Robert Charmas

Calorimetric studies of simple ion adsorption at oxide/electrolyte interface titration experiments and their theoretical analysis based on 2-pK charging mechanism and on the triple layer model

Abstract The expressions developed recently by us to describe the temperature dependence of the titration curves are used to describe the calorimetric effects accompanying ion adsorption at the oxide/electrolyte interface. For the purpose of illustration the data obtained by “calorimetric titration” of TiO 2 suspension in NaCl were subjected to a quantitative analysis, based on the developed equations. A proper choice of adsorption parameters leads to a quantitative agreement between both the experimental and theoretical titration curves, and between the experimental and theoretical heats of proton adsorption at different pH values. This quantitative analysis shows the risk of a simple interpretation on a qualitative level of the enthalpic effects accompanying ion adsorption. This is because the recorded enthalpic effects carry complicated information about the enthalpic effects of various surface (complexation) reactions occurring simultaneously.

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.

Adsorption of cadmium ions at the electrolyte/silica interface. II. Theoretical study of surface energetic heterogeneity

The equations developed by us for the triple layer surface complexation approach, taking into account energetic heterogeneity of surface oxygen, are applied here to study the heterogeneity influences on the adsorption of cadmium ions at the NaClO4/silica aqueous solution interface. The model of surface heterogeneity made it possible to fit in a quantitative way the data obtained in the four independent experiments: surface charge measurements (titration), measurements of electrophoretic mobility and monitoring radiometrically cation of inert electrolyte adsorption, together with cadmium ion adsorption at different pH values. Particularly the studies of cadmium ion adsorption show that the energetic heterogeneity of the silica surface oxygen should be taken into account.

A Thermodynamic Analysis of Ion Adsorption in the Metal Oxide/Electrolyte Systems in which PZC and CIP do not Coincide

AbstractA theoretical-numerical analysis of two adsorption systems composed of the same kind of oxide- TiO2, and of two different electrolytes, NaCl and CsCl is presented. For one kind of the electrolyte (NaCl), PZC and CIP coincide, whereas they are different for the other (CsCl) electrolyte. The analysis is carried out by applying the popular TLM model, and by drawing formal-mathematical consequences of CIP existence in both kinds of adsorption systems. The values of the adsorption parameters are found by fitting simultaneously the obtained theoretical expressions to both experimental titration isotherms, and to the individual isotherms of cation adsorption measured using radiometric methods. That theoretical-numerical analysis suggests, that the inequality PZC ≠

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

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On the Nature of the Energetic Surface Heterogeneity in Ion Adsorption at an Electrolyte/Oxide Interface. Theoretical Studies of the Correlations between Binding-to-Surface Energies of Various Surface Complexes

(pKinta1 + pKinta2) may be a general feature of the oxide/electrolyte systems including the systems in which PZC and CIP coincide.

A calorimetric-thermodynamic study of the energetic heterogeneity of water/oxide interfaces

In this article we study the effect of energetic heterogeneity of a crystalline surface on the adsorption of hydrogen ions (protons) from the liquid phase. In particular, we examine the influence of the shape of the adsorption energy distribution on the equilibrium isotherms of hydrogen ions. To that purpose, a few popular distribution functions, including rectangular, exponential, and asymmetric Gaussian are considered. Additionally, multimodal distribution functions, which may correspond to the adsorption on different crystal planes of the oxide, are also used. Lateral interactions between adsorbed charges are modeled using the potential function proposed by Borkovec et al., which accounts also for polarization of the liquid medium. The results presented here are obtained using both Monte Carlo (MC) simulations and theoretical calculations involving Mean Field Approximation (MFA). They indicate that increased energetic heterogeneity of the adsorbing surface may, in general, considerably change the behavior of the adsorption isotherms, regardless of the assumed distribution function. It is also shown that the predictions of the proposed theory are consistent with the data obtained from the MC simulations.

Calorimetric Effects of Simple Ion Adsorption at the Oxide-Electrolyte Interfaces: Seeking for a Simple Interpretation of the Data Obtained by Titration Calorimetry

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.