Preliminary note

The “Parsons-Zobel plot” for systems of suspended particles

There is a long-lasting uncertainty as to the structure of the metal oxide/water interface. Various structures have especially been invoked to rationalize the seemingly anomalous silica/water interface. But, none of them seems to be approved in a quantitative and systematic way. We show that the concentration ratio of monovalent counterions and coions around colloidal silica spheres, as detected by the cryo-XPS technique, follows fundamental theoretical predictions of either Boltzmann or Donnan distribution. It is suggested that the reason of the dual response is the formation of the swelling gel layer on the silica surface due to its longer contact with water.

Parsons–Zobel plots can in principle be used to estimate inner Helmholtz-layer capacitance values and electrochemical surface areas for mineral particles. Their application to aqueous suspensions of various minerals has been documented in the literature. For the experimental data used so far, the expected linear relationship between the overall and the diffuse-layer capacitances has been reported. The extracted values either have not been used at all subsequently in a surface complexation model to describe the mineral surface charge versus pH curves, or were found not to be suitable entirely for such purposes. In the latter case, the reported failure was not explained. In one part of the present paper, the Parsons–Zobel plot concept is tested with data generated from a surface complexation model, for which the interfacial structure closely corresponds to that assumed in the application of the Parsons–Zobel plot. From the analysis of the results it turns out that electrolyte binding and non-Nernstian surface potential–pH curves more or less strongly affect the outcome of Parsons–Zobel plots. Despite the fact that the analysis in this paper is restricted to iron(III) minerals only, it is concluded, in general, that the use of Parsons–Zobel plots with aqueous mineral suspensions to determine inner Helmholtz-layer capacitances for subsequent application to surface complexation models cannot be recommended, since the reasons for failure can be traced very nicely with applications to model-generated data. Such application requires the determination of further parameters, and it was found that low electrolyte binding and Nernstian slopes should be imposed. Of these two issues, the more important is electrolyte binding. For the surface complexation models, an inner Helmholtz-layer capacitance and weak electrolyte binding were required for a good fit to experimental data. The values of the electrolyte binding constants required to achieve this end are in conflict with the assumptions of the Parsons–Zobel plot (absence of specific adsorption). However, these parameters would not necessarily cause specific adsorption in terms of a classical colloid chemistry definition (i.e., would not shift isoelectric points). The electrochemical surface areas were found to be in good agreement with the value used to generate the data. Based on this, there is a potential for using the approach to determine surface areas in situ from titration curves. Consequently, in a second part of the paper, Parsons–Zobel plots are applied to experimental data with the objective of determining electrochemical surface areas in situ. Application to various sets of published experimental titration data for hydrous ferric oxide yielded consistently very large electrochemical surface areas for fresh samples. This can be explained by very small particles and/or inclusion of substantial amounts of water in the suspended particles. As would be expected, the electrochemical surface area for aged ferrihydrite was found to be substantially lower.

NiO and Co₃O₄ were prepared by thermal decomposition of the respective nitrate and mixed mechanically to give mixed oxides of any composition (‘physical’ mixtures). The point of zero charge (pzc) was determined by standard potentiometric titration. The surface area was measured by the BET method (‘dry’ area) as well as derived from titration curves using the Parsons–Zobel approach (‘wet’ area). The dependence of pzc on composition has been discussed in terms of mole, weight and surface area proportions, and compared with the behaviour of ‘chemical’ (atomically intimate) mixtures investigated previously. The surface area ratio appears to govern the pattern of the dependence of pzc on composition.

A systematic study of {silver ∣ inorganic and/or organic halide} interphases is described, based on cyclovoltammetric and impedance experiments carried out on polycrystalline electrodes of high reproducibility (controlled by comparison with monocrystalline ones), in water and in acetonitrile. The adsorption competition is brought into focus (i) between the three inorganic halides Cl⁻, Br⁻, and I⁻, with the possible interference of ions from the supporting electrolyte, and (ii) between organic and inorganic halides, especially iodides. Actually adsorption competition proves to be a determining factor in the electrocatalytic mechanism (a remarkable threshold effect being observed for the reduction potentials of organic bromides in correspondence with the limiting negative potential for iodide anion adsorption). Vice versa, the variation of the reduction potentials of a given reacting molecule with the background electrolyte provides significant clues on the nature of the specifically adsorbed species.

Zirconia powders were prepared by heating at 470°C hydrothermal precursors obtained by precipitation at increasing pH (from 2 to 12). The phase composition of the calcined products varied from pure monoclinic (at the acid extreme) to pure tetragonal (neutral-alkaline range). For the mixed phase samples, the relative enrichment in the two polymorphs was estimated by interpretation of X-ray diffractograms. Electrochemical determinations of the double layer reactivity of the pure and mixed phase oxides were performed and combined with characterisations of the surface state by X-ray photoelectron spectroscopy (XPS). Interpretations of the distinctive electrochemical features shown by the different polymorphs are proposed.

FITEQL requires a constant solid-to-liquid ratio in serial data for calculations involving different models of an electric double layer. Two dummy components, introduced in the present paper, make it possible to handle data with variable solid-to-liquid ratios without changing anything in the source code. Experimental data (nickel and gadolinium adsorption on alumina) obtained for solid-to-liquid ratios ranging from 2 to 200 g (310 to 31 000 m²) per litre are analyzed as an example.

A triple-layer model was used to calculate the distribution of electric potentials in the interfacial region. The effect of the solid-to-liquid ratio and ionic strength on nickel adsorption can be reproduced quantitatively by a model in which a AlONi⁺ surface species is formed, i.e., one proton is released per one adsorbed Ni²⁺ ion, and the adsorbed nickel is located about half way between the plane of the surface charge and the β plane (the plane in which ions of the supporting electrolyte are adsorbed).