Transition double layer effect

Experimental attempts to verify the dependence of the transfer coefficient on the electrode potential have been made with simple outer sphere redox electrode reactions (see refs. 5—19 in ref. 70a). Corrections to experimental values of the apparent transfer coefficient due to double layer effects are performed by the use of eqn. (109), but the value of a calculated from experimental data depends on the assumptions about the location of the centre of charge in the transition state in the Helmholtz layer [70b]. 

Most treatments of such double-layer effects assume that the microscopic solvation environment of the reacting species within the interfacial region is unaltered from that in the bulk solution. This seems oversimplified even for reaction sites in the vicinity of the o.H.p., especially since there is evidence that the perturbation of the local solvent structure by the metal surface [18] extends well beyond the inner layer of solvent molecules adjacent to the electrode [19]. Such solvent-structural changes can yield considerable influences upon the reactant solvation and hence in the observed kinetics via the work terms wp and wR in eqn. (7a) (Sect. 2.2). While the position of the reaction site for inner-sphere processes will be determined primarily by the stereochemistry of the reactant-electrode bond, such solvation factors can influence greatly the spatial location of the transition state for other processes. 

Taken together, these two terms, comprising the conventional "doublelayer effect, can be thought of as the influence of the surface upon the transition-state stability, presuming that the reactant-surface interactions in the transition state are an approximately weighted mean of those in the adjacent precursor and successor states. (The "appropriate weighting factor is the transfer coefficient aet.) This therefore constitutes the "thermodynamic catalytic influence of the surface, as distinct from the "intrinsic catalytic effect as defined above. The former, but not the latter, is conventionally termed the "double-layer effect, even though both, in fact, involve surface environmental influences upon the transition state stability. 

As discussed in Section 8.3.4, the presence of a finite double-layer capacity results in a charging current contribution proportional to dEldt (equation 8.3.11) and causes /f to differ from the total applied current, /. This effect, which is largest immediately after application of the current and near the transition (where dE/dt is relatively large), affects the overall shape of the E-t curve and makes measurement of r difficult and inaccurate. A number of authors have examined this problem and have proposed techniques for measuring T from distorted E-t curves or for correcting values obtained in the presence of significant double-layer effects. 

Mechanisms of electrochemical reactions of different systems, including transition metal complexes, were examined with a special attention paid to double layer effects and problems of generation and decay of intermediates which arise in such reactions. Electrodes modified with thin films of transition metal hexacyanoferrates and conducting polymers were investigated, also solid state electrochemistry in the absence of external supporting electrolyte were developed. Charge propagation rate in such mixed-valent solid systems and their electrocatalytic properties were studied. 

Figure B3.6.3. Sketch of the coarse-grained description of a binary blend in contact with a wall, (a) Composition profile at the wall, (b) Effective interaction g(l) between the interface and the wall. The different potentials correspond to complete wettmg, a first-order wetting transition and the non-wet state (from above to below). In case of a second-order transition there is no double-well structure close to the transition, but g(l) exhibits a single minimum which moves to larger distances as the wetting transition temperature is approached from below, (c) Temperature dependence of the thickness / of the enriclnnent layer at the wall. The jump of the layer thickness indicates a first-order wetting transition. In the case of a conthuious transition the layer thickness would diverge continuously upon approaching from below.

In the crudest approximation, the effect of the efectrical double layer on electron transfer is taken into account by introduction of the electrostatic energy -e /i of the electron in the acceptor into the free energy of the transition AF [Frumkin correction see Eq. (34.25)], so that corrected Tafel plots are obtained in the coordinates In i vs. e(E - /i). Here /i is the average electric potential at the site of location of the acceptor ion. It depends on the concentration of supporting electrolyte and is small at large concentrations. Such approach implies in fact that the reacting ion represents a probe ion (i.e., it does not disturb the electric held distribution). 

The principle behind this investigation is electrochromism or Stark-effect spectroscopy. The electronic transition energy of the adsorbed chromophore is perturbed by the electric field at the electric double layer. This is due to interactions of the molecular dipole moment, in the ground and excited states, with the interfacial electric field induced by the applied potential. The change in transition frequency Av, is related to the change in the interfacial electric field, AE, according to the following ... 

The effect of electrolyte concentration on the transition from common to Newton black films and the stability of both types of films are explained using a model in which the interaction energy for films with planar interfaces is obtained by adding to the classical DLVO forces the hydration force. The theory takes into account the reassociation of the charges of the interface with the counterions as the electrolyte concentration increases and their replacements by ion pairs. This affects both the double layer repulsion, because the charge on the interface is decreased, and the hydration repulsion, because the ion pair density is increased by increasing the ionic strength. The theory also accounts for the thermal fluctuations of the two interfaces. Each of the two interfaces is considered as formed of small planar surfaces with a Boltzmannian distribution of the interdistances across the liquid film. The area of the small planar surfaces is calculated on the basis of a harmonic approximation of the interaction potential. It is shown that the fluctuations decrease the stability of both kinds of black films. 

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