The Thermodynamics of Chromatography - The Distribution of Standard Energy Between Different Types of Molecular Interactions > The Concept of Complex Formation > Page 54

To obtain a value for (KEQ) then the pure stationary phase activity (aA), or the activity coefficient (gA) must be determined. It was argued that (gA) could be defined as follows,

This relationship depends on the assumption that two similar stationary phases, irrespective of their polarity, can be considered to differ by measuring the ratio of the activity coefficients of two noncomplexing solutes (this basically implies the solute is nonpolar and will only interact with the stationary phase by dispersion forces). If this were true then,

where () is the molar volume of the complexing phase.

Thus,                     

 

Similar substances were chosen to meet the assumptions of the theory that would offer approximately the same dispersive interactions but quite different polar interactions, for example di-n-octylmethylamine and n-octadecane. Consequently, retention differences were then assumed to be solely due to polar interaction and, thus, the 'complexation constant' could be calculated from retention data. Data obtained only agreed moderately well with values calculated by other techniques. It is clear, however, that the assumptions made in the development of the theory are, at the least, open to debate and although complexation may occur under certain circumstances, it is not necessary to evoke the concept to explain all polar interactions. From a chromatographic perspective, if it is accepted that dispersive forces cause solute retention by simple and direct interaction between molecules, without the need to invoke the concept of complexation, it is not clear why retention by polar interactions need to be considered differently.