Nonspecifically adsorbed ions

The electrified interface is generally referred to as the electric double layer (EDL). This name originates from the simple parallel plate capacitor model of the interface attributed to Helmholtz.1,9 In this model, the charge on the surface of the electrode is balanced by a plane of charge (in the form of nonspecifically adsorbed ions) equal in magnitude, but opposite in sign, in the solution. These ions have only a coulombic interaction with the electrode surface, and the plane they form is called the outer Helmholtz plane (OHP). Helmholtz s model assumes a linear variation of potential from the electrode to the OHP. The bulk solution begins immediately beyond the OHP and is constant in potential (see Fig. 1). 

Figure 1. Schematic picture of the metal/solution interphase in the case of nonspecific (a) and specific (b) anionic adsorption, x = 0, x = P and x = d are die electrode surface plane, the plane of closest approach for the specifically adsorbed anions, and that for the nonspecifically adsorbed ions. Curve 1 represents the potential-distance profile. In (b), curve 1 results from the combination of curve 2, expressing die contribution from the charge density as of the specifically adsorbed anions, and curve 3, expressing die contribution from die charge density Om on the metal. The potential difference, ft1 — d> across die inner layer is the same in (a) and (b). (Reprinted from Ref.7 with permission from the Am. Chem. Soc.)...

Distinction between Specific and Nonspecific Adsorbed Ions... 

The potential difference across the electrode/solution interface is dropped by the accumulation of ions of opposite charge in the solution immediately adjacent to the electrode surface in the electrochemical double layer. The spatial distribution of ions gives a potential profile across the double layer into the solution over a distance that is dependent upon the electrolyte concentration. Given this position-dependent potential profile, it is possible that species undergoing electrochemical reaction, which are assumed to reside in the outer Helmholtz plane of the electrical double layer adjacent to the substrate electrode (otherwise known as the plane of closest approach of nonspecifically adsorbed ions), may not actually be at ([is and hence would not experience the full electrical field corresponding to the electrode/solution potential difference. The result of this is that only a part of the measurable applied r] affects the Gibbs energy of activation of the process. The potential at the OHP with respect to solution, (t)s, is denoted t /i and is known as the potential of the (inner limit... 

The outer Helmholtz plane (OHP) refers to the distance of closest approach of nonspecifically adsorbed ions, generally cations. The interactions of the ions of the OHP with the surface are not specific and have the character of longer range coulombic interactions. Cations that populate the outer Helmholtz plane are usually solvated and are generally larger in size than the anions. 

Nonspecifically adsorbed ions are those ions which retain their primary solvation shells and which are concentrated adjacent to the electrode surface due to electrostatic forces only. However, because of thermal motion, the nonspecifically adsorbed ions are actually distributed in a layer extending some distance from the electrode surface. This layer is called the diffuse layer, and... 

Recapitulation of fundamental notions In the case of nonspecific adsorption it is assumed that ions retain their solvation shell, and in the position of closest approach to the interface they are separated from it by one or more solvent layers. The locus of the electrical centers of nonspecifically adsorbed ions in their position of closest approach is the outer Helmholtz plane (OHP). 

The position of this plane for the anion and the cation of the electrolyte may differ. It may also differ for the different cations and the different anions of solution. The region in which nonspecifically adsorbed ions are accumulated and distributed by the contrasting action of the electric field and thermal motion is called the diffuse layer. The region between the OHP and the interface is called the inner (compact) layer. 

The chemical composition of the emersed double layer was tested by ESCA. It was found for nonspecific adsorbing ions that the ex situ obtained data are in very good agreement with predictions from classical double-layer theory. 

Apart from the molecular-level information on the adsorbed species, such as chemical identity and orientation, and types of adsorbate-adsorbent and adsorbate-adsorbate interactions, which can reveal the origin of the electrode process, IR spectra of nonspecifically adsorbed ions present in the DL can be used for estimating the potential of zero charge (PZC) [642]. In particular, the absorption bands (especially, V5) of the NO anion in the DL will be broadened due to ion paring of nitrate anion with hydronium ions. Hence, they exhibit minimum FWHM at the PZC if the specific absorption of the ions is negligible. 

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