Grahame triple-layer model

Figure 4.11. Triple-layer model (Grahame) IHP, inner Helmholtz plane OHP, outer Helmholtz plane (, water dipole +, positive end of the dipole).

Thus, according to these theories, all univalent (1 1) electrolytes should behave the same way. However, this is not what was observed experimentally. Solutions of different 1 1 electrolytes (e.g., NaCl, NaBr, Nal, KI) show species-specific behavior. In order to interpret this specific behavior, Grahame (5) proposed a new model of the interphase the triple-layer model. The basic idea in the interpretation of the ion-specific behavior is that anions, when attracted into the interphase, may become dehydrated and thus get closer to the electrode. Each anion undergoes this to a different extent. This difference in the degree of dehydration and the difference in the size of ions results in the specific behavior of the anions. Ions that are partially or fully dehydrated are in contact with the electrode. This contact adsorption of ions allows short-range forces (e.g., electric image forces) to act between the metal elec-... 

The Triple-Layer Model (TLM) This model, first developed by Yates et al. (1974) and Davis (1978), is essentially an expanded Stem-Grahame model the specifically adsorbed ions are placed as partially solvated ions at a plane of closest approach. Additional capacitances are introduced. Subsequently, Hayes (1987) had to modify the earlier TLM by moving specifically adsorbed ions to the mean plane of the surface. 

Various types of SCM have been assessed namely, the diffuse-layer model (DLM) [27], the constant-capacitance model [28], the Stern model [29], and the triple-layer model (TLM) [30]. They differ in complexity from the simplest, DLM, which has four adjustable parameters, to the most complex, TLM, which includes seven adjustable parameters. The number of parameters is dependent on the hypothesis relative to the model. In various researches, the DLM is selected because of its simplicity and its applicability to various solution conditions [31]. It takes into account ionic strength effects on protolysis equilibria through the Gouy-Chapman-Stern-Grahame charge-potential relationship ... 

R. O. James and G. A. Parks, Characterization of aqueous colloids by their electrical double-layer and intrinsic surface chemical properties. Surface and Colloid Science 12 119 (1982). Perhaps the most complete review of the triple layer model from the perspective of Gouy-Chapman-Stem-Graham e double layer theory. 

A fourth model proposed, the Grahame model (Grahame, 1951), which is referred to as the triple-layer model, takes into consideration that ions could be dehydrated in the direction of the electrode and specifically adsorbed on the electrode. Thus, an inner layer between the electrode surface and the Helmholtz layer further modifies the structure of the double layer. The locus of electrical centers of unhydrated ions strongly attached to the electrode is called inner Helmholtz plane (IHP). Figure 5.5... 

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