Diffuse double layer ionic surface excesses

An adsorbed layer of water molecules at the interface separates hydrated ions from the solid surface. The interfacial electric double layer can be represented by a condenser model comprising three distinct layers a diffuse charge layer in the ionic solution, a compact layer of adsorbed water molecules, and a diffuse charge layer in the solid as shown in Fig. 5-8. The interfacial excess charge on the... 

In Eq. 30, Uioo and Fi are the activity in solution and the surface excess of the zth component, respectively. The activity is related to the concentration in solution Cioo and the activity coefficient / by Uioo =fCioo. The activity coefficient is a function of the solution ionic strength I [39]. The surface excess Fi includes the adsorption Fi in the Stern layer and the contribution, f lCiix) - Cioo] dx, from the diffuse part of the electrical double layer. The Boltzmann distribution gives Ci(x) = Cioo exp - Zj0(x), where z, is the ion valence and 0(x) is the dimensionless potential (measured from the Stern layer) obtained by dividing the actual potential, fix), by the thermal potential, k Tje = 25.7 mV at 25 °C). Similarly, the ionic activity in solution and at the Stern layer is inter-related as Uioo = af exp(z0s)> where tps is the scaled surface potential. Given that the sum of /jz, is equal to zero due to the electrical... 

In general, the phenomenological definition of specific adsorption depends on the model of the diffuse part of the double layer. However, at the potential of zero charge, the contribution of the nonspecific adsorption in the diffuse part of the double layer should be absent. According to the phenomenological definition of specific adsorption [53], at the point of zero charge it is said that there is specific adsorption if the measured surface excess of any ionic species is positive. 

In summary, since the surface excesses are obtained by integrating over the whole diffuse layer, the GC estimates are assumed to give reasonable values for these quantities. The estimates are often used in the analysis of double layer data involving ionic adsorption in the inner layer. 

Certain model assumptions are necessary in order to reveal the surface concentration of specifically adsorbed ions in the total surface excess F,-. Usually, the ionic component of the electrical double layer (EDL) is assumed to consist of the dense part and the diffuse layer separated by the so-called outer Helmholtz plane. Only specifically adsorbing ions can penetrate into the dense layer close to the surface (e.g. iodide ions), with their electric centers located on the inner Helmholtz plane. The charge density of these specifically adsorbed ions ai is determined by their surface concentration F Namely, for single-charged anions ... 

In short, ellipsometry applied to adsorption layers of ionic soluble surfactants does not measure the surface excess. The ellipsometric signal may show a non-monotonic behaviour which is caused by a redistribution of the ions between compact and diffuse layer. The data analysis within the classical model of a charged double layer yields an estimate of the prevailing ion distribution. 

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