Components of Charge and Potential in the Non-diffuse Region of the Electrical Double Layer: Potassium Iodide Solutions in Contact with Mercury at 25°<sup>1</sup>

129

The Stern-Grahame-Devannthan theory of the electrical double layer in aqueous systems is modified to include the so-called discreteness-of-charge effect of Esin and Shilcov and Ershler. This provides an explanation of a n~~m b e r of phenomena which are a t variance nrith the Stern theory. A simple method of incorporating the above effect into the Stern theory is suggested by the work of Grahame and is equivalent in principle to the discrete-ion approximation einployed by the Russian authors. It is shown that the effect can be interpreted in terms of a 'self-atmosphere' potential a t the counterions adsorbed in the Stern layer. This provides a new term in the energy of an adsorbed ion, which is very nearly proportional to the surface density of these ions and which had hitherto been included in the specific adsorptioi~ potential in the Stern adsorption isotherm. This energy is not small and accounts for the conclusion reached by Grahame that the adsorption potential varies strongly with the surface charge. Grahame found that the potential a t the plane separating the compact and diffuse parts of the double layer in the solution phase (i.e. the outer Hellnholtz plane) a t the mercuryaqueous electrolyte interface displays a lnaxi~nuin as the poteiltial across the interface is varied, and this property is reproduced by the theory. The effect of ion size on the adsorption isotherm is also considered.

Section: Introductionmentioning

confidence: 89%

The Discreteness-of-Charge Effect in Electric Double Layer Theory

Levine¹,

Bell²,

Calvert³

1962

129

“…oa, is the maximum possible coverage by adsorbed iodide ions and p,, the number of solvent molecules displaced by the reactant at the surface in the transition state. Since there is no evidence for surface coverage being significant in the analysis of data for iodide adsorption (22,26,27), the last term in eq. [3] may be disregarded, especially considering the extent of iodide adsorption under the present experimental conditions.…”

Section: Kineticmentioning

confidence: 99%

Discreteness-of-charge effects in electrode kinetics. I. The electroreduction of tetrathionate anion in the presence of specifically adsorbed iodide anions

Fawcett¹,

Markušová²

1982

W. RONALD FAWCETT and KVETA MARK'USOVA. Can. J. Chem. 60,2038Chem. 60, (1982.The kinetics of electroreduction of tetrathionate anion have been studied at a Hg electrode in aqueous solutions of NaI + NaF and KI + K F with an ionic strength of 0.25 M. The rate of reaction was observed to decrease as the iodide ion concentration was increased, and when the cation was changed from K+ to Na+ at constant electrode potential. Analysis of the double layer effects on the basis of the Frumkin model results in an overestimation of the repulsive effect of the adsorbed iodide anions. This result is interpreted on the basis of the discreteness-of-charge effect and the possible non-coincidence of the reaction plane and outer Helmholtz plane. Chem. 60,2038Chem. 60, (1982. On a Ctudie la cinitique de I'ClectrorCduction de l'anion thathionate au niveau d'une Clectrode au Hg dans des solutions aqueuses de NaI + NaF et de KI + KF avec une force ionique de 0,25 M. On a observe que la vitesse de reduction dicroit lorsqu'on augmente laconcentration en ion iodure et lorsqu'on passe du cation K+ au cation Na+ a potentiel d'tlectrode constant. L'analyse des effets de la couche double en fonction du modele de Frumkin conduit a une surevaluation tres marquee de I'effet rkpulsif des anions iodures absorbts. On interprete ce resultat en fonction du caractere discret de I'effet de charge et de la non-coincidence possible du plan de la reaction et du plan extkrieur de Helmholtz.[Traduit par le journal] Introduction Discreteness-of-charge effects in electrode kinetics have been discussed in detail in recent literature (1-14). One essential feature of the theory for these effects is that the rate of a simple redox process at an inert electrode is changed less than one would predict on the basis of the classical Frumkin theory of double layer effects (15,24) in the presence of specifically adsorbed non-reacting ions. For example, the rate of electroreduction of an anion such as tetrathionate at a mercury electrode is slower in the presence of specifically adsorbed iodide anions than in their absence (16,17), but the extent of deceleration is less than that estimated on the basis of the corresponding shift in the potential drop across the diffuse layer in the negative direction.The electroreduction of tetrathionate was first studied by ieiula (18)(19)(20) who showed that a minimum occurs on the current-potential curve at potentials negative of the point of zero charge (pzc) similar to that found in the electroreduction of other anions (21). The presence of a large double layer effect over a wide potential range for this system makes it a good candidate for more detailed examination. Foresti and Guidelli (16) studied the effects of adsorbed ions on the electroreduction of tetrathionate; however, these experiments were

“…If Rlo = 5, for exan~ple, corresponding to a ql of 8.22 pC/cm2, a ql of 10 or 12 pC/cin2 inight be required before hexagonal order were regained, and a value of q1 as small as 5 or 6 pC/cm2 might be required before the BuffStillinger approach would be applicable. As an example, for 1.0 N K I (z, = -I ) , the results of Grahaine (20) indicate that the charge on the electrode, q, might have to be less than , --9 p C /~m 2 for the Buff-Stillinger approach to apply and greater than , --6 ,LC/ cin2 for the hexagonal lattice model to apply. Between -9 and -6 &/cm2 neither model would theoretically apply well but the gap might be bridged reasonably accurately by extrapolation between the results of the two approaches.…”

Section: Thermal Averaging Calculationmentioning

confidence: 99%

Thermal Stability of an Adsorbed Array of Charges in the Einstein Approximation

Macdonald¹,

Barlow²

1965

For an infinite hexagonal array of ions adsorbed on a conducting plane of infinite extent, the thermal fluctuation from strict lattice ordering in the neighborhood of a given adion is considered. The ions are imaged in the uniform, conducting adsorbent and are assumed to move freely in the plane; they thus arrange themselves in a perfect hexagonal array a t absolute zero temperature. By use of the accurate planar potential seen by one adion moving in the field arising from a n infinite number of fixed hexagonally arrayed surrounding ions, the root-meansquare (r.m.s.) amplitude of planar vibration of the ion relative to its neighbors is approximately determined for several values of nearest neighbor distances between ions, rl. On the basis of these results, we find, for example, that for a distance, P, between the center of charge of a n ion and the imaging plane of 3 A, an ionic valence z, of unity, and a n effective dielectric constant of e, a hexagonal array with rl = 15 A is stable up t o a temperature, To, of approximately 1760/e OK while one with rl = 21 A is stable up to about 760/e OK. Results apply t o adsorption from either a gas or liquid phase and, as well, to a n array of real dipoles adsorbed on a nonconducting surface. The appropriate values for e are of the order of 2 and 6, respectively, for adsorption from gas or from aqueous electrolytes. For various rl's, explicit expressions for T o are obtained which depend on e, 8, and z, . -