Evaluation of surface excess isotherms in liquid chromatography

• The adsorption behavior of weak bases on BEH-C18 is heterogeneous at all pH.
• The EML model accounts poorly for changes in this behavior at increasing ionic strength.
• The adsorption behavior of weak bases on CSH-C18 is heterogeneous at pH ≥5.
• This behavior is practically homogeneous at pH ≤3.

Methods are proposed to calculate surface excess isotherms and to use them to derive adsorption isotherms in liquid chromatography. The consequences of these methods are discussed. The excess isotherm of isopropyl alcohol from its aqueous solutions on a C18 adsorbent was obtained using the minor disturbance method. The slope of the inflection tangent of the excess isotherm provides the position of the plane separating the adsorbed layer and the bulk phase, from which the adsorption isotherm was derived. At low concentrations of isopropyl alcohol, frontal analysis was used to derive the adsorption isotherm on the same adsorbent using an independent method. The isotherm was thus derived from both frontal analysis data and the minor disturbance method. The results obtained are compared. Our results show that the use of the same concentration unit for the calculation and the representation of the data is the only correct way to calculate the excess isotherms in practical applications of liquid chromatography.