Diffuse layer, Gouy-Chapman

Fig. 20.8 Gouy-Chapman diffuse layer model of the double layer...

The physical meaning of the g (ion) potential depends on the accepted model of an ionic double layer. The proposed models correspond to the Gouy-Chapman diffuse layer, with or without allowance for the Stem modification and/or the penetration of small counter-ions above the plane of the ionic heads of the adsorbed large ions. " The experimental data obtained for the adsorption of dodecyl trimethylammonium bromide and sodium dodecyl sulfate strongly support the Haydon and Taylor mode According to this model, there is a considerable space between the ionic heads and the surface boundary between, for instance, water and heptane. The presence in this space of small inorganic ions forms an additional diffuse layer that partly compensates for the diffuse layer potential between the ionic heads and the bulk solution. Thus, the Eq. (31) may be considered as a linear combination of two linear functions, one of which [A% - g (dip)] crosses the zero point of the coordinates (A% and 1/A are equal to zero), and the other has an intercept on the potential axis. This, of course, implies that the orientation of the apparent dipole moments of the long-chain ions is independent of A. 

The physical meaning of the g" (ion) potential depends on the accepted model of ionic double layer. The proposed models correspond to the Gouy Chapman diffuse layer, with or without allowance for the Stern modification and/or the penetration of small counterions above the plane of the ionic heads of the adsorbed large ions [17,18]. The presence of adsorbed Langmuir monolayers may induce very high changes of the surface potential of water. For example. A/" shifts attaining ca. —0.9 (hexadecylamine hydrochloride), and ca. -bl.OV (perfluorodecanoic acid) have been observed [68]. 

The Gouy Chapman diffuse layer model has been shown to describe adequately the electrostatic potential produced by charges at the surface of the membrane [137]. For a symmetrical background electrolyte, a and i// are related by ... 

Figure 2. The charge-potential curves for a Gouy-Chapman diffuse layer and an amphoteric surface 10 1 1 electrolyte, ApK = 2, N = 1 X 1018 nf2.

Figure 4. The charge-potential curves for overlapping Gouy-Chapman diffuse layers as a function of separation.

The earliest models used to describe the distribution of charges in the edl are, besides the Helmholz model, the Gouy-Chapman diffuse layer model and the Stem-Graham model. Details of these models are given in Westall and Hohl (1980), Schindler (1981, 1984) and Schindler and Stumm (1987). 

Inner Helmholtz Layer Outer Helmholtz Gouy-Chapman (diffuse) Layer... 

In the Gouy-Chapman diffuse layer the concentration-distance profile Is given by the Boltzmann distribution ... 

As we have seen, the electric state of a surface depends on the spatial distribution of free (electronic or ionic) charges in its neighborhood. The distribution is usually idealized as an electric double layer, one layer is envisaged as a fixed charge or surface charge attached to the particle or solid surface while the other is distributed more or less diffusively in the liquid in contact (Gouy-Chapman diffuse layer model, Figure 9.19). A balance between electrostatic and thermal forces is attained. 

Substantial efforts have been made to develop physicochemical models for ion exchange based on the Gouy-Chapman diffuse-layer theory (e.g., 9, 10). This work not only has provided insight into the role of diffuse-layer sorption in the ion-exchange process but also has pointed to the need to consider other factors, especially specific sorption at the surface. Consideration of specific sorption enables description of the different tendencies of ions to... 

Gouy and Chapman later suggested that the thermal energy of the ions would result in a diffuse layer on the solution side of the interface with the concentration of excess charges at a maximum close to the electrode surface and gradually decreasing with distance into the electrolyte (on the order of nanometers). However, the Gouy-Chapman diffuse layer model also does not match well with aU experimental data. 

It follows from the Gouy-Chapman diffuse-layer theory that if =... 

It is believed that the dou ble layer is composed of a rigid layer of adsorbed ions with a thickness equal to ionic diameters (the Stern or Helmoltz rigid layer) followed by a diffuse layer (Gouy-Chapman diffuse layer) extending up to several hundred microns into the solution. Below we describe the surface potentials of this double layer. 

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