Modelling the electric double layer at electrode/electrolyte interfaces

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Electrical double layer at the mercury-nitromethane solution interface

The differential capacity and the surface charge density curves as a function of the electrode potential for mercury/electrolyte solution in nitromethane interface are presented. For all the systems studied the capacity hump at the anodic potential region is observed. The height and the location of the hump considerably depends on the kind of anion. As a test of specific adsorption of ions in the systems studied the Esin-Markov effect was examined. The results indicated that anions appear to be specifically adsorbed from nitromethane in the order PF₆^?<ClO₄^?<Cl^?<SCN^?.     

X-ray study of the electric double layer at the n-hexane/nanocolloidal silica interface

The spatial structure of the transition region between an insulator and an electrolyte solution was studied with x-ray scattering. The electron-density profile across the n-hexane/silica sol interface (solutions with 5, 7, and 12 nm colloidal particles) agrees with the theory of the electrical double layer and shows separation of positive and negative charges. The interface consists of three layers, i.e., a compact layer of Na(+), a loose monolayer of nanocolloidal particles as part of a thick diffuse layer, and a low-density layer sandwiched between them. Its structure is described by a model in which the potential gradient at the interface reflects the difference in the potentials of "image forces" between the cationic Na(+) and anionic nanoparticles and the specific adsorption of surface charge. The density of water in the large electric field (approximately 10(9)-10(10) Vm) of the transition region and the layering of silica in the diffuse layer is discussed.     

Kinetics of electric double layer formation at the blocked spherical or cylindrical electrode/solid electrolyte interface under galvanodynamic and potentiodynamic conditions

The kinetics of charging of the blocked (inert) electrode/solid electrolyte interface is studied for spherical or cylindrical electrodes by the impedance method in two modes (galvanodynamic and potentiodynamic). The case of slow diffusion and adsorption-desorption is analyzed for species of one type, namely, defects of the solid-electrolyte rigid sublattice (minor carriers). The roles that both the slow lattice defects and the fast conduction ions play in the electric double layer formation are taken into account. Calculations involve the use of both the diffusion model of a spherical or cylindrical electrode in electrolyte (proposed by Jacobsen and West) and the ac circuit of an ideally polarizable planar electrode in a solid electrolyte (developed by Grafov, Ukshe, and Bukun).     

Temperature effects on electrical double layer at solid-aqueous solution interface

Despite the significant influence of solution temperature on the structure of electrical double layer, the lack of theoretical model intercepts us to explain and predict the interesting experimental observations. In this work, we study the structure of electrical double layer as a function of thermochemical properties of the solution by proposing a phenomenological temperature dependent surface complexation model. We found that by introducing a buffer layer between the diffuse layer and stern layer, one can explain the sensitivity of zeta potential to temperature for different bulk ion concentrations. Calculation of the electrical conductance as function of thermochemical properties of solution reveals the electrical conductance not only is a function of bulk ion concentration and channel height but also the solution temperature. The present work model can provide deep understanding of micro- and nanofluidic devices functionality at different temperatures.     

Adsorption of procaine at the mercury/electrolyte solution interface

The adsorption of the local anaesthetic procaine hydrochloride at the mercury/electrolyte interface solution is followed using capacitance measurements. The adsorption is studied at various procaine concentrations, in potassium chloride, potassium bromide or potassium fluoride used as supporting electrolytes, and at various pH values and temperatures. Procaine has basic properties with two acidity constants K. The results indicate the way the procaine molecules orientate at the interface. In all cases studied no hemimicelles or condensed film are observed.     

Lyotropic effects at the silver iodide/electrolyte solution interface

The lyotropic effect in the flocculation of a negatively charged AgI sol is explained on the basis of a double layer model without specific adsorption of cations. The constant potential model for the double layer interaction gives better results than the constant charge model. The theoretical results indicate furthermore that repeptization of the flocculated sol is possible.     

Planar electric double layer for a restricted primitive model electrolyte at low temperatures

Monte Carlo simulation and the modified Poisson-Boltzmann theory are used to investigate the planar electric double layer for a restricted primitive model electrolyte at low temperatures. Capacitance as a function of temperature at low surface charge is determined for 1:1, 2:2, 2:1, and 3:1 electrolytes. Negative adsorption can occur for 1:1 electrolytes at low surface charge with low electrolyte concentration. The 1:1 electrolyte diffuse layer potential as a function of surface charge displays a maximum at low densities. At high densities, the diffuse layer potential is negative with a negative slope. The Gouy-Chapman-Stern theory fails in this low-temperature regime, whereas the modified Poisson-Boltzmann theory is fairly successful in this regard.     

Repulsive interface forces in overlapping electric double layers in electrolyte solutions

The historical development of the problem of the electric interaction of particles in electrolyte solutions is comprehensively discussed. The existing approaches are divided into force-based methods, where the mechanical (ponderomotive) forces of the electric field are directly calculated, and energy-based methods calculating the free energy of the colloid system (at least the part of the free energy which is determined by the repulsive forces of electrical nature). The fundamental works of Langmuir, Derjaguin, Levine, Verwey and Overbeek are discussed in detail. At the same time, new original methods are proposed: the method of effective displacements; the formula of free energy of overlapping double layers.Special attention is paid to the analysis of electrostriction forces in liquids, particularly in electric double layers. The non-contradictory application of the concepts of classic macroelectrostatics is shown to result in the need to take into account electrostriction forces in overlapping double layers. The main formulas are given for force and energy of repulsion in flat surfaces with a constant density of the electric charge on them. These formulas are derived with electrostriction forces taken into account. A number of the theoretical results are new.Some experiments are discussed in measuring repulsive forces in colloid systems. A qualitative agreement is established between the experimental results of Ottewill et al. and the theory of electrostriction forces in double layers.     

Electrochemistry at conductor/insulator/electrolyte three-phase interlines: A thin layer model

A thin layer model is proposed to assist in the understanding of the electrochemical conversion of insulator to conductor at the conductor/insulator/electrolyte three-phase interline (3PI) when the influence of mass diffusion in the electrolyte phase is negligible. The model predicts, under potentiostatic conditions, a linear variation of the current or the length of the 3PI with time. When polarization is sufficiently large, the logarithm of the current/time ratio or the 3PI-length/time ratio, according to the model, increases linearly with the applied potential. These predictions were tested against and agreed very well with two practical systems: the electroreduction of solid AgCl to Ag in aqueous KCl and of solid SiO2 to Si in molten CaCl2. Kinetic parameters were derived from experimental data using the model. Particularly, the electron transfer coefficient, alpha, was found to be about 0.29 for the reduction of AgCl to Ag in the aqueous KCl solution at room temperature but about 10(-2) for the reduction of SiO2 to Si in molten CaCl2 at 850 degrees C.     

The glass/electrolyte solution interface

A hydrated layer is formed on most glass/electrolyte solution interfaces. Arguments for the existence of such a layer are discussed, and concentration profiles of some ions are presented. In some cases (SiO₂ glass after short contact with water at room temperature; vitreous CaSiO₃) a hydrated layer does not exist. These results are applied to the concepts of the origin of glass electrode potentials.     

Diffuse double layer interaction between two parallel plates with constant surface charge density in an electrolyte solution

Summary The double layer interaction between two parallel metallic plates with adsorbed surface charge of constant density in a symmetrical electrolyte solution is considered. Numerical calculations using the nonlinearPoisson-Boltzmann equation are made. Analytic expressions for the interaction are also obtained by using theDebye-Hückel approximation. The double layer interaction is found to be influenced by true charges induced on the plate surface due to electrostatic induction. This influence becomes important at small plate separations. It is also shown that the force and potential energy of interaction between metallic plates are small compared with those between insulating plates.

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Zusammenfassung Die Wechselwirkung der Doppelschichten zwischen zwei parallelen metallischen Platten mit der konstanten Dichte der adsorbierten Oberflächenladung in einer symmetrischen Elektrolytlösung wird betrachtet. Numerische Berechnungen werden mittels der nichtlinearenPoisson-Boltzmannschen Gleichung ausgeführt. Durch Benutzung der Debye-Hückel-Näherung werden auch analytische Ausdrücke für die Wechselwirkung gewonnen. Es zeigt sich, daß die Wechselwirkung der Doppelschichten von reellen Ladungen, die an der metallischen Oberfläche durch elektrostatische Induktion induziert werden, beeinflußt wird. Dieser Einfluß wird besonders wichtig für kleine Plattenabstände. Die Kraft und die potentielle Energie der Wechselwirkung zwischen metallischen Platten sind kleiner als die zwischen Isolierplatten.

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With 4 figures and 3 tables     

On the adsorption of cetyldimethylbenzylammonium chloride at the mercury/electrolyte solution interface

The adsorption of cetyldimethylbenzylammonium chloride (CDBACl) on the hanging mercury electrode is studied in various supporting electrolytes at various temperatures from 1 to 50 degrees C. A condensed film with low capacitance is formed at negative potentials at transition temperatures below approximately 40 degrees C. The decrease of the temperature favors the film formation, and increases the width of the capacitance pit, while its value remains practically constant. Hysteresis phenomena are also observed during different scan directions. Capacitance-time curves at the potentials where the film is formed show in some cases a nucleation and growth mechanism with induction time and studied by the Avrami formulation. At high temperatures an increase of the capacitance with time is observed depending on the CDBACl concentration and slightly on the electrolyte used, and is attributed to the formation of hemimicelles. At high negative potentials a second narrow region with lower capacitance values is observed. This is easily observed at very high temperatures, while it is absent at lower temperatures. It depends upon the concentration of CDBACl and the electrolyte used. The results are different from those obtained for the adsorption of cetyltrimethylammonium bromide on mercury, indicating the importance of interaction between the hydrophobic chains.     

Studies on the electrical double layer structure at the water/air interface

Effective dipole moments (calculated from experimental data of surface tension and electric surface potential) of some homologous normal alcohols and carboxylic acid were found to vary linearly with the number of carbon atoms in the hydrocarbon chain. Values of effective dipole moments were used for the determination of the effective dipole moments of water molecules , and the dielectric permittivity of the water subphase (₁), as well as in the vicinity of the hydrophobic part of adsorbed molecule (₂). The latter was found to decrease with the increase of the hydrocarbon chain length. Knowing the effective dipole moment of surface water dipoles, the average orientation angle () of water molecules at the inteface was estimated. The calculated potential drop of water varies within the range –0.038 to –2.38 V for two extreme orientations of water dipoles at the surface.