Polarization Catastrophe and Other Artifacts

Although the study of the transition region poses no difficulties, it was for two decades a source of artifacts, big misunderstandings, and 

The first indication that something went wrong with the molecular modeling of charged interfaces was detected by Cooper and Harrison in 1975 and it was the prediction of infinite or negative capacities by all molecular models. This unrealistic situation was cailedpolarization 

In order to clarify this point, let us consider the plots of 0 at 

The dependence of the interfacial tension y upon A ) at constant lnPA c can be calculated by means of the adsorption isotherm, Eq. (23), 

Therefore, at each pair (y, A(j ) at a certain In PaX value we can calculate the quantity y + and plot it as a function of A( ). Note that the y values of this plot contain an arbitrary constant, which nevertheless does not affect the results. In the transition region this plot exhibits two characteristic loops, which can be easily eliminated. The maximum in this plot, after the elimination of the transition loops, gives the value of Parson s function = y + ct A( ) at a given Therefore, the surface coverage 6 at a certain value of is obtained by numerical differentiation of the values with respect to In PaX. These values of 

---

We should point out that if we use the potential drop A( ) (or E) to express the properties of the adsorbed layer, the above peculiarity disappears and we observe the expected behavior, which is the decrease in the adsorption of a neutral solute in the presence of specifically adsorbed anions. Thus, the experimental and theoretical study of the effect of the specific adsorption on the adsorption features of an organic compound gives the first indication that the two electrical variables, and A(j), are not equivalent in expressing the properties of the adsorbed layer and that the use of as the independent electrical variable should be handled with care. This issue of the equivalence of the two electrical variables is also discussed from another point of view in the section Polarization Catastrophe and Other Artifacts below. 

It is seen that the polarization catastrophe is in fact an innocent artifact, because it is now easily detected and amended. In contrast, there are other artifacts appearing in the adsorption isotherms when is used as the independent electrical variable which are difficult to be identified. Note especially the artifact curve (2) in Figure 18, which does not exhibit any sign of phase transition. However, even these artifacts can be amended either by the use of the generalized ensemble A or the procedure suggested above. At any rate we observe once more that the electrode charge density when it is used as an independent electrical variable, is a difficult variable, which should be handled with much care. 

---