An electrochemical study into the interaction between complement-derived peptides and DOPC mono- and bilayers

Electrochemical methods employing the hanging mercury drop electrode were used to study the interaction between variants of the complement-derived antimicrobial peptide CNY21 (CNYITELRRQH ARASHLGLAR) and dioleoyl phosphatidylcholine (DOPC) monolayers. Capacitance potential and impedance measurements showed that the CNY21 analogues investigated interact with DOPC monolayers coating the mercury drop. Increasing the peptide hydrophobicity by substituting the two histidine residues with leucine resulted in a deeper peptide penetration into the hydrophobic region of the DOPC monolayer, indicated by an increase in the dielectric constant of the lipid monolayer (Deltaepsilon = 2.0 after 15 min interaction). Increasing the peptide net charge from +3 to +5 by replacing the histidines by lysines, on the other hand, arrests the peptide in the lipid head group region. Reduction of electroactive ions (Tl+, Pb2+, Cd2+, and Eu3+) at the monolayer-coated electrode was employed to further characterize the types of defects induced by the peptides. All peptides studied permeabilize the monolayer to Tl+ to an appreciable extent, but this effect is more pronounced for the more hydrophobic peptide (CNY21L), which also allows penetration of larger ions and ions of higher valency. The results for the various ions indicate that charge repulsion rather than ion size is the determining factor for cation penetration through peptide-induced defects in the DOPC monolayer. The effects obtained for monolayers were compared to results obtained with bilayers from liposome leakage and circular dichroism studies for unilamellar DOPC vesicles, and in situ ellipsometry for supported DOPC bilayers. Trends in peptide-induced liposome leakage were similar to peptide effects on electrochemical impedance and permeability of electroactive ions for the monolayer system, demonstrating that formation of transmembrane pores alone does not constitute the mechanism of action for the peptides investigated. Instead, our results point to the importance of local packing defects in the lipid membrane in close proximity to the adsorbed peptide molecules.     

Solvent dependent dimercaptothiadiazole monolayers on gold electrode for the simultaneous determination of uric acid and ascorbic acid

Dimercaptothiadiazole compound, 2,5-dimercapto-1,3,4-thiadiazole (DMcT) forms ‘thin’ monolayers on Au electrode when it was adsorbed from methanol, ethanol or DMSO solutions while it forms ‘thick’ layers on Au electrode from an aqueous solution under identical experimental conditions. Thick DMcT layers formed from aqueous solution effectively blocks the redox reaction of couple in contrast to thin DMcT monolayers. The monolayer thickness did not vary when structurally related DMcT compounds, 5-methyl-1,3,4-thiadiazole-2-thiol or 5-amino-1,3,4-thiadiazole-2-thiol was adsorbed from aqueous and non-aqueous solutions. This indicates that the presence of two thiol groups in DMcT plays a crucial role in the formation of thick and thin DMcT layers on Au electrode when it was adsorbed from aqueous and non-aqueous solutions. Methanol, ethanol, or DMSO solution of DMcT is considered as strong acid because these solvents are able to deprotonate DMcT into DMcT⁻ and thus thin monolayers formed on Au electrode. The deprotonating ability of these solvents was further verified from the observed absorption spectrum characteristic of DMcT⁻ species. On the other hand, an aqueous solution of DMcT is less acidic due to weak deprotonation of DMcT by water and thus DMcT forms thick layer on Au electrode. Interestingly, thin DMcT monolayers formed from non-aqueous solvents separates the voltammetric signals of uric acid and ascorbic acid while thick DMcT layers formed from aqueous solution fails to separate them.     

Square-wave voltammetry of 5-fluorouracil

The redox reaction of 5-fluorouracil (FU) at a hanging mercury drop electrode (HMDE) is studied by means of square-wave voltammetry (SWV). It is demonstrated that the redox reaction proceeds according to the scheme: L²⁻(aq)L²⁻(ads)+Hg(l)HgL(s)+2e⁻, which involves both chemisorption of FU on the electrode surface and creation of a sparingly soluble compound with the electrode material. The overall response exhibits properties of a surface process in which both the reactant and the product of the redox reaction are immobilized on the electrode surface. The square-wave voltammetric response of FU possesses features typical of surface confined processes such as ‘split SW peaks’ and a ‘quasi-reversible maximum’. The proposed electrode mechanism is studied theoretically. The numerically calculated response under conditions of SWV is in qualitative agreement with the experimental data. Comparing the theoretical and the experimental data, the kinetic parameters of the redox reaction investigated are estimated. The standard rate constant appears to be within the interval 54≤k_s/s⁻¹≤108, the adsorption constant is K=10 cm⁻¹, and the transfer coefficient is α=0.54±0.01. The effect of the Cu(II) ions on the adsorptive SWV response of FU is discussed from an analytical point of view. It is demonstrated that SWV is a particularly appealing technique, which enables determination of FU at an ultra-trace concentration level. A linear calibration plot was established at 10⁻¹¹ mol l⁻¹ concentration level with a correlation coefficient of R²=0.992. The detection limit is 7.7×10⁻¹² mol l⁻¹. The reproducibility of the results in terms of the relative standard deviation ranges from 0.9 to 3.2%.     

Adsorption of ethyl xanthate anions on a silver electrode. An in-situ FTIR study

The adsorption of ethyl xanthatc ion (EX⁻) on a silver electrode in sodium sulphate, phosphate buffer (pH 6.8) and borate buffer (pH 9.2) solutions was studied in situ using Fourier transform IR reflection spectroscopy. The measurements were carried out using a thin-layer flow cell, which allows a continuous supply of the electroactive species into the thin layer. The voltammetric behaviour of the silver electrode in EX⁻-containing solution is characterized by the formation of silver ethyl xanthate (AgEX), which is preceded by a prewave due to chemisorption of ethyl xanthate. An IR band at 1220 cm ⁻¹, assigned to chemisorbed ethyl xanthate, was observed in spectra obtained in the chemisorption region. All IR bands characteristic of AgEX were observed in spectra of a silver electrode surface covered by a few monolayers of AgEX. The in-situ IR measurements also showed that the adsorption of EX⁻ starts directly on the positive side of the potential of zero charge of silver (−0.7 V/SHE).     

Photocurrent kinetics at the electron emission from metal into electrolyte solution X. Discharge of short-lived intermediate species

Electrode reactions of intermediate species (IS), generated by a short pulse of laser photoemission (LPE), result in the time-dependent change of emitted charge Q(t). Analytical expressions for the kinetic curves Q(t) are derived by solving non-stationary diffusion equations for e⁻_aq and IS. For the IS adsorption Gibbs energy less than −25 kJ mol ⁻¹, kinetic curves are exponential over the very wide range of electrode reaction rate constant W, from 1 up to 10⁷ s⁻¹. The dependence Q(t) ∝ t⁻¹² is typical for the case of activated adsorption of IS or their discharge from the non-adsorbed state. Voltammograms of IS generated by pulse radiolysis, modulated photolysis and pulsed or alternating photoemission current are demonstrated to be described by similar expressions. The difference between half-wave and equilibrium potentials depends on the reactant and product lifetimes and rates of desorption. A characteristic trapezoid of Tafel lines is introduced as a new way to characterize completely the kinetics of two-electron electrode reactions. The relations obtained were applied to the analysis of hydrogen evolution reactions and carbon dioxide and formaldehyde reduction, where hydrogen atoms and organic radicals HCO₂ and CH₂OH adsorbed on a mercury electrode participate as IS.     

Metallic and non-metallic redox response of conducting polymers

The potentiometric response of electrodes coated with polypyrrole or poly(N-methylpyrrole) films with different doping anions was studied in solutions containing the redox couples: Fe(CN)₆^3−/4−, Ru(NH₃)₆^3+/2+ and Fe(Ill)/Fe(II). The stable potential measured with the electrodes was the potential of the redox couple. The response time was instant for polypyrrole doped with dodecylsulphate ions, PPy(DS) and slow for the polymers doped with mobile anions. On the basis of electrochemical measurements and chemical analysis by EDAX spectroscopy it was found that with the PPy(DS) electrode the potentiometric response was of the ‘metallic’ type, with no change in the oxidation state of the bulk polymer. With the other polymer systems studied reduction or oxidation of the polymer bulk took place when it was in contact with a redox couple in the solution.     

Voltammetric quantification and speciation of mercury compounds

The use of a carbon paste electrode modified with a thiolic resin for the determination of inorganic mercury and organomercury compounds, present simultaneously in a sample, is described. The compounds are first preconcentrated at the electrode surface by means of a purely chemical reaction with the modifier on the electrode surface. The high affinity of the modifier for the mercury compounds ensures low limits of detection and determination. Differentiation between several mercury species is possible by control of the reduction potential applied to the working electrode. This selective reduction results in the formation of atomic mercury at the electrode surface which can be determined with a very high sensitivity by means of its re-oxidation wave in cyclic voltammetry. Optimization of the instrumental parameters and evidence for the reduction processes are discussed. Analysis of inorganic mercury in the presence of methylmercury, with a detection limit of 4 μg Hg 1⁻¹, and of methylmercury in the presence of inorganic mercury, with a detection limit of 2 μg Hg 1⁻¹, is described in detail. In both cases the preconcentration time is 6 min. Other organomercury species can also be quantified. Application of the method to environmental aquatic samples is discussed.     

Application of electrochemical impedance spectroscopy to the study of dioleoyl phosphatidylcholine monolayers on mercury

Electrochemical impedance spectroscopy has been applied to the analysis of the behavior of monolayers of dioleoyl phosphatidylcholine (DOPC) on a mercury electrode. Experiments were carried out in electrolytes KCl and NaCl (0.1 mol dm(-3)) and Mg(NO3)2 (0.05 mol dm(-3)), and the frequency dependence of the complex impedance was estimated between 65 000 and 0.1 Hz at potentials -0.4 to -1.5 V versus Ag/AgCl 3.5 mol dm(-3) KCl at uncoated and coated electrode surfaces. Experiments were also carried out in the presence of gramicidin A (gA). Between the potentials of -0.4 and -0.7 V, the DOPC monolayer behaves as an almost ideal capacitor with little frequency dispersion. At more negative potentials, the impedance data show the formation of defects (-0.7 to -0.85 V), ingression of electrolyte into the layer (capacitance peak approximately -0.935 V), reorientation of phospholipid-water structures (capacitance peak approximately -1.0 V), and initiation of phospholipid desorption (approximately -1.3 V). gA interaction with the phospholipid monolayer at -0.4 V is shown as an additional low-frequency element. A general "one capacitor model" in a RC series equivalent circuit is developed incorporating the frequency dispersion of the capacitance, distribution of the time constants of the dispersion, and a coefficient related to the interface between the solution and the coated electrode. This latter coefficient is the most robust and decreases at potentials approaching those coincident with the DOPC phase transitions.     

Alkanethiol monolayers at reduced and oxidized zinc surfaces with corrosion protection: a sum frequency generation and electrochemistry investigation

In this work, octadecanethiol (ODT) was demonstrated to form ordered monolayers at either electrochemically reduced or oxidized Zn surfaces, by means of sum frequency generation (SFG) spectroscopy, cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS). The SFG spectra of ODT-modified Zn electrodes featured three methyl group resonances in the C-H vibrational region (2800-3100 cm(-1)). A significant decrease in interfacial capacitance and an increase in charge-transfer resistance were observed in EIS measurement after ODT modification. The alkane chain tilt angle of ODT within a monolayer at the Zn surface was estimated as 0 degrees with respect to the surface normal by interfacial capacitance measurement via EIS. CV and SFG investigation revealed that ODT monolayers undergo reductive desorption from the Zn electrode in 0.5 M NaOH at -1.66 V (vs SCE) and in 0.5 M NaClO4 at -1.62 V. The integrated charge consumed to the desorption of ODT is determined as 87 mC/cm2 from the reductive peak on CV curve, resulting in a coverage of 9.0 x 10(-10) mol/cm2 (5.4 x 10(14) molecules/cm2) if assuming the reduction follows a one-electron process. ODT monolayers show corrosion protection to underlying zinc at the early immersion stage in base, salt, and acid media. However, the protection efficiency was reduced with immersion time due to the presence of defects within the monolayers.     

Breaking the barrier to fast electron transfer

A study of the electron transfer for a non-glycosylated redox variant of GOx is reported, immobilised onto an electrode via a polyhistidine tag. The non-glycosylated variant allows the enzyme to be brought closer to the electrode, and within charge transfer distances predicted by Marcus' theory. The enzyme-electrode-hybrid shows direct very fast reversible electrochemical electron transfer, with a rate constant of ~ 350 s^− 1 under anaerobic conditions. This is 2 orders of magnitude faster than the enzyme-free flavin adenine dinucleotide (FAD). These results are discussed in the context of the conformation of FAD in the active site of GOx. Further data, presented in the presence of oxygen, show a reduced electron transfer rate (~ 160 s^− 1) that may be associated with the oxygen interaction with the histidines in the active site. These residues are implicated in the proton transfer mechanism and thus suggest that the presence of oxygen may have a profound effect in attenuating the direct electron transfer rate and thus moderating ‘short-circuit’ incidental electron transfer between proteins.     

MD studies of electrolyte solution|liquid mercury interfaces

We report molecular dynamics (MD) computer simulations of a single lithium or iodide ion near a water|liquid mercury interface. The ion–mercury and the water–mercury potentials are derived from ab initio calculations of an ion or a water molecule and a mercury cluster consisting of seven, nine or 10 atoms. The flexible BJH water model and a mercury–mercury potential derived from pseudopotential theory are employed. The ion–water potentials are also based on ab initio calculations. The structural properties at the interfaces are described in terms of various density profiles and the ion–mercury radial distribution functions (RDF). An analysis of the induced rearrangements of the mercury atoms at and below the surface is also performed. Finally, the spectral densities of the hindered translational motions of the ions parallel and perpendicular to the mercury surface are reported. We conclude that, while the I⁻-ion is contact adsorbed on the mercury surface, the Li⁺-ion is not.     

Novel lipoate-selective membrane sensor for the flow injection determination of alpha-lipoic acid in pharmaceutical preparations and urine

A potentiometric lipoate-selective sensor based on mercuric lipoate ion-pair as a membrane carrier is reported. The electrode was prepared by coating the membrane solution containing PVC, plasticizer, and carrier on the surface of graphite electrode. Influences of the membrane composition, pH, and possible interfering anions were investigated on the response properties of the electrode. The sensor exhibits significantly enhanced response toward lipoate ions over the concentration range 1 × 10⁻⁷ mol L⁻¹ to 1 × 10⁻² mol L⁻¹ with a lower detection limit of (LDL) of 9 × 10⁻⁸ mol L⁻¹ and a slope of −29.4 mV decade⁻¹, with S.D. of the slope is 0.214 mV. Fast and stable response, good reproducibility, long-term stability, applicability over a pH range of 8.0–9.5 is demonstrated. The sensor has a response time of ≤12 s and can be used for at least 6 weeks without any considerable divergence in its potential response. The proposed electrode shows good discrimination of lipoate from several inorganic and organic anions. The CGE was used in flow injection potentiometry (FIP) and resulted in well defined peaks for lipoate ions with stable baseline, excellent reproducibility and reasonable sampling rate of 30 injections per hour. The proposed sensor has been applied for the direct and FI potentiometric determination of LA in pharmaceutical preparations and urine; and has been also utilized as an indicator electrode for the potentiometric titration of LA.     

Electrolyses of organic compounds with microelectrodes with renewable surfaces: Comparison of dropping mercury and mercury pool electrodes

Controlled potential electrolysis with a dropping mercury electrode (DME), which is a microelectrode with a periodically renewed surface, is usually carried out in small volumes (0.5–1.0 ml) of diluted solutions (10⁻³–10⁻⁴M). Such electrolysis can be used for identification of products and determination of their yields in the elucidation of mechanisms of polarographic reduction of organic compounds. Controlled potential electrolysis with a mercury pool electrode, which is a large-scale electrode with a surface that is not renewed, is usually used in coulometry and preparative electrolysis of larger volumes (10–100 ml) of more concentrated (10⁻²–10⁻³M) solutions, when the goal is preparation of a larger amount of electrolysis product. Identification of products and determination of yields when the DME is used involves microchemical techniques. Frequently, information obtained by the DME and Hg pool electrolysis is similar. In some instances the nature of products formed, the number of electrons transferred, and the shape of current-voltage curves obtained by the micro- and macroelectrodes differ. Such differences can involve a number of waves, their potentials, the ratio of wave heights, and shapes of the waves. These dissimilarities can be caused by differences in time scale, reaction conditions, adsorption and other interactions at the electrode surface, concentration, and pH control. Concentration changes of starting material, intermediates, and products in the course of electrolysis can be followed spectrophotometrically or using polarographic current-voltage curves. Time dependence of current-voltage curves in the course of electrolysis can indicate the role of chemical reactions accompanying the electrolysis. Comparison of current-voltage curves and the nature of electrolysis products obtained with micro- and macroelectrodes can contribute to a better understanding of electrode processes and to development of better analytical methods.     

Self-assembled monolayers and electrostatically self-assembled multilayers of polyalkylviologens on sulfonate-modified gold and indium–tin–oxide electrodes

Self-assembly (SA) of polyviologens with linear alkyl spacers PVn (n = 3–10) on sulfonate-primed gold and ITO electrode surfaces has been investigated by CV, QCM, XPS and AFM. The polymers form stable and relatively dense monolayers even from 10⁻⁴ M PVn concentration. The viologen surface density is decreased as n is increased but for n = 10 and on alkylsulfonate-primed surfaces a strong adsorption takes place, with decreased redox potential and very narrow cyclic voltammogram of the absorbed layer due to organized structures on the surface.Stable and regular electrostatically self-assembled (ESA) multilayers are built on sulfonate-primed surfaces with PV3 and polysulfonates. From XPS analysis, the composition of the PVn/polysulfonate multilayers corresponds to a 1:1 ratio of polyanion and polycation charges, i.e., no extra non-polymeric ion is present, independently from the alkane chain length. The rate of electron transfer within the multilayers (diffusion coefficient = ca. 10⁻⁹ cm² s⁻¹) is higher than for analogous bulk materials.     

Ellipsometric and microwave reflectivity studies of current oscillations during anodic dissolution of p-Si in fluoride solutions

Periodic current oscillations during anodic dissolution of monocrystalline p-Si(100) in buffered ammonium fluoride solutions (0.1 mol dm⁻³ fluoride, pH 4.5) were investigated using a flow cell in order to eliminate mass transport limitations. The flow cell was designed to permit simultaneous in-situ ellipsometry, impedance and potential modulated microwave reflectivity measurements. Analysis of the ellipsometric response showed that the current oscillations are accompanied by a synchronous variation of the overall oxide thickness with an amplitude of 4.5±0.1 nm. Analysis of the relationship between the total oxide thickness and the current during the oscillation cycle shows that to a first approximation the rate of chemical dissolution of anodic oxide remains constant. Oscillations of the electrode admittance and potential modulated microwave reflectivity were also measured. The imaginary component of the admittance is related to the oscillation in thickness of a narrow inner region of ‘dry’ oxide and to changes in the accumulation capacitance. The oscillation in the potential modulated microwave reflectivity is interpreted in terms of the changes in the density of holes accumulated at the p-Si SiO₂ interface.     

XPS investigations of the electrochemical double layer on silver in alkaline chloride solutions

The electrochemical double layer on Ag in alkaline NaCl solutions was examined ex situ with X-ray photoelectron spectroscopy (XPS). The specimens were removed from the electrolyte with hydrophobic surfaces and under potential control. The potential dependent surface concentrations of the adsorbed anions (Cl⁻, OH⁻), cations (Na⁺), the surface excess charge and the amount of adsorbed water were determined and compared to the results obtained for acidic NaCl solutions. The distinct differeness found between both electrolytes were discussed in terms of a specific adsorption of hydroxide ions in the basic Cl⁻-electrolyte; i.e., the OH⁻-surface concentration has to be considered for a proper determination of the cationic excess charge and the potential of zero charge. In addition, the initial stages of silver (1) oxide formation were examined with XPS.     

Influence of gel-phase microdomains and lipid rafts in lipid monolayers on the electron transfer of a lipophilic redox probe: dioctadecylviologen

Gel-phase microdomains and lipid rafts form spontaneously in monolayers of lipid mixtures of dioleoylphosphatidylcholine (DOPC), palmitoylsphingomyelin (PSM) and cholesterol (Chol), self-assembled on mercury. The influence of microdomains on the electron transfer properties of 2 mol% dioctadecylviologen (DODV), incorporated in these lipid monolayers, was investigated by cyclic voltammetry. In pure DOPC, the DODV molecules tend to aggregate, giving rise to strong attractive lateral interactions. With an increase in the PSM mole fraction in DOPC/PSM binary mixtures, the edges of the resulting gel-phase microdomains act as docking sites for the DODV molecules, decreasing lateral interactions and modifying the DODV redox properties. A similar behavior is shown by lipid rafts formed by adding Chol to the above binary mixtures. By varying the DOPC/PSM molar ratio, the midpoint between the peak potentials of the DODV reduction and oxidation peaks shifts in parallel with the surface dipole potential of the lipid mixture. This behavior indicates that the formal (half-reduction) potential of a redox pair, as measured versus a given reference electrode, may include a surface dipole potential if one or both members of the redox pair are embedded in a medium different from the bulk phase containing the reference electrode.     

Effect of various additives on the performance of a newly developed PVC based potentiometric sensor for anionic surfactants

A new poly(vinyl chloride) PVC membrane electrode to determine monomer concentrations of dodecylbenzenesulphonate ions (DBS⁻) based on a neutral ion-pair carrier complex of dodecyltrimethylammonium–dodecylbenzenesulphonate (DTA⁺–DBS⁻), is reported here. The electrode exhibits a slope of 51.25 mV per decade for DBS⁻ ions. The DBS⁻ ion selective electrode (ISE) can determine monomer units down to concentrations as low as 3.32 × 10⁻⁴ M. The effect of three kinds of additives, i.e. alcohols, glycols and triblock polymers on the performance of the surfactant selective electrode is studied systematically. The effect of foreign anions along with primary ions on the performance of ion-selective electrode is investigated in terms of potentiometric selectivity coefficients, which were determined using the fixed interference method (FIM) at 1.0 × 10⁻² M concentration of foreign anions. The sensor responds well to the surfactant ions in the presence of additives at lower concentration. The Gibbs free energy of micelle formation (ΔG_m) of sodium dodecylbenzenesulphonate (SDBS) in the presence of various additives is calculated and found to vary differently with respect to the increase in the amount of additives. The sensor worked in the acidic pH range with a short response time of 30 s. The lifetime of the sensor is more than three months. The sensor was further used to determine the amount of DBS⁻ in local detergents. This method of determining anionic surfactants was found to be quite accurate when compared with classical methods.     

Application of Voltammetric Method of Total Chromium Determination in the Presence of Cupferron for Selective Determination of Cr(VI) in Water Samples

A voltammetric method of Cr(VI) determination in a flow system based on the combination of selective accumulation of the product of Cr(VI) reduction on hanging mercury drop electrode and a very sensitive method of chromium determination in the presence of cupferron previously described is proposed. The calibration graphs were linear from 3 × 10⁻⁹ to 3 × 10⁻⁸ and from 5 × 10⁻¹⁰ to 5 × 10⁻⁹ mol L⁻¹ for accumulation times of 120 and 600 s, respectively. The detection limit for the accumulation time of 600 s was 9 × 10⁻¹¹ mol L⁻¹. The relative standard deviation was 5.1% (n = 5) for Cr(VI) concentration 1 × 10⁻⁸ mol L⁻¹ and the accumulation time of 120 s. The influence of foreign ions commonly present in water samples is presented. The validation of the method was made by studying the recovery of Cr(VI) from spiked natural water samples.     

Silver-water interactions: Part I. Model of the inner layer at the metal/water interface

Silver-water interactions, as expressed by the surface potential of the water molecules and the modification of the surface potential of the metal, are determined, at the potential of zero charge, as a function of the superficial structure of the electrode by means of the two more significant approaches, that based on the potential of zero charge-work function relation and that using the differential capacity of the inner-layer and its different components. The surface potential of water g^s(dip) is estimated from the inner-layer capacity while the modification of the surface potential of the metal δ_χ^M is obtained from theoretical calculations. The comparison of the [δ_gc^M − g^S(dip)] values with those deduced from the experimental pzc and work function is more than satisfactory. g^S(dip) is proposed to be equal to 0.19, 0.17 and 0.15 V, and −δ_χ^M equal to 0.41, 0.35 and 0.33 V, respectively for the (111), (100) and (110) faces of silver. Consequently, the strength of the silver-water interactions decreases from (111) to (110).The proposition of a large but electrode charge σ-independent capacitance contribution of the metal to the differential capacity of the inner layer Cⁱ is advanced from theoretical estimates and from the quantitative analysis of the Cⁱ(σ) curve for the mercury/water interface. The Cⁱ(σ) curve continues to represent the σ dependence of g^S(dip) as for the model with a σ- and metal-independent δ_χ^M. The basic change is that the capacity at fixed orientation of the water molecules C(ion) can no longer be identified with the minimum value of Cⁱ at high negative σ. A distance of 0.05 nm between the metal and the water molecules is proposed in order to interpret the low value of C(ion) equal to 8 μF cm⁻².The Cⁱ maximum located at the potential of zero charge for the three low-index faces of silver, is attributed to a maximum value of ∂g^S(dip)/∂σ, whatever the value of g^S(dip) for σ = 0 may be. On the other hand, the proposed estimates of the capacitance contribution of silver lead to an identical value of C(ion) and consequently to an identical structure of the inner layer at fixed orientation of the water molecules for the (111) face of silver as for mercury and the other sp metals. The same close-packed arrangement of the metal atoms at the surface of the electrode would be responsible for this identity.