Capacitance calculations with solvent contributions

Another example of specific ion adsorption was discussed in terms of the formation of interfacial ion pairs between ions in the aqueous and the organic phase. The contribution of specific ionic adsorption to the interfacial capacitance can be calculated using the Bjerrum theory of ion-pair formation. The results show that a phase boundary between two immiscible electrolyte solutions can be described as a mixed solvent region with varying penetration of ion pairs into it, depending on their ionic size. The capacitance increases with increasing ionic size in the order Ii+ < Na+ < K" " < Rb < Cs" ". Yufei et al. [22] found that significant specific ion adsorption occurs at the interface between two immiscible electrolytes... 

With ex = e2 = 1, capacitances were very small compared to experimental ones, so that Cs(dip) would have to be much smaller than generally accepted values to get agreement with experiment. With a dielectric present but no 6-function barrier, more reasonable results were obtained. The addition of the barrier changed 6 M, but had little effect on capacitances. Of course, these calculations are of interest only in comparing the contributions of two metals, or in investigating the importance of modifications of the model, since there is no solvent. 

The observed capacitance, C, is a series combination of the inner layer capacity, C and the diffuse layer capacity. Equation (5.36) predicts an inverted parabolic dependence between and 02- Furthermore, the capacity at the minimum decreases with decreasing concentration and is centred symmetrically about the potential of zero charge for a z z valent electrolyte. Consequently, the diffuse layer contribution only becomes apparent in dilute solutions, as is shown for NaF solutions in Fig. 5.5. As the inner layer contains only solvent molecules, its capacity is independent of NaF concentration so that once C is known as a function of potential then the capacity curves at all other concentrations can be calculated. This has been confirmed experimentally. Similar equations have been... 

---