1pK and 2pK Protonation Models in the Theoretical Description of Simple Ion Adsorption at the Oxide/Electrolyte Interface: A Comparative Study of the Predicted and Observed Enthalpic Effects Accompanying Adsorption of Simple Ions

Abstract

There is developed a quantitative theoretical description of enthalpies of surface protonation of the (hydr)oxide/electrolyte solution interface assuming the lpKcharging mechanism. The predictions obtained by adopting the1pK basic Stern model (BSM) of the interface are compared with the experimental data (potentiometric titration curves and heats of proton adsorption) and with the results obtained recently by adopting the 2pK triple-layer model (TLM) of the interface. It is found that the 1pK BSM gives similar results to the 2pK TLM, but in some cases the latter appears to be more flexible than the 1pK BSM. The conditions for existence of the common intersection point were consistently applied in the calculations to reduce the number of parameters used. The obtained values of heats of proton adsorption are compared with the values available in the literature. The found heats are exothermic and range between +17 kJ/mol (silica) and +46 kJ/mol (alumina) in the case of 1pK BSM. The calculated values of the proton adsorption entropy (TAS) are almost constant for the investigated systems and equal to +3 kJ/mol. This means that enthalpic effects play a main part in the process of proton adsorption on metal (hydr)oxides and silica.