Stern-Graham model

Figure 7.4. Schematic model of the Electrical Double Layer (EDL) at the metal oxide-aqueous solution interface showing elements of the Gouy-Chapman-Stern-Grahame model, including specifically adsorbed cations and non-specifically adsorbed solvated anions. The zero-plane is defined by the location of surface sites, which may be protonated or deprotonated. The inner Helmholtz plane, or [i-planc, is defined by the centers of specifically adsorbed anions and cations. The outer Helmholtz plane, d-plane, or Stern plane corresponds to the beginning of the diffuse layer of counter-ions and co-ions. Cation size has been exaggerated. Estimates of the dielectric constant of water, e, are indicated for the first and second water layers nearest the interface and for bulk water (modified after [6]).

Fuerstenau") was the first who used the Stern-Grahame model of EDL to describe the adsorption of long-chain surfactants for the equilibrium in heterogeneous systems. The adsorption density in the Stem plane is given by the equation... 

Fig. 10.14 Schematic diagram of the double layer according to the Gouy-Chapman-Stern-Grahame model. The metal electrode has a net negative charge and solvated monatomic cations define the inner boundary of the diffuse layer at the outer Helmholtz plane (oHp).

Figure 12. Diagram of inner region of the double layer showing outer Helmholtz (OHP) plane with oriented solvent dipoles interacting with electrostatically adsorbed solvated ions [schematic based on Stern-Grahame model (Ref. 95) BDM model (Ref. 60) includes an extra layer of solvent dipoles between the metal surface and OHP of cations].

In accordance with the Stern-Grahame model of the EDL structure the values of C are determined by both the diffuse and compact-layer properties, the latter being dependent on the metal properties. However, in very dilute solutions of a surface-inactive electrolyte the dominant contribution to C near the p.z.c. (at the capacitance minimum) is given by the diffuse layer, C = Cgc(0, c). Therefore the ratio of capacitances in these conditions should be close to the RF for the surface of the solid metal M ... 

FIGURE 11.1 Schematic representation (based on Figure 3.13, Gouy-Chapman-Stern-Grahame model) of the approach of an ion (right) to a charged surface (left), where an electric potential gradient is present. 

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