Calorimetric effects accompanying ion adsorption at the charged metal oxide/electrolyte interfaces: effects of the correlations between the binding-to-surface energies of various surface complexes.

As has been shown in our earlier publications, a theoretical description of ion adsorption at the real, energetically heterogeneous oxide/electrolyte interfaces, involves necessary assumption about the correlations between the adsorption energies of the various surface complexes formed at this interface. So far, only two extreme models have been considered in such theoretical descriptions: one model assuming that high correlations exist, and the other assuming that practically no correlations exist. The purpose of this paper is to develop yet another description based on a model assuming that some partial correlations exist between the adsorption energies of the various surface complexes. The other purpose of this publication is to test these three models by analysing some experimental data reported so far in literature. Such data sets must include necessary information about the enthalpic effects accompanying ion adsorption. This is because enthalpic effects are known to be more sensitive to the mechanistic models underlying a theoretical description of adsorption systems. With such requirements in mind, we have selected three adsorption systems: Al2O3/NaCl, TiO2/NaCl and silica/NaCl, for our analysis. Our theoretical-numerical analysis of the behaviour of these adsorption systems suggests that either none or partial correlations exist between the adsorption energies of the various surface complexes. However, that analysis also shows, that the present accuracy of the experimental data does not allow us to draw more elaborate conclusions.

Study of proton adsorption at heterogeneous oxide/electrolyte interface. Prediction of the surface potential using Monte Carlo simulations and 1-pK approach.

Adsorption of protons on a heterogeneous solid surface is modeled using the Monte Carlo (MC) simulation method. The surface of an oxide is assumed to consist of adsorption sites with pK assigned according to a quasi-Gaussian distribution. The influence of the electrostatic interactions combined with the energetic heterogeneity of the surface is examined and the MC results are compared with the predictions of the analytical 1-pK approach. The surface potential behavior is examined using both "experimental" MC results and "theoretical" results obtained from the application of 1-pK model. The results are compared qualitatively with experimental determination of the surface potential of metal oxide surfaces. They confirm that the relation between the surface potential and the pH of bulk solution should not be described by the Nernst equation but by the equation with the parameter linearly reducing Nerstian potential. The values of this parameter are examined with respect to degree of surface energetic heterogeneity and site density of the surface.