Surface Diffusion Control Research Articles

Zur kinetik der metallionenadsorption: I. Polarisationsmodell der metalladsorbatbildung

A polarisation model of Me z+ adsorption at solid foreign electrodes is proposed, considering Me z+ transfer and surface diffusion to be the rate determining steps. Surface heterogeneity is accounted for by assuming the substrate to be composed of individual homogeneous regions of limited size, the Me z+ transfer occurring preferentially at line discontinuities such as grain or subgrain boundaries or step lines of atomic height. An explicit expression for the potentiostatic charge—time curve is derived. Short-time approximations of both potentiostatic and galvanostatic response signals are given as convolution products of the input functions with power series of t 1/2. All odd powers of t 1/2 disappear in the case of Dolin—Ershler RC-behaviour without surface diffusion control. The t −1/2 and t 1/2 terms of the potentiostatic and galvanostatic series, respectively, are present only if adsorption equilibrium is established instantaneously at the line discontinuities. From the t 0 and t terms, respectively, the overall transfer resistance may be calculated.

Vapor pressure osmometry: A model accounting for the solute dependence of the calibration constant

A model describing the operation of the vapor pressure osmometer has been developed to account for the solute dependence of the calibration constant. This model envisions the existence of a diffusion-controlled surface concentration that differs from the concentration of the drop as a whole. Three limiting cases approximated for the surface concentration depend on the relative magnitudes of thermistor self-heating and solution concentration. Quantitative predictions of the dependence of d(ΔR)/d(V2) (the variation of thermistor resistance difference with thermistor self-heating) on both solute molecular weight and solution concentration are in good agreement with experimental data.

A model for crystal growth rates under surface diffusion control in impure solution

A model is developed for the growth rate of crystals growing in impure solutions under the influence of surface diffusion. In this model, a surface active impurity can adsorb on the faces of the steps of a crystal and reduce the area of the face available for adsorption of solute molecules. The model is then applied successfully to the data of Smythe for sucrose crystals growing in the presence of raffinose as impurity. However, more data are required before the model can be accepted.

Performance of charring ablative materials in the diffusion-controlled surface combustion regime.

Ablative behavior of low and high density phenolic nylon in diffusion controlled surface combustion regime

The Dependence of Charge Transfer and Surface Diffusion Rates on the Structure and Stability of an Electrode Surface: Copper

Experiments have been carried out to ascertain the effect on the parameters of the Cu deposition reaction of varying the surface configuration, of the substrate. Electrodeposited, He‐quenched, H2‐quenched, dissolved, oxide film and chemically reduced surfaces were used. The observations were made by means of cathodic transients. An examination was also made of the change of activity of an electrode surface for times in the range 10−3 sec‐10 min after production in situ of a fresh surface.The charge transfer process per real sq. cm., is about ten times higher on electrodes prepared by quenching from the liquid than from those of other methods. The velocity of surface diffusion is about ten times lower on the quenched electrodes than on other electrodes. The concentration of adions on the quenched electrodes is much smaller than those on other substrates. The double layer capacity is approximately the same on He‐quenched and electro‐deposited electrode but much higher on oxide film and chemically reduced surfaces. There is a rapid fall of the velocity of surface diffusion on exposure of a fresh surface produced by anodic dissolution to a solution and no fall for oxide film electrodes. When the He‐quenched electrodes were submitted to a successive series of cathodic pulses, such that Cu deposition was succeeded by H2 evolution in each pulse, io, Cd. I. and τ all increased linearly with number of pulses, τ increased about ¼ the common rate of increase of io and Cd. I.The rate‐determining step on electrodeposited, oxide film, dissolved and chemically reduced surfaces is shared at low current density by surface diffusion and transfer process: at high current density, it is predominantly by transfer process. On He‐ and H2‐quenched electrodes, surface diffusion control is predominant at low current density and transfer process at high current density. The change of rate constant for surface diffusion with time on exposure of a fresh surface to the solution is inconsistent with adsorption of impurities from solution and consistent with an equilibration of the adion concentration among the different planes on the surface. The dependence of io on substrate is probably a secondary effect of a larger concentration of adions on electrodeposited surfaces. Oxide film, chemically reduced, dissolved, and H2‐evolved electrodes all have similar surface configuration to that of the electrodeposited electrodes. The surface adion concentration arises from the dominance of 100 planes in the surfaces of these substrates. The remarkable effects of successive cathodic pulses arise from the increase in surface area attendant on the production of Cu in fine particle form by a secondary mechanism. The anomalous linear dependence of τ on the number of pulses is consistent with a horn‐like shape of the pores.

Kinetics of the Deposition and Dissolution of Silver

Galvanostatic measurements, made on spherical silver electrodes in highly purified aqueous acid solutions of silver perchlorate at current densities up to 10 amp cm—2, gave overpotential as a function of time and Faradaic current density at constant total current. Variables were total current density, concentration of silver ions and state of the electrode surface. The theory of the surface diffusion kinetics was developed to encompass high current density regions and the surface adion concentration (cad0) evaluated. A theoretical analysis of the steady state of surface diffusion control of the metal-ion exchange reaction shows that the current is distributed nonuniformly between growing sites, with constant potential over the electrode surface. An equation previously developed by Rojter, Juza, and Polujan has been modified and shown to be applicable to galvanostatic transients when the transfer reaction is rate controlling. The symmetry factor β, of electrode kinetics, is shown to be potential dependent, and, in systems of sufficiently high i0 values, to tend to zero at high overpotential. This results in a limiting current density for a transfer controlled process. The tendency towards such a current was observed and shown to be quantitatively consistent with the discussion of β.