Stern inner region

Finally we shall argue that present-day theories of the nonprimitive models of the electric double layer have considerable difficulty in treating properly ion adsorption in the Stern inner region at metal-aqueous electrolyte interfaces and we suggest that this region is a useful concept which should not be dismissed as unphysical. Indeed Stern-like inner region models continue to be used in colloid and electrochemical science, for example in theories of electrokinetics and aqueous-non-metallic (e.g., oxide) interfaces. 

From their calculations of the surface excess entropy and volume of the electric double layer at a mercury-aqueous electrolyte interface, Hill and Payne (HP) [147] postulated an increase in the number of water molecules in the Stern inner region as the surface charge a of about 30 piC/m2, which is consistent with the results of TC on a silver surface obtained some 30 years later. HP used an indirect method to determine the excess entropy and volume by measuring the dependence on temperature and pressure of the double layer capacitance at the mercury-solution interface. 

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].

A charged particle in solution provokes an increased eoneentration of counter ions (ions of opposite eharge to that of the partiele) close to its surface, generating an electrical double layer around this partiele. The liquid layer surrounding the particle exists as two parts an inner region (the Stern... 

The liquid layer surrounding the particle exists as two parts (i) an inner region (Stern layer) where the ions are strongly bound and (ii) an outer (diffuse) region where they are less firmly associated. Within the diffuse layer there is a notional boundary inside which the ions and particles form a stable entity. When a particle moves (e.g., due to gravity), ions within the boundary move with it The ions beyond the boundary do not travel with the particle. The potential at this boundary (surface of hydrodynamic shear) is the zeta potential (Fig. 8.5). 

Most of the counterions form a diffuse concentration profile away from the particle surface. There are two models that describe this diffusive layer. In the diffusive double-layer model, the concentration of this diffuse region of counterions decreases gradually away from the surface (Figure 13.2a) (Hamley 2000). In the Stern model, the interface between the inner region of the counterion environment is a sharp plane (Stern plane) and the inner region consists of a single layer of counterions called the Stern layer (Figure 13.2b) (Hamley 2000). 

One other aspect of nonprimitive electric double layer theories which is particularly relevant to the inner Stern region are the models for the water molecule and the ions. The simplest models for a water molecule and an ion are a hard-sphere point dipole and point charge, respectively. A more realistic model of the hard-sphere water molecule would include quadrupoles and octupoles and also polarizability. However the hard-sphere property is best avoided and replaced, for example, by a Lennard-Jones potential. An alternative to a multipolar water model are three point charge sites associated with the atoms within the water molecule. 

Parsons and Zobel plot — In several theories for the electric - double layer in the absence of specific adsorption, the interfacial -> capacity C per unit area can formally be decomposed into two capacities in series, one of which is the Gouy-Chapman (- Gouy, - Chapman) capacity CGC 1/C = 1 /CH + 1 /CGC. The capacity Ch is assumed to be independent of the electrolyte concentrations, and has been called the inner-layer, the - Helmholtz, or Stern layer capacity by various authors. In the early work by Stern, Ch was attributed to an inner solvent layer on the electrode surface, into which the ions cannot penetrate more recent theories account for an extended boundary region. In a Parsons and Zobel plot, Ch is determined by plotting experimental values for 1/C vs. 1/Cgc- Specific adsorption results in significant deviations from a straight line, which invalidates this procedure. 

Conversely, according to the description of the electrical double layer based on the Stern-Gouy-Chapman (S-G-C) version of the theory [24], counter ions cannot get closer to the surface than a certain distance (plane of closest approach of counter ions). Chemically adsorbed ions are located at the inner Helmholtz plane (IHP), while non-chemically adsorbed ions are located in the outer Helmholtz plane (OHP) at a distance x from the surface. The potential difference between this plane and the bulk solution is 1 ohp- In this version of the theory, Pqhp replaces P in all equations. Two regions are discernible in the double layer the compact area between the charged surface and the OHP in which the potential decays linearly and the diffuse layer in which the potential decay is almost exponential due to screening effects. 

Also the choice of the electrostatic model for the interpretation of primary surface charging plays a key role in the modeling of specific adsorption. It is generally believed that the specific adsorption occurs at the distance from the surface shorter than the closest approach of the ions of inert electrolyte. In this respect only the electric potential in the inner part of the interfacial region is used in the modeling of specific adsorption. The surface potential can be estimated from Nernst equation, but this approach was seldom used In studies of specific adsorption. Diffuse layer model offers one well defined electrostatic position for specific adsorption, namely the surface potential calculated in this model can be used as the potential experienced by specifically adsorbed ions. The Stern model and TLM offer two different electrostatic positions each, namely, the specific adsorption of ions can be assumed to occur at the surface or in the -plane. 

Stern [4] introduced the concept of the nondiffuse part of the double layer for specifically adsorbed ions, the remainder being diffuse in nature this is shown schematically in Figure 7.4, where the potential is seen to drop linearly in the Stern region, and then exponentially. Grahame distinguished two types of ions in the Stern plane, namely physically adsorbed counterions (outer Helmholtz plane) and chemically adsorbed ions that lose part of their hydration shell (inner Helmholtz plane). 

Core level spectra indicate that Al atoms strongly interact with conjugated compounds, affecting the n-clcctron sterns. This behavior is confirmed by the evolution of the UPS spectra (Fig. 3). The n electronic states which are most relevant in determining the electronic properties of these materials arc located in the 4-9 cV region (relative to the vacuum level), while the inner part of the spectrum mostly contains the o dcctronic stales. In the ease of DPT, the two UPS... 

After 20 years. Stern [23] modified these models by including both a compact and a diffuse layer. At the same time, Grahame [24] divided the Stern layer into two regions (i) an inner Helmholtz plane consisting of a layer of adsorbed ions at the surface of the electrode and (ii) an outer Helmholtz plane (referred to as Gouy plane as well), which is formed by the closest approach of diffuse ions to the electrode surface. From the Grahame model, the capacitance C of the double layer is described by Equation 8.1 as follows ... 

Helmholtz, or Stern layer capacity by various authors. In the early work by Stern, Ch was attributed to an inner solvent layer on the electrode surface, into which the ions cannot penetrate more recent theories accoimt for an extended boundary region. In a Parsons and Zobel plot, Ch is determined by plotting experimental values for 1/C vs. 1/Cgc- Specific adsorption results in significant deviations from a straight line, which invahdates this procedure. 

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