Specifically adsorbed ions

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. 

The outer Helmholtz plane (OHP) refers to the distance of closest approach of non specifically 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. 

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.

Even in the absence of Faradaic current, ie, in the case of an ideally polarizable electrode, changing the potential of the electrode causes a transient current to flow, charging the double layer. The metal may have an excess charge near its surface to balance the charge of the specifically adsorbed ions. These two planes of charge separated by a small distance are analogous to a capacitor. Thus the electrode is analogous to a double-layer capacitance in parallel with a kinetic resistance. 

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. 

Adsorption of ions from the solution. There are two types of ionic adsorption from solutions onto electrode surfaces an electrostatic (physical) adsorption under the effect of the charge on the metal surface, and a specific adsorption (chemisorption) under the effect of chemical (nonelectrostatic) forces. Specifically adsorbing ions are called surface active. Specific adsorption is more pronounced with anions. 

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. 

Pettinger, B., Philpott, M. R. and Gordon, J. G. (1981) Contribution of specifically adsorbed ions, water, and impurities to the surface enhanced Raman spectroscopy (SERS) of Ag electrodes. 

The charge density of specifically adsorbed ions (assuming that anions are adsorbed specifically while cations are present only in the diffuse layer) for a valence-symmetrical electrolyte (z+ = — z = z) is... 

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. 

Equation (2.33) now defines the double layer in the final model of the structure of the electrolyte near the electrode specifically adsorbed ions and solvent in the IHP, solvated ions forming a plane parallel to the electrode in the OHP and a dilfuse layer of ions having an excess of ions charged opposite to that on the electrode. The excess charge density in the latter region decays exponentially with distance away from the OHP. In addition, the Stern model allows some prediction of the relative importance of the diffuse vs. Helmholtz layers as a function of concentration. Table 2.1 shows... 

A closer inspection of the predominant peak in the conductance histogram at G0 (=77.5 pS) reveals that its position and magnitude depend on the applied electrode potential, as well as on the strength of anion adsorption (Fig. 11). The peak position shifts in the presence of weakly specifically adsorbed ions (e.g., C104-, SO)2 ) to value smaller than G0. 

In the presence of specifically adsorbed ions, e.g., for halides at potentials E > Epzc, the trend changes and the position of the 1 G0 peaks shifts to higher values than the quantum conductance unit. The up-shifts follow the order of adsorption strength, Cl- < Br < I-. 

Figure 1. A schematic representation of the synthesis of the electrochemical double layer in UHV a) adsorption of specifically adsorbed ions without solvent b) addition of hydration water c) completion of the inner layer d) addition of solvent multilayers, e) model for the double layer at an electrode surface in solution.

Titration of a suspension of a-FeOOH (goethite) in absence of specifically adsorbable ions. 

The Triple Layer Model. This model developped by Yates et al. (1974) and Davis et al. (1978) uses a similar idea as the Stern model the specifically adsorbed ions are... 

The net charge at the hydrous oxide surface is established by the proton balance (adsorption of H or OH" and their complexes at the interface and specifically bound cations or anions. This charge can be determined from an alkalimetric-acidimetric titration curve and from a measurement of the extent of adsorption of specifically adsorbed ions. Specifically adsorbed cations (anions) increase (decrease) the pH of the point of zero charge (pzc) or the isoelectric point but lower (raise) the pH of the zero net proton condition (pznpc). 

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... 

For adsorption at the IHP, mass action and material balance equations could be set up for specifically adsorbed ions yielding equations similar to those for the surface plane... 

M.A. Anderson, Control of surface and. potentials on nanoporous Ti02 films by potential-determining and specifically adsorbed ions, Langmuir 16 (2000) 6094-6101. 

There are a number of more loosely defined terms for different types of adsorption that are related to the form of surface complexation. Specifically adsorbed ions are held in inner-sphere complexes whereas non-specifically adsorbed ions are in outer-sphere complexes or the diffuse-ion swarm. Readily exchangeable... 

Investigators tend to report either the pzc or the iep, but ideally, when specifically adsorbing ions are present, both parameters should be measured. 

In the triple layer model, the potential determining ions are located at the oxide surface with the specifically adsorbing ions and the ion pairs in the inner Helmholz... 

Ions with a weak solvation shell, anions in general, lose a part of or the complete solvation shell in the double layer and form a chemical bond to the metal surface. The adsorption is termed specific since the interaction occurs only for certain ions or molecules and is not related to the charge on the ion. The plane where the center of these ions are located is called the inner Helmholtz layer. In the specific adsorption, ions are chemically bound to the surface and the interaction has a covalent nature. In the case of non-specific adsorption, in which an electrostatic force binds ions to the surface, the coverage of ions is below 0.1 -0.2 ML due to electrostatic repulsion between the ions. In contrast, the coverage of specifically adsorbed ions exceeds this value, and a close-packed layer of specifically adsorbed ions is often observed. Specifically adsorbed ions are easily observed by STM [22], indicating that the junction between the electrode surface and the inner Helmholtz layer is highly... 

Solvent displacement, and isotherms. 954. 955 Solvent excess entropy at the interface, 912 Solvent interactions, 923, 964 Soriaga, M., 1103, 1146 Specifically adsorbed ions, 886 Spectrometer, 797 Spikes, electrodeposition. 1336 Spillover electrons, of metal, 889 Spiral growth, electrodeposition, 1316, 1324, 1326, 1324,1328 s-polarized light, 802 Srinivasan, S 1439,1494 Standard electrode potential American convention, 1354 convention, 1351 rUPAC convention, 1355 prediction of reactions, 1359 the zinc-minus and copper-plus convention, 1352... 

This countercharge can accumulate in the electrolyte side of the interface by coulombic non-specific interactions as well as by specifically adsorbed ions and dipoles and it can extend, to some degree, into the electrolyte, especially at low ionic concentrations. 

In the presence of specifically adsorbed ions, the situation is more complicated [18] but there still is a minimum in the C—E curve for A02 = 0. The pzc shifts to more negative potentials when anions are specifically adsorbed due to the dipole formed by the adsorbed anion and the corresponding cation left behind at x2 (Fig. 2). 

Gouy—Chapman theory and involves coulombic and possibly specific interactions due to weak electron orbital overlapping. The amount of specifically adsorbed ions at the electrode generally varies linearly with the charge at the electrode and logarithmically with the ion concentration in the solution. Further evidence of specific adsorption of ions at electrodes is the Esin—Markov effect, i.e. the shift in the pzc due to specific adsorption of ions [6]. 

Partial charge transfer during adsorption is difficult to evaluate because separation of the charge transferred to the electrode and that part of the charge transferred across the double layer to give specifically adsorbed ions cannot be done through measurements of the total charge in the external circuit. Vetter and Schultze [102] defined the electrosorption valence as... 

The excess surface charge in the presence of specifically adsorbed ions was found by measuring the amount of selenite adsorbed and the amount of hydroxyl displaced into the solution. The quantity of OH" displaced (A) was estimated for constant pH (a hypothetical situation) from the curves for titration of goethite, goethite plus selenite, and selenite alone by the equation. 

Studies of specific adsorption of a series of anions to be discussed in detail elsewhere (5), have shown that the maximum specific adsorption at any pH, r0(1)H>, is related to the pKD of the anion acid and the charge on the adsorbing species. From these studies essential requirements for specific adsorption of anions, or exchange of specifically adsorbed ions, seem to be as follows ... 

---