Electrostatic adsorption frequency potential

An attempt to describe the response of the EQCM in the double-layer region only on the basis of the properties of the diffuse double layer was undertaken in Ref. 61. In doing so, the specific adsorption of ions and the potential-dependent specific interactions of the solvent with the metal were entirely ignored. Under these assumptions one could think of three reasons leading to the observed dependence of frequency on potential (1) dependence of the surface tension on potential, (2) electrostatic adsorption of charged species, and (3) a local change in viscosity in the diffuse double layer. 

For the film thickness, as a first approximation, one can take that Lf = K. Another simplifying assumption is that the viscosity changes abruptly at the boundary between the film and the solution. Estimation of the viscosity of the film as a function of potential is very difficult, since electro-neutrality is not maintained in the diffuse double layer, and it is difficult to take into account the influence of the electric field in the double layer on the viscosity of the film. Instead, the viscosity of the film, tjf, can be taken as a parameter, to fit the theoretical curve to the experimental results. To do this one substracts from the observed frequency shift the contribution of the mass effect caused by electrostatic adsorption of ions [Eq. (56)]. 

Once the mass of replaced water molecules has been taken into account (or it was assumed a priori that this was negligible), the response of the EQCM can be used to compare surface excess Fa, obtained from the frequency shift with that derived from other techniques. However, the situation is complicated by the fact that the absolute value of the frequency shift caused by adsorption of organic substances is usually small, similar to that caused by electrostatic adsorption of ions in the diffuse double layer over the same range of potential, as seen, for example, in Fig. 9. Thus, the effect of the diffuse double layer must be considered as a background, and a suitable correction should be made. 

One complication which may be present, when the Helmholtz model is in other respects appropriate, is that of specific adsorption. If one of the mobile species is to some extent chemically bound rather than being simply electrostatically bound to the metal electrode, Cji may show a dependence on the dc bias potential. Indeed this is the normal method of inferring specific adsorption. Another possibility in this case is that dl exhibits different high frequency and low frequency limits because at high frequencies the specific adsorption being an activated process is too slow to follow changes in interface potential. A further complication which is often present in real systems is the presence of an oxide layer on the surface of the metal electrode. Such an oxide layer can generate a potential... 

Oscillations may exert a strong effect on adsorption processes in the frictional contact. Adsorption of particles on the electrode with a certain potential is known [23] to occur at a finite speed. Under low oscillation frequencies the adsorption manages to follow the potential and participate in the variation of the interfacial layer structure. At high frequencies the adsorption mechanism does not work, giving place to electrostatic charging of the layer as a condenser, i.e. the generation of the double electric layer (DEL). A mechanical model of the interfacial DEL has been elaborated by Shepenkov [24]. It follows from the model that, if a periodic mechanical force acts on the double layer from the side of the liquid or electrode, the electrode potential will vary periodically with the same excitation frequency. 

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