Oxide-solution interface model

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. 

Many models, which could be classified as "surface complexation models (6-8)," have been used to describe reactions at the oxide-solution interface. Although there are differences in the way these models are formulated, they all have two features in common ... 

Using a simple amphoteric model for the mineral surface, we have demonstrated the role specific chemical binding reactions of potential determining Ions In determining the electrical properties and thermodynamics of the oxide/solution interfaces. A by-product of our study Is that under appropriate conditions, an amphoteric surface can show marked deviations from ideal Nernstlan behaviour. The graphical method also serves to Illustrate the... 

Returning to our introductory remarks about the existence of various models for the oxide/solution interface, It may be appropriate to point out that the results of very relevant experiments based on electrokinetic measurements are often not used in conjunction with titration data. Granted that there may be additional difficulties in identifying the precise location the slipping plane and hence the significance of the electrokinetic c potential may be open to debate, both titration and electrokinetic data ought to be combined where possible to elucidate the behaviour of the oxide/solution Interface. 

Figure 2.5 Schematic representation of the Au/MPS/PAH-Os/solution interface modeled in Refs. [118-120] using the molecular theory for modified polyelectrolyte electrodes described in Section 2.5. The red arrows indicate the chemical equilibria considered by the theory. The redox polymer, PAH-Os (see Figure 2.4), is divided into the poly(allyl-amine) backbone (depicted as blue and light blue solid lines) and the pyridine-bipyridine osmium complexes. Each osmium complex is in redox equilibrium with the gold substrate and, dependingon its potential, can be in an oxidized Os(lll) (red spheres) or in a reduced Os(ll) (blue sphere) state. The allyl-amine units can be in a positively charged protonated state (plus signs on the polymer...

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. 

Hayes, KF. Papelis, C. Leckie, J.O. (1988) Modeling ionic strength Effects on anion adsorption at hydrous oxide/solution interfaces. J. Colloid Interface Sd. 78 717—726 Hayes, KF. Roe, A.L. Brown, G.E. Hodgson, KO. Leckie, J.O. Parks, G.A. (1987) In-situ X-ray absorption study of surface complexes Selenium oxyanions on a-FeOOH. Sdence 238 783-786... 

A more complicated model situation is demanded if one thinks of the equivalent circuit for an electrode covered with an oxide film. One might think of A1 and the protective oxide film that grows upon it during anodic polarization. One has to allow for the resistance of the solution, as before. Then there is an equivalent circuit element to model the metal oxide/solution interface, a capacitance and interfacial resistance in parallel. The electrons that enter the oxide by passing across the interfacial region can be shown to go to certain surface states (Section 6.10.1.8) on the oxide surface, and they must be represented. Finally, on the way to the underlying metal, the electron... 

Hayes, K.F. and Leckie, J.O. (1987) Modelling ionic strength effects of cation adsorption at hydrous oxide/solution interfaces./. Colloid Interface Sci., 115, 564-572. 

Complexation Modelling of Ion Adsorption at the Oxide/Solution Interface (Technical Report 306). Deptartment of Civil Engineering, Stanford University, CA. 

Papelis C., Hayes K. F., and Leckie J. O. (1988) HYDRAQL A Program for the Computation of Chemical Equilibrium Composition of Aqueous Batch Systems Including Surface-complexation Modeling of Ion Adsorption at the Solution Oxide/Solution Interface. 306. Environmental Engineering and Science, Department of Civil Engineering, Stanford University, Stanford, CA, 131pp. 

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. 

To describe in detail such a specific system as the metal oxide/solution interface, it is necessary to prepare a model describing dependences between potential and surface charge and draw up reactions, the occurrence of which leads to the changes of surface charge o-The reaction equations describing an equilibrium state between the surface and solution as well as values of equilibrium constants of these reactions provide detailed information... 

Constant Capacitance Model The constant capacitance model of the oxide-solution interface (Schindler et al., 1976 Stumm et al., 1980) contains the following assumptions ... 

Papelis, C., K.F. Hayes, and J.O. Leckie. 1988. HYDRAQL A program for the computation of chemical equilibrium composition of aqueous batch systems including surface-complexation modeling of ion adsorption at the oxide/solution interface. Department of Civil Engineering Technical Report no. 306, Stanford Univ. Menlo Park, CA. 

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. 

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