Properties Of Electrical Double Layer Research Articles

Complex Geometry and Electric Double Layers

ABSTRACTThe properties of electric double layers near curved surfaces of arbitrary shape and genus are obtained exactly within the Debye-Hückel theory by means of multiple-scattering expansion. For smooth membranes, geometric and topological feature of the electrostatic free energy then emerge through convergent expansions in inverse powers of the principal radii of curvature. Some consequences for the electrostatic stability of various membrane shapes are considered. We also study the effects of surface singularities, e.g., wedges, on the thermodynamics of electric double layers near a rough colloid. Each wedge yields an additive contribution to the free energy that is a functionof the angle. A probabilistic Brownian representation of is given, which is entirely similar to that of vibration eigenmodes given by M. Kac long ago in “Canone hear the shape of a drum?” [Amer. Math. Monthly 73S, 1 (1966)]. The analysis yields a universal scaling law for the free energy of a rough colloid with its fractal Minkowski dimension.

Geometrical and topological aspects of electric double layers near curved surfaces.

The properties of electric double layers near closed-curved surfaces of arbitrary shape and genus are obtained exactly within the Debye-H\"uckel approximation by means of multiple-scattering expansions. Geometric and topological features of the electrostatics and thermodynamics emerge in a straightforward way through convergent expansions in powers of the ratio of the screening length to the principal radii of curvature. Some consequences of these results for the electrostatic contribution to the stability of structures of various shapes are considered.

Effect of concentration of surface acidic functional groups on electric double-layer properties of activated carbon fibers

The relationship between electric double-layer properties in an organic electrolytic solution and a concentration of surface acidic functional groups (surface acidity) of a phenolic resin-based activated carbon fiber (ACF) was characterized in a system of ACF/electrolytic solution/ACF The lower surface acidity of ACF resulted in the lower apparent leakage current of the electric double layer. The capacitance and the dc resistance of the layer were almost independent of the surface acidity. Further heat treatment of ACF in nitrogen atmosphere decreased the surface acidity of ACF. Types of C-O bonds in surface acidic functional groups were identified as carboxy and ester groups for ACF as-prepared and as phenolic hydroxy and quinone groups for ACF heat treated. The ACF obtained from the phenolic resin-based fibers showed an extremely low surface acidity in comparison with other kinds of ACF. The polarizable electrodes of the phenolic resin-based ACF resulted in electric double-layer capacitors with high capacitance and low leakage current.

Electrochemistry of a model for patchwise heterogeneous surfaces: The rutile-hematite system

The electrical double layer properties of aqueous suspensions of rutile and hematite and of mixtures of these two oxides have been studied. The surface charge σ 0 of the oxides and their mixtures as a function of pH in various KNO 3 concentrations and the isoelectric points (IEP) of rutile and hematite have been measured. From shifts of the points of zero charge (PZCs) and the IEPs of the pure oxides as a function of the KNO 3 concentration and by the analysis of the components of charge of the double layer it could be concluded that for both oxides specific adsorption of K + and NO − 3 occurs. The σ 0(pH) and the capacitance curves, C∗(pH), indicate that rutile and hematite behave similarly but not identically. Modeling of the σ 0(pH) and the C∗(pH) curves of both oxides with the one p K-Stern-Gouy-Chapman (SGC) model gives good results provided K + and NO − 3 are allowed to adsorb. The experimental σ 0(pH) and C∗(pH) data obtained for six different mixtures of rutile and hematite can be described as a weighed average of the individual component properties. This indicates that the mixtures can be treated as a patchwise heterogeneous system. Modeling of the mixtures on the basis of the one p K-SGC model extended to patchwise heterogeneity with the parameters obtained for the pure oxides results in a satisfactory description of σ 0(pH), C∗(pH), and the shift of a mixture PZC as a function of both the heterogeneity and the KNO 3 concentration. Qualitative agreement is found between the model predictions and the experimentally observed shift of the capacitance minimum.

Electric Double-Layer Properties of Carbon Films Formed by RF Sputtering of Glass-Like Carbon Target

Electric double-layer properties of a carbon film formed by RF sputtering of the glasslike carbon target were characterized. The target was obtained by carbonization of a phenolic resin-based plate material. A stable sputtering of the carbon target was performed at Ar gas pressure of 2.5×10-3-1.2×10-2 Torr to form carbon films on an etched aluminum foil substrate: the thickness of the foil was 35μm. Sputtering time and RF power were 15-180 min and 100-150 W, respectively. The rate of film formation was 0.1-0.2 μm/h at 100 W of sputtering power and film obtained had a carbon content of over 90 %. A conductive carbon film was obtained on the substrate with temperatures above 250°C. SEM observation of the film revealed that the carbon film consisted of ac-cumulated carbon particles with a number of small grains and the Ar gas pressure during sputtering significantry affected to the structure of the film: the higher Ar gas pressure resulted in an increase of the number of grains with smaller size. The capacitance of the electric double-layer formed between the carbon film and an organic electrolytic solution increased with increasing Ar gas pressure during sputtering.pressure during sputtering.

Electro adsorption, the electrical double layer and their relation to dimensional changes of carbon electrodes

Among the many features and properties of carbons and graphites, the electrical double layer and electroadsorption properties are the least known. Combined with the readily attainable high surface areas, and the “custom designed” pore system, these features suggest new and intriguing promises in 1. (1) understanding the electrochemistry of carbon and graphite surfaces; 2. (2) electrically controlled adsorption and separation processes; 3. (3) electrical energy storage and delivery. In the present communication the thermodynamics of the electrical double layer is briefly outlined. Specific attention is paid to the measurement of dimensional changes of the electrode which is proportional to the surface tension changes. Coupling this parameter with electrical measurements enables discrimination between the electrical double layer charge that resides at the interface and controls electroadsorption, and faradaic or electrochemical charge that crosses the interface and induces changes in the bulk. Unlike metallic interfaces which exhibit fast electrical double layer charging rates, there are slow sites on the surfaces of carbon with relaxation times in the range of few minutes. Since according to previous studies graphite electrodes acquire slow sites as well, at least part of these sites seems to originate at the graphitic side of the basal planes, and are not associated with chemical functional groups.

Modification of the Electric Double Layer Properties of the Interface Mica/Aqueous Solution by Adsorption of Polyelectrolytes

AbstractMeasurements of streaming potentials with an rectangular channel formed by two mica plates and spacers are used to study (a) the properties of the electric double layer at the interface mica/aqueous alkali chloride solution and (b) modifications of these properties by adsorption of polyelectrolytes (poly‐4‐vinyl‐pyridinium bromide, heparine, poly‐vinyl sulfate). The results obtained for the unmodified

Interfacial electrochemistry of haematite (α-Fe 2O 3): homodisperse and heterodisperse sols

A comparison is made between the properties of electrical double layers on classical, heterodisperse haematite prepared by neutralization of a ferric salt solution by the addition of base and on homodisperse sol of the same material precipitated from an acidified ferric salt solution at elevated temperatures. The consequences of heating and rehydration are also considered. Direct measurement of the surface charge, in conjunction with electrophoresis and streaming potential studies are followed by a thermodynamic analysis of the ionic components of charge. A detailed picture of the double layer composition is obtained. Homodisperse and classical sols have very similar surface electrochemical properties, but the homodisperse sols tend to be less prone to specific adsorption. Both sols have a pristine point of zero charge of 9.5 provided they are properly purified. Heating of the sols leads to dramatic changes in the surface structure.

Electrical double layer in solid electrolytes—silver ion conductors

Published data and the results of authors' investigations of the electrical double layer properties in silver ion conducting solid electrolytes are discussed. These are classical systems with point defects AgBr,AgCl (pure and doped), β-AgI and superionic conductors α-AgI and RbAg 4I 5. The comparative analysis of the results for the different systems is made. The double layer structure on graphite electrodes in crystals with Frenkel defects corresponds to the Helmholtz layer. In the case of metallic electrodes (Pt,Au) it was found that double layer properties are strongly influenced by the electrode nature; it means that the specific absorption of electrolyte ions plays the essential part. Some results may be satisfactorily explained in terms of the intrinsic surface disorder model.

Thermal and radiation control of the electrical double layer properties of silica and glass

High temperature or γ-radiation pretreatment of pyrex glass and vitreous silica has been found to change the isoelectric point of the oxide, and decrease, substantially, the magnitude of the zeta potential at any pH. The changes observed can be adequately explained in terms of a decrease in the number of surface hydroxyl groups existing on the metal oxide. Model calculations of the zeta potential as a function of the number of surface hydroxyl groups, indicate that the surface density of hydroxyl groups after pretreatment is of the order of less than one hydroxyl group per nm 2 This value is affirmed by independent contact angle measurements made on γ-irradiated quartz plates.

Electrical double layers. II. Monte Carlo and HNC studies of image effects

The effect of surface polarization (i.e., image forces) on the properties of electrical double layers is studied by means of Monte Carlo calculations on a primitive model electrolyte next to a planar charged surface bounding a semi-infinite dielectric. Two cases are considered, that of a conducting material for which an ion is attracted by its own image and that of an insulator for which the self-image force is repulsive. At low surface charge densities the image forces cause quite dramatic changes in the ionic densities near the wall; the effect on the electrostatic potential is small but increases with surface charge density. The modified Poisson–Boltzmann theory of Outhwaite is quite successful in describing the Monte Carlo results for the range of parameters studied. A screened self-image model of image effects is also considered for which both Monte Carlo calculations and numerical solution of the HNC equation have been obtained.

The electrical double-layer properties of the mica (muscovite)-aqueous electrolyte interface

Electrophoresis and streaming potential data are reported for crushed and sheet muscovite mica respectively. For streaming potential measurements a newly designed radial flow apparatus was used. Measurements on crushed mica show that both aluminium and silicon leach out of the mica. Leached aluminium and silicon may readsorb to confer increased positive and negative charge respectively on the mica. Electrophoresis data indicate that leaching of aluminium occurs more rapidly than of silicon. Aging experiments on sheet mica show leaching effects to be much slower than on crushed mica. Streaming potential measurements on freshly cleaved mica sheets showed that (i) the zeta- potential depended strongly on electrolyte (KCl) concentration; (ii) the zeta-potential was relatively independent of pH and (iii) monovalent cations were adsorbed in the sequence H+ > Cs+ > K+ > Na+ > Li+, whilst Ca2+ adsorbed more strongly than K+. It is proposed that the structure of the electrical double layer at the mica/electrolyte interface results from the distribution of all ions between the diffuse layer, the Stern plane (hydrated) and more critically the lattice holes of the silica-alumina basal plane.

Interfacial properties and equilibria in the apatite-aqueous solution system

Thermodynamic calculations of the solubility of hydroxyapatite have been carried out and the results are presented in different types of equilibrium diagrams in order to interpret observed interfacial behavior in this system. For predicting the electrical double-layer properties, especially the conditions for zero surface charge, diagram with activities of predominant calcium and phosphorous species as the axes and pH as the third independent variable have been found to be most useful. Conditions for zero charge at the surface are predicted based on the assumption that the affinity for the surface of all the lattice ions in solution is nearly equal. The different kinds of equilibrium diagrams have been found useful to explain some hitherto unexplained observations in the literature. Calculations have also been made to determine the effect of fluoride ions on the thermodynamic properties of the system. Experimental data are presented to support the calculations.

The adsorption of aqueous heavy metals on inorganic minerals

Two models have been used to explain the in-situ control of heavy metals: (1) solubility controls, where precipitation of a solid phase occurs under varying metal and ligand concentrations, and (2) surface chemical controls, where adsorption or exchange occurs at the solid/solution interface. Based on experiments presented in this paper, surface chemical controls can account for the removal of heavy metals, particularly Zn(II), from metal solutions which are undersaturated with respect to the hydroxide, oxide, or other controlling solid phases. Adsorption isotherms are presented for varying solution pHs, total metal ion concentrations, ionic strengths, and mineral substrates. The minerals chosen for illustration are SiO 2, TiO 2, FeOOH, Al 2O 3, MnO x and HgS which range widely in surface acidity, electrical double layer properties, specific surface areas, and surface functional groups.

Application of nonlinear a.c. methods in the investigation of the electrical double layer properties

Application of nonlinear a.c. methods in the investigation of the electrical double layer properties

Application of nonlinear A.C. methods in the investigation of the electrical double layer properties

This paper presents the results of an investigation of the electrical double layer properties for the systems Hg/NaF, Cd/NaF, Hg/Na 2SO 4 carried out by use of nonlinear a.c. methods, based on the quadratic approximation of the current–voltage characteristic of the cell, the two-frequency method, and the amplitude demodulation method. By use of these methods it is possible to measure the potentials of zero charge of metals at higher electrolyte concentrations than is possible with the cell impedance measurement method.

Adsorption of benzenesulphonate ion at the mercury aqueous solution interface

Adsorption of potassium benzenesulphonate (KBS) at the mercury electrode from mixed solutions containing an inert electrolyte (KF) of 0.3 and 1 mol l −1 ionic strength has been studied. Mixed electrolyte solutions investigated were of a composition: xM KBS+(0.3− x ) M KF and xM KBS+(1− x ) M KF. The results obtained show the influence of ionic strength on the adsorption process and on some electrical double layer properties. It is pointed out that specific adsorption of fluoride ion can only play a restricted rôle in the adsorption process of the benzene-sulphonate ion.

Adsorption of benzenesulphonate ion at the mercury aqueous solution interface

Summary Adsorption of potassium benzenesulphonate (KBS) at the mercury electrode from mixed solutions containing an inert electrolyte (KF) of 0.3 and 1 mol l−1 ionic strength has been studied. Mixed electrolyte solutions investigated were of a composition: xM KBS+(0.3−x) M KF and xM KBS+(1−x) M KF. The results obtained show the influence of ionic strength on the adsorption process and on some electrical double layer properties. It is pointed out that specific adsorption of fluoride ion can only play a restricted role in the adsorption process of the benzene-sulphonate ion.

Functional representation of properties of the electrical double layer around a spherical colloid particle

Functional representation of properties of the electrical double layer around a spherical colloid particle