Diprotic surface groups model

The monoprotic surface group model can be described in terms of the more familiar diprotic surface group model. In the diprotic model, the surface is thought of as an ensemble of Ng diprotic surface groups, which, under the condition of zero protonic charge, are occupied by Ns protons. 

Two models of surface hydrolysis reactions and four models of the electrical double layer have been discussed. In this section two examples will be discussed the diprotic surface group model with constant capacitance electric double layer model and the monoprotic surface group model with a Stern double layer model. More details on the derivation of equations used in this section are found elsewhere (3JL). ... 

If the constraint that is small is removed in interpretation of the data, one may consider the physical nature of the interface and other forms of experimental data in deciding what combinations of parameters are appropriate for describing the interface. In particular, the relationship of the diprotic surface group model to the monoprotic surface group model can be examined. 

Surface complexation models for the oxide-electrolyte interface are reviewed two models for surface hydrolysis reactions are considered (diprotic surface groups and monoprotic surface groups) and four models for the electric double layer (Helmholtz,... 

Gouy-Chapman, Stern, and triple layer). Methods which have been used for determining thermodynamic constants from experimental data for surface hydrolysis reactions are examined critically. One method of linear extrapolation of the logarithm of the activity quotient to zero surface charge is shown to bias the values which are obtained for the intrinsic acidity constants of the diprotic surface groups. The advantages of a simple model based on monoprotic surface groups and a Stern model of the electric double layer are discussed. The model is physically plausible, and mathematically consistent with adsorption and surface potential data. 

Diprotic Surface Groups. According to the diprotic model, the surface is represented as an ensemble of identical diprotic weak acid groups, which react according to ... 

Diprotic Surface Groups. Most of the recent research on surface hydrolysis reactions has been interpreted in terms of the diprotic surface hydrolysis model with either the triple layer model or the constant capacitance model of the electric double layer. The example presented here is cast in terms of the constant capacitance model, but the conclusions which are drawn apply for the triple layer model as well. 

In order to maintain the complexity of the model at a level consistent with the resolution of the experimental data, the reactivity of these surface groups has been described by relatively simple models (i) as diprotic weak acids, and (ii) as monoprotic... 

The representation of surface groups as diprotic weak acids is appealing because it includes a modest degree of complexity (two acidity constants), allows convenient representation of the condition of zero surface excess of hydrogen ion, and is still quite manageable mathematically. However, it must be borne in mind that this model is still a grossly simplified representation of the actual surface. It remains to be shown that this simplification is significantly better than any other simplification. 

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