Extended thermodynamic approach to ion interaction chromatography: a thorough comparison with the electrostatic approach, and further quantitative validation

The most reliable literature experimental results, concerning retention behavior of charged molecules, in the presence of an ion-interaction reagent (IIR), were used to obtain a further quantitative validation of a new theory. The present work emphasizes the fact that the extent to which electrostatic interactions, ion pair formation in the adsorbed and the mobile phases, and adsorption competitions are one more important than the other depends on experimental conditions. Further insight into the meaning of the linearity of the log k vs. log [IIR] plot, which is common to many theoretical models, is given. The experimental conditions under which the linearity of this plot can be expected not only practically, but also theoretically, are elucidated. The dependence of the ratio of retention factors with and without IIR in the eluent on the analyte nature, which cannot be predicted by the electrostatic approach, was explained and tracked. The difference between the actual surface potential and that predicted by the electrostatic approach is also rationalized. The model is also theoretically shown to be able to elucidate the enantioselective retention mechanism, in the presence of chiral counter ions.