Helmholtz inner plane

The compact layer can be structured into what is called an inner Helmholtz plane... 

There is, or may be, an iimer layer of specifically adsorbed anions on the surface these anions have displaced one or more solvent molecules and have lost part of their iimer solvation sheath. An imaginary plane can be drawn tlirough the centres of these anions to fomi the inner Helmholtz plane (IHP). 

Eigure 3 schematically depicts the stmcture of the electrode—solution interface. The inner Helmholtz plane (IHP) refers to the distance of closest approach of specifically adsorbed ions, generally anions to the electrode surface. In aqueous systems, water molecules adsorb onto the electrode surface. 

Fig. 1. The structure of the electrical double layer where Q represents the solvent CD, specifically adsorbed anions 0, anions and (D, cations. The inner Helmholtz plane (IHP) is the center of specifically adsorbed ions. The outer Helmholtz plane (OHP) is the closest point of approach for solvated cations or molecules. O, the corresponding electric potential across the double layer, is also shown.

FIGURE 1-11 Schematic representation of the electrical double layer. IHP = inner Helmholtz plane OHP = outer Helmoltz plane. 

The inner layer (closest to the electrode), known as the inner Helmholtz plane (IHP), contains solvent molecules and specifically adsorbed ions (which are not hilly solvated). It is defined by the locus of points for the specifically adsorbed ions. The next layer, the outer Helmholtz plane (OHP), reflects the imaginary plane passing through the center of solvated ions at then closest approach to the surface. The solvated ions are nonspecifically adsorbed and are attracted to the surface by long-range coulombic forces. Both Helmholtz layers represent the compact layer. Such a compact layer of charges is strongly held by the electrode and can survive even when the electrode is pulled out of the solution. The Helmholtz model does not take into account the thermal motion of ions, which loosens them from the compact layer. 

Grahame introdnced the idea that electrostatic and chemical adsorption of ions are different in character. In the former, the adsorption forces are weak, and the ions are not deformed dnring adsorption and continne to participate in thermal motion. Their distance of closest approach to the electrode surface is called the outer Helmholtz plane (coordinate x, potential /2, charge of the diffuse EDL part When the more intense (and localized) chemical forces are operative, the ions are deformed, undergo partial dehydration, and lose mobility. The centers of the specifically adsorbed ions constituting the charge are at the inner Helmholtz plane with the potential /i and coordinate JCj < Xj. 

Quantitative calculations are beset by a number of difficulties when specific adsorption occurs. Since the ions at the inner Helmholtz plane are localized (contrary to those at the outer Helmholtz plane, which are smeared out ), it is not correct to... 

The effects of the anions (i.e., their specific adsorbabilities) increase in the order F < Cr < Br < I . This trend is due to the fact that the solvation energy decreases with increasing crystal radius as one goes from F to I , and the transfer of the ions to the inner Helmholtz plane is facilitated accordingly. The opposite picture is seen for surface-active cations (e.g., [N(C4H5)4]+) the descending branch of the ECC is depressed, and the PZC shifts in the positive direction. 

Reactant concentrations Cyj in the bulk solution, as well as the Galvani potential between the electrode and the bulk solution (which is a constituent term in electrode potential E), appear in kinetic equations such as (6.8). However, the reacting particles are not those in the bulk solution but those close to the electrode surface, near the outer Helmholtz plane when there is no specific adsorption, and near the inner Helmholtz plane when there is specific adsorption. Both the particle concentrations and the potential differ between these regions and the bulk solution. It was first pointed out by Afexander N. Frumkin in 1933 that for this reason, the kinetics of electrochemical reactions should strongly depend on EDL structure at the electrode surface. 

Until the advent of modem physical methods for surface studies and computer control of experiments, our knowledge of electrode processes was derived mostly from electrochemical measurements (Chapter 12). By clever use of these measurements, together with electrocapillary studies, it was possible to derive considerable information on processes in the inner Helmholtz plane. Other important tools were the use of radioactive isotopes to study adsorption processes and the derivation of mechanisms for hydrogen evolution from isotope separation factors. Early on, extensive use was made of optical microscopy and X-ray diffraction (XRD) in the study of electrocrystallization of metals. In the past 30 years enormous progress has been made in the development and application of new physical methods for study of electrode processes at the molecular and atomic level. 

By means of the thermodynamic theory of the double layer and the theory of the diffuse layer it is possible to determine the charge density ox corresponding to the adsorbed ions, i.e. ions in the inner Helmholtz plane, and the potential of the outer Helmholtz plane 2 in the presence of specific adsorption. 

The adsorption of ions is determined by the potential of the inner Helmholtz plane 0n while the shift of Epzc to more negative values with increasing concentration of adsorbed anions is identical with the shift in 0(m). Thus, the electrocapillary maximum is shifted to more negative values on an increase in the anion concentration more rapidly than would follow from earlier theories based on concepts of a continuously distributed charge of adsorbed anions over the electrode surface (Stern, 1925). Under Stern s assumption, it would hold that 0(m) = 0X (where, of course, 0X no longer has the significance of the potential at the inner Helmholtz plane). 

According to Ershler, 0X must appear in the expression for the electrochemical potential of ions adsorbed on the inner Helmholtz plane. If their electrochemical potential is expressed by the equation... 

Anions may exhibit a tendency toward specific adsorption at the O/S interface. This may be related in some way to the complexing affinity. This effect, occurring at the inner Helmholtz plane of the electrochemical double layer, may significantly change the charge transfer situation [cf. Section III(5(iii))]. 

By contrast, anions have larger radii and tend to be more weakly hydrated. In addition, they are able to form relatively strong ionic/covalent bonds to the surface of the metal electrode and, as a result, frequently find it energetically feasible to shed their inner hydration sphere, or at least part of it, and adsorh directly on the surface. The plane formed hy the nuclei of anions directly adsorbed on the metal surface is termed the inner Helmholtz plane (IHP). 

As a result of the above considerations, the Helmholtz model of the interface now shows two planes of interest (see Figure 2.8). The inner Helmholtz plane (IHP) has the solvent molecules and specifically adsorbed ions (usually anions) the outer Helmholtz plane (OHP), the solvated ions, both cations and anions. It can be seen from Figure 2.8 that the dielectric in the capacitor space now comprises two sorts of water that specifically adsorbed at the electrode surface and that lying between the two Helmholtz planes. Continuing the analogy with capacitance, these two forms of water act as the dielectric in two capacitors connected in series. 

There is compelling evidence that reducing agent oxidation and metal ion reduction are, more often than not, interdependent reactions. Nonetheless, virtually all established mechanisms of the electroless deposition fail to take into account this reaction interdependence. An alternative explanation is that the potentials applied in the partial solution cell studies are different to those measured in the full electroless solution studies. Notwithstanding some differences in the actual potentials at the inner Helmholtz plane in the full solution relative to the partial solutions, it is hard to see how this could be a universal reason for the difference in rates of deposition measured in both types of solution. 

In contrast, Lambert has proposed that the shift is due to the Stark effect exerted by the electric field at the inner Helmholtz plane on the intra-molecular charge distribution of the adsorbed CO molecule (19). 

The fact that linear CO species are observed at 0.05 V in the absence of C.H.CN (cf. spectrum a) indicates that H.O molecules at the inner Helmholtz plane are not able to displace CO out of its linear configuration at the same potential. This may be due to a re-orientation of the adsorbed HjO as a function of potential, with the positive end of the molecular dipole becoming attracted to the surface as the electrode potential is made more negative. This would reduce the ability of the H O molecule to donate electron density from its oxygen atom, and would also Increase the ability of its hydrogen atoms to compete for accepting electron density from the metal. 

In-line extrusion, 19 543 In-line filtration, 15 827 Inline motionless mixers, 16 711-716 In-motion checkweighers, 26 245 Inner-Helmholtz plane (IHP), 3 419 Inner transition-metal perchlorates, 18 278 Inner tubes, butyl rubber for, 4 434, 453 Innohep, 4 95t 5 175... 

Fig. 17.2. The distribution of charges at the internal wall of a silica capillary. x is the length in cm from the center of charge of the negative wall to a defined distance, 1 = the capillary wall, 2 = the Stern layer or the inner Helmholtz plane, 3 = the outer Helmholtz plane, 4 = the diffuse layer and 5 = the bulk charge distribution within the capillary.

Beyond the surface plane is a layer of ions attracted to the surface by specific chemical interactions. The locus of the center of these ions is known as the inner Helmholtz plane (IHP). The charge in this plane, which results from the specifically adsorbed ions is denoted by a2, and the electrostatic potential at the IHP by The species usually assigned to this plane include... 

According to this model, and in the absence of specific adsorption, the adsorbed solvent molecules are located in the inner Helmholtz plane, the thickness of which is determined by the radius of the molecule. At the same time, solvated ions define the location of the outer Helmholtz plane. Other ions, charged oppositely to the surface charge, are smeared out in the diffuse layer. 

Chemisorption of anions at the electrode interface involves dehydration of hydrated anions followed by adsorption of dehydrated anions which, then, penetrate into the compact double layer to contact the interface directly, this result is called the contact adsorption or specific adsorption. The plane of the contact adsorption of dehydrated anions is occasionally called the inner Helmholtz plane... 

In the course of ionic contact adsorption on the interface of metal electrode, hydrated ions are first dehydrated and then adsorbed at the inner Helmholtz plane in the compact layer as shown in Fig. 5-27 and as described in Sec. 5.6.1. In the interfacial double layer containing adsorbed ions, the combined charge of motal and adsorbed ions = z eF on the metal side is balanced with the... 

Pig. 5-27. Contact ion adsorption on metal electrodes in aqueous solution IHP = inner Helmholtz plane OHP = outer Helmholtz plane i,d = adsorbed ion ih = hy-dratedion oM = charge on the metal electrode o i = charge of adsorbed ions o i = charge of excess hydrated ions in solution. [From Bockris-Devanathan-MuUer, 1963.]... 

Fig. 6-99. An interfacial electric double layer on semiconductor electrodes a = charge of surface states 0.1 = interfadal charge of adsorbed ions IHP = inner Helmholtz plane.

The reaction of electron transfer at electrodes in aqueous electrolytes proceeds either with hydrated redox particles at the plane of closest approach of hydrated ions to the electrode interface (OHP, the outer Helmholtz plane) or with dehydrated and adsorbed redox particles at the plane of contact adsorption on the electrode interface (IHP, the inner Helmholtz plane) as shown in Fig. 7-2. 

Fig. 7-2. Electron transfer of hydrated redox particles and of dehydrated adsorbed redox particles across an electrode interface (a) electron transfer of hydrated redox particles, (b) electron transfer of dehydrated and adsorbed redox particles on electrodes. (RED., OX,q) = hydrated redox particles (RED.d, OX.d) = dehydrated and adsorbed redox particles on electrode OHP = outer Helmholtz plane, IHP = inner Helmholtz plane.

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