Modeling of the Oxide-Solution Interface

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

To the previous phenomenological considerations, it is necessary to add a quantitative approach that could link the characteristics of the medium with those of the surface. More specifically, the acid-base characteristics of the surface and the charge variation as a function of pH must be characterized. Unfortunately, very few surface-specific experimental quantities are accessible to experiment, and the structural details of the surface are usually unknown or impossible to determine. Building a model of the oxide-solution interface is therefore required in order to understand the behavior of particles in suspensions. 

In the absence of a good technique able to characterize reaction sites, surface acidity is considered to be an average value. The constants and (or and K ) related to amphoteric sites are often treated as adjustable parameters in models of the oxide-solution interface. A model of multiple site complexation has recently allowed a better description of the surface of oxide colloids, as well as an interpretation of their behavior. 

Modeling of the Oxide-Solution Interface 001 face (AI2OH)... 

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. 

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. 

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. 

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.

Several theories have been postulated recently for the structure of the oxide-solution interface.81 These approaches may be conveniently classified into two general categories—classical and non-classical models. 

While it appears that no totally satisfactory model for the oxide-solution interface has emerged to date, progress is obviously being made in this area. Dispersion, hydration, hydroxylation, and acid-base properties (and, in particular, the variation of the latter with change in the oxidation state of the central metal ion) are all factors which must be combined before a model of general validity is obtained. 

---