Counteranion Basic analyte interaction with

As was shown above, the chaotropic effect is related to the influence of the counteranion of the acidic modifier on the analyte solvation and is independent on the mobile-phase pH, provided that complete protonation of the basic analyte is achieved. Analyte interaction with a counteranion causes a disruption of the analyte solvation shell, thus affecting its hydrophobicity. Increase of the analyte hydrophobicity results in a corresponding increase of retention. This process shows a saturation limit, when counteranion concentration is high enough to effectively disrupt the solvation of all analyte molecules. A further increase of counteranion concentration does not produce any noticeable effect on the analyte retention. 

With the increase of the counteranion concentration, the solvation of the protonated basic analyte decreases. The primary sheath of water molecules around the basic analytes is disrupted, and this decreases the solvation of the basic analyte. The decrease in the analyte solvation increases the analyte hydrophobicity and leads to increased interaction with the hydrophobic stationary phase and increased retention for the basic analytes. 

The optimal counteranion of the acid will have its negative charge dispersed or delocalized throughout its structure, thus having less ability to hydrogen bond. The lower the pA" of the acid the stronger it is, and it may be ionized at pHs usually used for reversed-phase HPLC. However, weak acids may not be fully ionized at a certain pH of the mobile phase, and this will have an effect on their interaction with the basic analytes. Only the ionized form of the acid participates in the ion association. The following are some acids typically used in HPLC and their ionization properties. 

When these counteranions are present in the eluent and interact with the protonated basic analyte they tend to disrupt the solvation of the basic analyte. These anions can be classified as chaotropic counteranions. ... 

It is an effect caused primarily by the concentration and type of counteranion. As a result of addition of acid there is an increase in the concentration of the counteranion of the acid and a simultaneous decrease in the pH. pH is a factor affecting the protonation of the analyte. Only when the analyte is protonated it can undergo ionic association with the counteranion of the acid that was used. Hence, when the protonated base interacts with the counteranion this leads to changes in its solvation and increase in its hydrophobicity. At higher concentrations of the acidic counteranion, the protonated basic analyte is desolvated to a greater extent. This ultimately causes an increase in analyte retention. 

---