Inner Part of the Double Layer

Kunimatsu, K. and Bewick, A. (1986) Electrochemically modulated infrared spectroscopy of adsorbed water in the inner part of the double layer part 1. Oxygen-hydrogen stretching spectra of water on gold in 1M perchloric acid. fnd. J. Technol., 24, 407-412. 

The value of the electric permittivity of water in the inner part of the double layer is commonly accepted as equal to 6. A much higher capacity of the inner layer at the Ga/solution interface was explained by the weak interaction of gallium with water, leading to a high value of As shown... 

According to the capacitor model of the double layer, assuming constant thickness and electric permittivity, the dependence of AG° on <7m should be linear. " Deviations from linearity can be viewed as resulting from changes of X2 and/or e in the inner part of the double layer. A linear plot ofAG° vs. is observed for adsorption of ions and thiourea. ... 

Naneva and Popov et al. [4, 5] have studied Cd(OOOl) grown electrolytically in a Teflon capillary in NaF aqueous solution. A value of fpzc equal to —0.99 V (versus saturated calomel electrode (SCE)) was evaluated from minimum potential (Amin) on the differential capacity C-E curves obtained in dilute electrolyte. The zero charge potential was found to be practically independent of the crystallographic orientation. The Apzc and the irmer layer capacity of Cd(OOOl) single crystals were determined in KF solution as a function of temperature [5]. The positive values of AApzc/AT indicated that the water dipoles in the inner part of the double layer were orientated with their negative part to the electrode surface. It was found that the hydrophilicity of the electrodes was increasing in the order Cd(OOOl) < Ag(100)[Pg.768]

Tab. 1 Surface density of atonns (<7at) measured at the potentials of zero charge (PZC), and double-layer capacitances of the inner part of the double layer (Q) for different crystal planes of Ag (lattice constant d = 0.2889 nm)...

The electric field which actually affects the charge transfer kinetics is that between the electrode and the plane of closest approach of the solvated electroactive species ( outer Helmholtz plane ), as shown in Fig. 2.2. While the potential drop across this region generally corresponds to the major component of the polarization voltage, a further potential fall occurs in the diffuse double layer which extends from the outer Hemlholtz plane into the bulk of the solution. In addition, when ions are specifically absorbed at the electrode surface (Fig. 2.2c), the potential distribution in the inner part of the double layer is no longer a simple function of the polarization voltage. Under these circumstances, serious deviations from Tafel-like behaviour are common. 

Electrokinetic measurements at 25°C on silver iodide in 10 3 mol dm-3 aqueous potassium nitrate give d /d(pAg) = -35 mV at the zero point of charge. Assuming no specific adsorption of K+ or NO3 ions and no potential drop within the solid, estimate the capacity of the inner part of the electric double layer. Taking the thickness of the inner part of the double layer to be 0.4 nm, what value for the dielectric constant near to the interface does this imply Comment on the result. 

The inner part of the double layer may include specifically adsorbed ions. In this case, the center of the specifically adsorbed ions is located between the surface and the Stem plane. Specifically adsorbed ions (e.g., surfactants) either lower or elevate the Stem potential and the zeta potential as shown in Figure 4.31. When the specific adsorption of the surface-active or polyvalent counter ions is strong, the charge sign of the Stem potential will be reversed. The Stem potential can be greater than the surface potential if the surface-active co-ions are adsorbed. The adsorption of nonionic surfactants causes the surface of shear to be moved to a much longer distance from the Stem plane. As a result, the zeta potential will be much lower than the Stem potential. 

Especially for polar solvents the dielectric permeability may be reduced under the influence of the electric field in the double layer. We Introduced this dielectric saturation In secs. 1.5. Id and I.5.3e. The consequence Is a reduced screening power of the solvent, especially In the inner part of the double layer where the field is high. 

Electrokinetics yield Information on double layers which are slightly out of equilibrium and also on the distribution of charge and potentled. For instance, from the surface conductivity K the product of Ionic concentrations and mobilities in (essentially the Inner part of) the double layer is obtainable. See further sec. 3.13 and chapter 4. 

Two main aspects are considered here. First is the coupling of the potential drop caused by the preferential orientation of dipoles and by the inhomogeneity of ion distribution. Second is the specificity of the structure of the inner part of the double layer formed by ionic surfactants at the water-air interface. 

At comparable thicknesses of the dipole and diffuse layers their coupling becomes important and has so far been underestimated. However, the Debey length increases with decreasing electrolyte concentration and the diffuse layer becomes independent. The counter-ion distribution in the inner part of the double layer can be described as localised or non-localised adsorption dependent on some conditions. 

The treatment given above of the diffuse double layer is based on the assumption that the ions in the electrolyte are treated as point charges. The ions are, however, of finite size, and this limits the inner boundary of the diffuse part of the double layer, since the center of an ion can only approach the surface to within its hydrated radius without becoming specifically adsorbed (Fig. 6.4.2). To take this effect into account, we introduce an inner part of the double layer next to the surface, the outer boundary of which is approximately a hydrated ion radius from the surface. This inner layer is called the Stern layer, and the plane separating the inner layer and outer diffuse layer is called the Stern plane (Fig. 6.4.2). As indicated in Fig. 6.4.2, the potential at this plane is close to the electrokinetic potential or zeta ( ) potential, which is defined as the potential at the shear surface between the charge surface and the electrolyte solution. The shear surface itself is somewhat arbitrary but characterized as the plane at which the mobile portion of the diffuse layer can slip or flow past the charged surface. 

The character of the inner part of the double layer can vary from hydrophobic to hydrophilic the field strength in this area can vary due to ion adsorption. 

In this work, the activities are approximated by concentrations, so that h = < h and x- = u = where c is the electrolyte concentration. The potentials at the respective planes are related through the integral capacitances of the inner part of the double layer, Qj and that of the outer part, Cqj by... 

Zeta Potential Zeta (Q potential is a parameter used to describe the electrophoretic mobility of colloidal particles. Charged colloidal particles are slightly different from ions in that colloidal particles are surrounded by an electric double layer which is similar but not identical to the ionic atmosphere. The inner part of the double layer moves as a unit in transport experiments. The ( potential is the surface potential of the inner part of the double layer, as shown in Figure 13.10. It is defined as... 

Two things are evident from Table IV, the first is that the change in Clover the given concentration range is much smaller than the change in C or in Q the second is that in concentrated solutions most of the capacity is accounted for by the capacity of the inner part of the double layer and, therefore, most of the potential drop occurs there. We use these deductions in order to test the validity of the Stern model, i.e. whether it is correct to divide the double layer into two parts which are related to each other like capacitors in series. 

In the region of small surface coverages with organic molecules the composition of the solvent which fills the inner part of the double layer changes very little with change in ethanol content of the solution. 

As already mentioned above, the composition of the solvent in this case changes at the Helmholtz layer on one side, in the inner layer, water is present, and on the other side the binary solvent. Therefore, if a depolarizer molecule in State II is located in the inner part of the double layer in the region of small coverages, its activity does not change when ethanol is added to the solution if, however, it is located on the other side of the Helmholtz layer, the activity of the State II decreases when ethanol is added. Consequently, if the transition state (like the initial state I) is located outside the limits of the inner part of the double layer, the f i/f function should not depend on the ethanol concentration, and addition of ethanol should not affect the half-wave potential. If, however, the transition state is located in the inner part of the double layer, its activity coefficient is practically independent of the ethanol concentration, and it follows from Eqs. (14) and (27) that... 

When this relationship was first obtained [43], it was also assumed that the activity coefficient of the transition state does not depend on the concentration of ethanol however, for some incompletely understood reasons, the authors linked the formula obtained to the reduction of particles situated outside the limits of the inner part of the double layer. 

We will call a molecule which has entered the inner part of the double layer an adsorbed molecule. Only those molecules which are situated outside the inner part of the double layer will be regarded as unadsorbed molecules. 

A consideration of the decreased concentration of ions in the inner part of the double layer explains why large shifts in i x-potential lead to an increase in the concentration of discharging ions by several orders without any appreciable increase in their total adsorption. According to this model, Aq must be almost equal to the shift (if we do not make the improbable assump-... 

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