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Inner Layer Capacitor at the Solid/Liquid Interface

Interpretation of the interfacial equilibrium at the solid/liquid interface based on the Surface Complexation Model requires introduction of the concept of the constant capacitance capacitor in the inner part of the electrical interfacial layer (EIL). There are two possible variations of this concept. The common approach is to define capacitance by the surface charge density in the inner part of the layer (s0). This concept is related to the simplified scheme of the serially connected capacitors within EIL. Another possibility was examined in this article. If one assumes that capacitors are connected parallel, the surface charge density of the plane in which associated counterions are located (sb) should be introduced into the definition of capacitance. The problem was analyzed on the basis of experimental findings as well as by numerical simulation. As assumed, it was found that both concepts provide almost the same result if the degree of counterion association is high. In the case of a low degree of counterion association, at a low electrolyte concentration, no pronounced discrepancy in surface charge density and surface potential was obtained by using these two different concepts. This unexpected result could be explained by considering the mutual dependency of the parameters determining the equilibrium in EIL. It was concluded that it is very difficult to distinguish between these two concepts experimentally, so both of them are still equally applicable.

surface charge; electrical interfacial layer; capacitor; capacitance

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