Do you have examples of the separation of carboxylic acids at a mobile phase pH way below their pKa point? Is it necessary to have carboxylic acid ionized for IE separation?
In fact, to do pure ion-exchange separation, the acids need to be in ionized form. However, at a pH below the pKa of the carboxylic acid, the ionization is not significant and the molecules become significantly less polar. In this case they can be retained by the RP mechanism. When molecules end up within a hydrophobic stationary phase layer, the meaning of the pH is different than in aqueous solution, and electrostatic interaction still takes place. This is an example of an RP method with some contribution of IE interaction to improve selectivity.
