Effects of Ionisation on Adsorption from Solution

Abstract

The adsorption of γ-picoline (4-methyl pyridine) at the silica–water interface, as a function of pH, is reported. A general model for the adsorption is developed, which allows for the possibility that the ionised (HPic+) and the neutral (Pic) forms of the molecule may both adsorb. For the limit of zero coverage, explicit expressions are derived which give a simultaneous fit of adsorption and electrokinetic measurements, and the model is extended to consider the complete adsorption isotherms. The magnitudes of the initial slopes of the isotherms, and the occurrence and position of a maximum in the adsorption with pH, are explained. The two forms of the picoline molecule have specific adsorption potentials of ∼–4 kT, with the neutral molecule slightly the more strongly adsorbed by ∼ 0.3 kT.