, e.g.,

 

 

In an attempt to explain subtle interactive differences, polar interactions can be divided into weak, moderate and strong interactions that, in the literature, have been, somewhat arbitrarily, given terms such as p/p interactions, dipole/dipole interactions, and hydrogen bonding, e.g.,

 

Nevertheless, it is important to realize that p/p interactions, dipole/dipole interactions, and hydrogen bonding,  are not different types of interaction but are all polar interactions but of different strength. In fact, many more terms have been introduced to describe subtly different enthalpic and entropic contributions to retention. Again one should remember Einstein's comment on first order effects being simple. However, in fairness, it must be said that, in many cases, these extra terms are often introduced to take into account many second order effects. One final point, standard energy is a bulk property, and in this approach, where a portion is allotted to a particular type of interaction, e.g., dispersive, it will include all dispersive interactions throughout the whole molecule. It will account for the net effect of many interactions involving all dispersive groups and all other groups (including polar) that can offer dispersive interactivity and not solely for a specific interaction that arises from a particular exclusively dispersive group.