Retention mechanisms general model

A number of theoretical approaches explained the retention by a simple size-exclusion mechanism presented in the Introduction. Such explanations were associated with more or less complicated pore shapes that were considered as a system of cylindrical, conical, etc., holes that spherical solute molecules can penetrate. The size distribution of the pores was not given much importance. Nevertheless, experiments whose objective was to correlate the molar mass dependence of the retention volume of different solute molecules with the pore size distribution curves did not provide satisfactory results. Although more sophisticated models of this type were more successful when compared with the experiments, they are not considered now as they are not sufficiently general and do not accruately describe the physical reality. 

In certain cases, a values of 30 or more have been found, which then correspond to A(AG) values in the range of 2 kcal/mol (8.4 kJ/mol). Generally, such values are obtained owing to very low retention of the first enantiomer eluted. This means that a very enantioselective sorption process is operating in the column, i.e., one of the enantiomers is virtually unbound by the CSP for steric reasons. Such phenomena are not easily explained by the three-point interaction model, but rather indicate the operation of a sort of chiral steric exclusion mechanism, more in line with a steric fit concept involving only one binding interaction. ... 

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