Double Layer at the Pt(111)–Aqueous Electrolyte Interface: Potential of Zero Charge and Anomalous Gouy–Chapman Screening

We report, for the first time, the observation of a Gouy–Chapman capacitance minimum at the potential of zero charge of the Pt(111)‐aqueous perchlorate electrolyte interface. The potential of zero charge of 0.3 V vs. NHE agrees very well with earlier values obtained by different methods. The observation of the potential of zero charge of this interface requires a specific pH (pH 4) and anomalously low electrolyte concentrations (<10^?3 m ). By comparison to gold and mercury double‐layer data, we conclude that the diffuse double layer structure at the Pt(111)‐electrolyte interface deviates significantly from the Gouy–Chapman theory in the sense that the electrostatic screening is much better than predicted by purely electrostatic mean‐field Poisson–Boltzmann theory.     

The double layer at the interface between two immiscible electrolyte solutions: Part II. Structure of the water/nitrobenzene interface in the presence of 1:1 and 2:2 electrolytes

From fast galvanostatic pulse measurements at 25°C the capacitance of the water/nitrobenzene interface was evaluated as a function of the interfacial potential difference Δ_o^w? for systems consisting of NaBr, LiCl or MgSO₄ in water and tetrabutylammonium tetraphenylborate, tetraphenylarsonium tetraphenylborate or tetraphenylarsonium dicarbollylcobaltate in nitrobenzene. The modified Verwey—Niessen model, in which an inner layer of solvent molecules separates two space-charge regions (the diffuse double layer), describes the structure of the water/nitrobenzene interface well at electrolyte concentrations above ca. 0.02 mol dm^?3, provided that the ions are allowed to penetrate into the inner layer over some distance. For all the systems studied the zero-charge potential difference was found at Δ^w_o?_pzc ≈ 0 on the basis of the standard potential difference Δ^w_o?⁰_TMA + = 0.035 V for tetramethylammonium cation which was used as a reference ion. At zero surface charge a comparison was made with the theoretical capacitance calculated using the mean spherical approximation for a model consisting of two ion and dipole mixtures facing each other. The effect of ion penetration on the interfacial capacitance was estimated from the solution of the linearized Poisson-Boltzmann equation for a triple dielectric model with a continuous distribution of the point ions. The concentration-independent inner layer potential difference and capacitance can only be inferred from the capacitance data if the ion size effect is taken into account. A non-iterative procedure based on the hypernetted-chain equation was used for the evaluation of the potential drop across the diffuse double layer. The extend of the penetration into the inner layer appears to be a function of ion solvation, e.g. the more hydrated ion the less extensive ion penetration is likely.     

Description of <Emphasis Type="Italic">n</Emphasis>-butanol adsorption at the Hg/(H<Subscript>2</Subscript>O + NaF) interface in terms of the model of three parallel capacitors combined with the classical theory of diffuse layer

The curves of differential capacitance of the electrical double layer dense part in the Hg/(H₂O + NaF + n-C₄H₉OH) are calculated by using the equations of the model of three parallel capacitors, with the corresponding adsorption parameters fitted. The curves of full differential capacitance in the system are calculated, basing on the obtained results combined with the classical theory of the diffuse layer, for the following concentrations of the supporting electrolyte: 0.003, 0.01, 0.03, 0.1, 0.3, and 1 M. By using the curves’ regression analysis it is shown that they agree very well with the model of two parallel capacitors, when six effective adsorption parameters are appropriately selected (provided the linear potential dependence of the effective attraction constant is allowed for). The dependences of all model’s effective parameters on the NaF concentration are found; they correspond well to the analogous experimental dependence in the system under study.     

On the relationship between charge distribution, surface hydration, and the structure of the interface of metal hydroxides

The double layer structure of metal (hydr)oxides is discussed. Charge separation may exist between the minimum distance of approach of electrolyte ions and the DDL domain. The corresponding capacitance value of the outer Stern layer is similar to the capacitance value of the inner Stern layer. The extended Stern model implicitly supports a hydration structure at the near-surface with some discrete layering of water and electrolyte ions. The significance of dipole orientation is analyzed theoretically. Dipole theory in combination with a calculated ion charge distribution is compared with the experimental overall charge distribution. Ion charge distribution for various oxyanions has been calculated applying the Brown bond valence concept to the geometry of surface complexes that have been optimized with MO/DFT calculations. The comparison is done in detail for silicic acid adsorption on goethite. In addition, results are discussed for arsenite, carbonate, sulfate, and phosphate, using the same approach. The dipole correction depends on the charge introduced in a neutral surface by ion adsorption, which differs for the various ions studied. The fractional correction factor phi derived for the experimental data agrees with the theoretical value phi(m)=0.17+/-0.02. On an absolute scale, the dipole corrections are usually limited to the range about 0-0.15 v.u. The CD values calculated with MO/DFT are not particularly sensitive (approximately 0.03 v.u.) to the precise Fe-octahedral geometry, which suggests that a calculated CD is a reasonable approximation in ion adsorption modeling for ill-defined Fe-oxides like HFO and natural Fe oxide materials of soils.     

Electrochemistry Investigation on the Graphene/Electrolyte Interface

In this study, the graphene/electrolyte interface was investigated in detail by the electrochemical method. The total interface capacitance was calculated from the electrochemical impedance spectroscopy (EIS) results, and its responses to different concentrations and pHs were also investigated. The minimum point of the capacitance was found to shift to right (vs. Ag/AgCl) as the concentration increased, and to left with increasing pH. As a comparison, gold/electrolyte interface was investigated in the same way, but didn’t show the similar behavior. To further explore the interface capacitance, proper models were proposed to fit the EIS results. Two constant‐phase elements (CPE) were used in the graphene‐electrolyte interface model to represent the double‐layer capacitance (CPE_dl) and the quantum capacitance (CPE_q), respectively. The CPE_q exhibited its sensitivity to concentration and pH, while CPE_dl did not. This study gives a new aspect to detect concentration and pH, and may provide a reference for the graphene‐based supercapacitor research.     

Double Layer at the Pt(111)–Aqueous Electrolyte Interface: Potential of Zero Charge and Anomalous Gouy–Chapman Screening

We report, for the first time, the observation of a Gouy–Chapman capacitance minimum at the potential of zero charge of the Pt(111)-aqueous perchlorate electrolyte interface. The potential of zero charge of 0.3 V vs. NHE agrees very well with earlier values obtained by different methods. The observation of the potential of zero charge of this interface requires a specific pH (pH 4) and anomalously low electrolyte concentrations (<10⁻³ m ). By comparison to gold and mercury double-layer data, we conclude that the diffuse double layer structure at the Pt(111)-electrolyte interface deviates significantly from the Gouy–Chapman theory in the sense that the electrostatic screening is much better than predicted by purely electrostatic mean-field Poisson–Boltzmann theory.     

Electrical double layers at the oil/water interface

This review presents the historical development and current status of the theory of the electrical double layer at a liquid/liquid interface. It gives rigorous thermodynamic definitions of all basic concepts related to liquid interfaces and to the electrical double layer. The difference between the surface of a solid electrode and the interface of two immiscible electrolyte solutions (ITIES) is analyzed in connection to their electrical properties. The most important classical relationships for the electrical double layer are presented and critically discussed. The generalized adsorption isotherm is derived. After a short review of the classical Gouy-Chapman and Verwey-Niessen models, more recent developments of the double layer theory are presented. These include effects of variable dielectric permittivity, nonlocal electrostatics, hydration forces, the modified Poisson-Boltzmann equation and the ion-dipole plasma. The relative merits of different theories are estimated by comparing them with computer simulation of the ITIES and electrical double layer. Special attention is given to the structure of ITIES and its variation due to adsorption of ions and amphiphilic molecules.     

Electrochemistry,ion adsorption and dynamics in the double layer: a study of NaCl(aq) on graphite

Graphite and related sp² carbons are ubiquitous electrode materials with particular promise for use in e.g., energy storage and desalination devices, but very little is known about the properties of the carbon–electrolyte double layer at technologically relevant concentrations. Here, the (electrified) graphite–NaCl(aq) interface was examined using constant chemical potential molecular dynamics (CμMD) simulations; this approach avoids ion depletion (due to surface adsorption) and maintains a constant concentration, electroneutral bulk solution beyond the surface. Specific Na⁺ adsorption at the graphite basal surface causes charging of the interface in the absence of an applied potential. At moderate bulk concentrations, this leads to accumulation of counter-ions in a diffuse layer to balance the effective surface charge, consistent with established models of the electrical double layer. Beyond ∼0.6 M, however, a combination of over-screening and ion crowding in the double layer results in alternating compact layers of charge density perpendicular to the interface. The transition to this regime is marked by an increasing double layer size and anomalous negative shifts to the potential of zero charge with incremental changes to the bulk concentration. Our observations are supported by changes to the position of the differential capacitance minimum measured by electrochemical impedance spectroscopy, and are explained in terms of the screening behaviour and asymmetric ion adsorption. Furthermore, a striking level of agreement between the differential capacitance from solution evaluated in simulations and measured in experiments allows us to critically assess electrochemical capacitance measurements which have previously been considered to report simply on the density of states of the graphite material at the potential of zero charge. Our work shows that the solution side of the double layer provides the more dominant contribution to the overall measured capacitance. Finally, ion crowding at the highest concentrations (beyond ∼5 M) leads to the formation of liquid-like NaCl clusters confined to highly non-ideal regions of the double layer, where ion diffusion is up to five times slower than in the bulk. The implications of changes to the speciation of ions on reactive events in the double layer are discussed.

CμMD reveals multi-layer electrolyte screening in the double layer beyond 0.6 M, which affects ion activities, speciation and mobility; asymmetric charge screening explains concentration dependent changes to electrochemical properties.     

The modelling of solvent structure in the electrical double layer

A Civilized Model electrolyte is one in which the ions and solvent molecules are regarded as distinct molecular species and treated on an equal basis. Recent efforts to use a Civilized Model to study the effects of solvent structure in the properties of the electrical double layer are discussed. By modelling the electrolyte as a simple ion-dipole mixture, it is possible to gain valuable insight in areas such as: 1) the successes and limitations of the Gouy-Chapman-Stern picture; 2) the derivation as opposed to a postulation of the Stern layer; 3) the influence of the charged surface on the magnitudes of the apparent Stern capacities (e.g. the “low” capacitances of the mercury/solution interface vs. the “high” capacitance of inorganic oxides); 4) the effect of solvent structure on the potential profile in the diffuse layer; 5) the interpretation of the electrokinetic potential; and 6) the role of solvent orientation on the x potential.     

A new surface structural approach to ion adsorption: tracing the location of electrolyte ions

Electrolyte ions differ in size leading to the possibility that the distance of closest approach to a charged surface differs for different ions. So far, ions bound as outersphere complexes have been treated as point charges present at one or two electrostatic plane(s). However, in a multicomponent system, each electrolyte ion may have its own distance of approach and corresponding electrostatic plane with an ion-specific capacitance. It is preferable to make the capacitance of the compact part of the double layer a general characteristic of the solid-solution interface. A new surface structural approach is presented that may account for variation in size of electrolyte ions. In this approach, the location of the charge of the outersphere surface complexes is described using the concept of charge distribution in which the ion charge is allowed to be distributed over two electrostatic planes. It was shown that the concept can successfully describe the pH dependent proton binding and the shift in the isoelectric point (IEP) in the presence of variety of monovalent electrolyte ions, including Li(+), Na(+), K(+), Cs(+), Cl(-), NO(-)(3), and ClO(-)(4) with a common set of parameters. The new concept also sheds more light on the degree of hydration of the ions when present as outersphere complexes. Interpretation of the charge distribution values obtained shows that Cl(-) ions are located relatively close to the surface. The large alkali ions K(+), Cs(+), and Rb(+) are at the largest distance. Li(+), Na(+), NO(-)(3), and ClO(-)(4) are present at intermediate positions.     

Electrochemical impedance spectroscopy of mesoporous Al-stabilized TiO<Subscript>2</Subscript> (anatase) in aprotic medium

High-frequency electrochemical impedance spectroscopy was used to investigate the mesoporous film of Al-stabilized TiO₂ on F-doped SnO₂ support in 1 M Li(CF₃SO₂)₂N in ethylene carbonate/dimethoxyethane (1:1 v/v). Kinetic parameters, viz. charge transfer resistance and chemical diffusion coefficient, were determined. Charge transfer resistance increased with time of contact of electrode in the above aprotic electrolyte solution. The increase followed exponential dependence, whereas the double layer capacitance, simultaneously, decreased exponentially with time. These effects were discussed in terms of the solid–electrolyte interface, which undergoes chemical changes upon contact with the electrolyte solution. Adel Attia is currently on leave from the Department of Physical Chemistry, National Research Center, El-Tahrir St., Dokki 12622, Cairo, Egypt.     

On the effects of ion-wall chemical association on the electric double layer: a density functional approach for the restricted primitive model at a charged wall

We describe a density functional theory for the restricted primitive model of ionic fluid at a charged wall with active sites to which ions can bond. The theory is an extension of our recent approach [Pizio et al., J. Chem. Phys. 121, 11957 (2004)] and is focused in the effects of specific adsorption of ions on the wall, besides the electrostatic phenomena. In order to solve the problem, we use the first-order thermodynamic perturbation theory of chemical association developed by Wertheim [J. Chem. Phys. 87, 7323 (1987)]. The microscopic structure of the electric double layer and the amount of adsorbed charge are investigated. Also, the temperature dependence of capacitance is analyzed. The capacitance depends on the kind of ions that form associative bonds with the surface sites and is determined by a net charge acting on the diffuse layer. The shape of the temperature dependence of capacitance essentially depends on the association energy and the density of bonding sites.     

Monte-Carlo simulation of mixed electrolytes next to a plane charged surface

Monte-Carlo simulations of the electric double layer are performed for two electrolyte mixtures next to a plane, uniformly charged, surface. Simulations are made at parameters corresponding to a Poisson-Boltzmann theory which is corrected to include the excluded volume effects of the ions. The corrected Poisson-Boltzmann theory is found to have some deficiencies. The structural properties disagree with simulation results while the theory can be adjusted to give agreement with experiment for the relative concentration excesses of small univalent counterions.     

The capacity versus potential dependence of the electrical double layer of the mercury/electrolyte solution of the dicarboxylic ions

The effect of dicarboxylic ions on the capacity hump was investigated by measuring the double layer capacity of the mercury/water electrolyte solution interface. Further experimental evidence was found of the internal rotation effect of the molecule in the double layer electric field. When discussing the effect of dicarboxylic ions on the capacity hump, it is assumed that the latter is due not only to dipole-ion, dipole-electrode and electrode-ion interactions but moreover occurs as the result of intra-ionic electrostatic interaction and the ability of the ion/or its fragment/to reorient in the electric field of the interface.     

A local density functional approximation for the ion distribution near a charged electrode in the restricted primitive model electrolyte

A local HNC/MSA approximation is developed and applied to the 1:1 restricted primitive model electrolyte. Improvement of ion density profiles in front of a charged electrode is achieved by employing ion—ion direct correlation functions from homogeneous systems of non-neutral composition as found locally in the inhomogeneous double layer. This approximation is related to the density functional approach for inhomogeneous fluids. The local HNC/MSA method predicts, at higher surface charges, layering of counterions and charge inversion as seen in Monte Carlo data, a strong increase of the surface potential with charging and a maximum in the double layer capacitance.     

Differential capacitance of ionic liquid interface with graphite: the story of two double layers

We combine electronic density functional theory for the screening properties of graphite with a mean-field theory of the double layer in ionic liquids to reveal what underpins the morphology of the voltage dependence of electrical capacitance of a flat graphite/ionic liquids interface.     

Potential dependent organization of water at the electrified metal-liquid interface

In situ infrared visible sum frequency generation spectroscopy (SFG) is used to examine the structure of water at the Ag-water interface in NaF and KF electrolyte solutions. Water is observed in environments associated with both the electrode surface and the diffuse double layer. Peaks are observed that are correlated with low-order water, water interacting with electrolyte ions, specifically adsorbed water to the electrode surface, and hydronium. Spectra obtained from a thiol-modified Ag surface enabled discrimination between surface-bound water and that in the double layer. The water organization is dependent on applied potential, with the observed intensities for specifically adsorbed and ion solvating water diminishing near the pzc.     

Ionic liquid near a charged wall: structure and capacitance of electrical double layer

We study the effects of ion size asymmetry and short-range correlations on the electrical double layer in ionic liquids: we perform molecular dynamics simulations of a model ionic liquid between two "electrodes" and calculate the differential capacitance of each as a function of the electrode potential. The capacitance curve has an asymmetric "bell-shape" character, in qualitative agreement with recent experiments and the mean- field theory (MFT) which takes into account the limitation on the maximal local density of ions. The short-range ionic correlations, not included in the MFT, lead to an overscreening effect which changes radically the structure of the double layer at small and moderate charging. With the radius of cations taken to be twice as large as anions, the position of the main capacitance maximum is shifted positively from the potential of zero charge (PZC), as predicted by MFT. An extension of the theory (EMFT), however, reproduces the simulated capacitance curve almost quantitatively. Capacitance curves for real ionic liquids will be affected by nonspherical shape of ions and sophisticated pair potentials, varying from liquid to liquid. But understanding the capacitance behavior of such model system is a basis for rationalizing those more specific features.