Oxide-solution interface electrostatic models

Westall, J. C. and H. Hohl, 1980, A comparison of electrostatic models for the oxide/solution interface. Advances in Colloid Interface Science 12, 265-294. 

Reactions at the Oxide-Solution Interface Chemical and Electrostatic Models... 

The nature of the problem in establishing a mechanistic model of the oxide-electrolyte interface, in which chemical and electrostatic energies are described explicitly, can be appreciated by consideration of the adsorption reaction depicted in Figure 2. The adsorption of a hydrogen ion from the bulk of a monovalent electrolyte is considered. The oxide-solution interface is divided conceptually into four regions the bulk oxide (not shown in the figure), the oxide surface at which the adsorption reaction takes place, the solution part of the double layer containing the counterions, and the bulk of solution. 

Figure 4. Electrostatic models for the surface-electrolyte solution interface. These models were conceived for metal surfaces but have been used for oxide surfaces as well.

The main, currently used, surface complexation models (SCMs) are the constant capacitance, the diffuse double layer (DDL) or two layer, the triple layer, the four layer and the CD-MUSIC models. These models differ mainly in their descriptions of the electrical double layer at the oxide/solution interface and, in particular, in the locations of the various adsorbing species. As a result, the electrostatic equations which are used to relate surface potential to surface charge, i. e. the way the free energy of adsorption is divided into its chemical and electrostatic components, are different for each model. A further difference is the method by which the weakly bound (non specifically adsorbing see below) ions are treated. The CD-MUSIC model differs from all the others in that it attempts to take into account the nature and arrangement of the surface functional groups of the adsorbent. These models, which are fully described in a number of reviews (Westall and Hohl, 1980 Westall, 1986, 1987 James and Parks, 1982 Sparks, 1986 Schindler and Stumm, 1987 Davis and Kent, 1990 Hiemstra and Van Riemsdijk, 1996 Venema et al., 1996) are summarised here. 

Westall, J. C., and Hohl, H. (1980) A Comparison of Electrostatic Models for the Oxide Solution Interface, Adv. Colloid Interface Sci. 12, 265-294. 

Figure 10,18 Schematic plot of surface species and charge (a) and potential ) relationships versus distance from the surface (at the zero plane) used in the constant capacitance (CC) and the diffuse-layer (DL) models. The capacitance, C is held constant in the CC model. The potential is the same at the zero and d planes in the diffuse-layer model i/fj). Reprinted from Adv. Colloid Interface Sci. 12, J. C. Westall and H. Hohl, A comparison of electrostatic models for the oxide/solution interface, pp. 265-294, Copyright 1980 with kind permission of Elsevier Science-NL, Sara Burgerhartstraat 25, 1055 KV Amsterdam, The Netherlands.

Westall, j. C. 1986. Reactions at the oxide-solution interface Chemical and electrostatic models. In Geochemical processes and mineral surfaces, ed J. A. Davis and K. F. Hayes. Am. Chem. Soc. Symp. Ser. 323, pp. 54-78. Washington DC Am. Chem. Soc. 

Pivovarov, S., Acid-base properties and heavy and alkaline earth metal adsorption on the oxide-solution interface Non-electrostatic model, J. Colloid Interf. Sci., 206, 122, 1998. 

Westall, J. C. (1986). Reactions at the oxide-solution interface chemical and electrostatic models. In Geochemical Processes at Mineral Surfaces, ed. 

Another extreme is to neglect the exponential term in Eq. (5.24) at all. This leads to overestimated effect of the pH on tjo (assuming a fixed value). A model neglecting the surface potential is physically unrealistic, but non-electrostatic models of adsorption at solid-aqueous solution interface can be found even in very recent literature. According to the prevailing opinion the actual surface potential is between the above two extremes (Nemst potential and 0 = 0). The electrostatic models of oxide - inert electrolyte solution interface were discussed in detail by Westall and Hohl [25]. In this section the most common electrostatic models are combined with the 1-pK model in order to illustrate their ability to simulate the actual surface charging data. 

The boundary condition for potential at the pore wall requires special consideration. Basically, this boundary condition defines the electrostatic reaction conditions in the pore. It captures the interaction between charged metal walls and protons in solution. The boundary condition should relate

relation requires a model of the metal-solution interface. This problem is complicated by the formation of adsorbed oxygen species, particularly difficult in the potential region for ORR, where various Pt oxides are formed at the surface. Oxide formation modulates the metal surface charge. It leads to pseudocapacitance effects, which make an accurate determination of ctmCz) impossible with the current level of understanding. 

Numerous studies have attempted to elucidate the role of Mo in the passivity of stainless steel. It has been proposed from XPS studies that Mo forms a solid solution with CrOOH with the result tiiat Mo is inhibited from dissolving trans-passively [9]. Others have proposed that active sites are rapidly covered with molybdenum oxyhydroxide or molybdate salts, thereby inhibiting localized corrosion [10]. Yet another study proposed that molybdate is formed by oxidation of an Mo dissolution product [11]. The oxyanion is then precipitated preferentially at active sites, where repassivation follows. It has also been proposed that in an oxide lattice dominated by three-valent species of Cr and Fe, ferrous ions will be accompanied by point defects. These defects are conjectured to be canceled by the presence of four- and six-valent Mo species [1]. Hence, the more defect-free film will be less able to be penetrated by aggressive anions. A theoretical study proposed a solute vacancy interaction model in which Mo " is assumed to interact electrostatically with oppositely charged cation vacancies [ 12]. As a consequence, the cation vacancy flux is gradually reduced in the passive film from the solution side to the metal-film interface, thus hindering vacancy condensation at the metal-oxide interface, which the authors postulate acts as a precursor for localized film breakdown [12]. 

The model Introduced by Brunelle [27] has served as a practical guide to rationalize the interfacial chemistry during adsorption of catalytic precursors on oxides. The model assumes electrostatic attraction of adsorbing ions by a homogeneous oxide surface. Experimental evidence, indeed, has shown that adsorption of cations/anions takes place from solutions with pH higher/lower than a certain pH, called FZQ where the net charge of the surface is assumed to be zero. When experimental data have contradicted the concept of ion attachment at the interface by electrostatic forces only the effect of some "specific" or "chemical" interaction has been invoked. 

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