Helmholz double layer

Fig. 20.2 Helmholz double layer (H.D.L.) consisting of a plate of excess negative charges on the surface of the metal and a counterbalancing plate of excess positive charges (cations) in solution, the double layer as a whole being electrically neutral. The double layer can be regarded as equivalent to a capacitor in which the plates are separated by a distance ...

Two alternative structures for the simple Helmholz double layer (Fig. 20.11) have been provided ... 

Figure 1.5 shows a schematic representation of the double layer at a planar solid-liquid interface. The potential drop across the Helmholz layer is shown as linear (in the presence of specific adsorption, it will not be completely linear), followed by a tailing-off of the potential into the diffuse layer. For concentrated solutions (>0.1 M) the diffuse layer is typically a nanometer or less, while for dilute solutions it may be tens or even hundreds of nanometers. 

The only models that exist for double layer structure in ionic liquids suggest a Helmholz layer at the electrode/solution interface [103, 104], If the reduction potential is below the point of zero charge (pzc) then this would result in a layer of cations approximately 5 A thick across which most of the potential would be dropped, making it difficult to reduce an anionic metal complex. Hence, the double layer models must be incorrect. 

In semiconductor electrodes, we have a space charge layer in addition to an electrical compact double layer (Helmholz layer) at the electrode interface. The electrode potential, then, is the sum of the space charge potential, Ac[>sc, and the interfacial potential, AH ... 

The reason for this state of affairs may be seen in past emphasis on surface phenomenological studies which attempted to model the metal surface as an array of surface atoms with some valences saturated by subsurface metal atoms and other valences saturated by ions or molecules making up the environment. This model led to the description of the interface in terms of the Helmholz and Guy-Chapman double layer theories, and inhibitors were visualized as interfering with the double layer structure through adsorption on the surface atoms of the metal, thereby altering the electrochemical reaction rates which are governed by the energetics of the double layer. While this model has been... 

Under the formation of double layer in interface Pt - water solution of NaCl a model was proposed that for positive charged surfaces an ionic parts of double layer are construeted of halohenide with effective radius 32> A. Then effective thickness of the Helmholz layer d is equal to ion radii and the potential drops within this thickness. 

Another difference between an electrochemical reaction and a catalytic reaction is that a so-called electrical double layer will form as the appearance of electrostatic potential gradient in the interface of electrolyte solution and electrode (conductor). Graham summarized in more detail the electrical double layer in 1947. He considered that this electrostatic potential, i.e. the double layer potential, is different from the electrode potential. He also discussed and observed in detail the double layer potential of Hg-electrode-water solution system. He found that it could not observe such potential when electrode reaction occurred while the ideal polarization happened in a wide range of electrode potential if there was no electrode reaction. Hg is a liquid and it is thus easy to observe its surface tension and calculate the relationship between surface tension and double layer potential. Therefore, its structure is clearer. The structure of electrical double layer is composed of Helmholz layer and diffusion layer. The Helmohloz face is located between Helmholz layer and diffusion layer. The external of Helmohloz face is diffusion double layer. The model of Helmholz electrical double layer corresponds to simple parallel-plate capacitor. According to its equation, it can quantitatively describe the structure of diffusion double layer. 

As shown in Table 6.1 (a), where the supporting electrolyte anion and solvent are constant, a decrease in the capacitance was observed with increasing number of alkyl carbons, following the trend mentioned above. In Table 6.1 (b), the capacitance values decreased with increasing solvent volume. These results clearly indicate that the electrical double-layer capacitance values for diamond electrodes decrease, whereas the thickness of the Helmholz layer increases, with increasing volume of the solvated cations. 

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