Atypical titration curves for GaAl₁₂ Keggin-ions explained by a joint experimental and simulation approach

Although they have been widely used as models for oxide surfaces, the deprotonation behaviors of the Keggin-ions (MeAl₁₂⁷⁺) and typical oxide surfaces are very different. On Keggin-ions, the deprotonation occurs over a very narrow pH range at odds with the broad charging curve of larger oxide surfaces. Depending on the Me concentration, the deprotonation curve levels off sooner (high Me concentration) or later (for low Me concentration). The leveling off shows the onset of aggregation before which the Keggin-ions are present as individual units. We show that the atypical titration data previously observed for some GaAl₁₂ solutions in comparison to the originally reported data can be explained by the presence of Ga₂Al₁₁ ions. The pK_a value of aquo-groups bound to octahedral Ga was determined from ab initio molecular dynamics simulations relative to the pure GaAl₁₂ ions. Using these results within a surface complexation model, the onset of deprotonation of the crude solution is surprisingly well predicted and the ratio between the different species is estimated to be in the proportion 20 (Ga₂Al₁₁) : 20 (Al₁₃) : 60 (GaAl₁₂).