Interactions of bipyridylium cations with adsorbed halides on the DME: The effect on the charge transfer rate

The influence of specific adsorption of halides on adsorption and electrode kinetics of dimethylbipyridylium and dibenzylbipyridylium cations on the DME is investigated. The charge transfer rate enhancement is due to adsorbed Cl^? and Br^? ions.     

Study on the chemisorption kinetics of Methylene Blue using SERS technique

SERS technique was used to study the chemisorption kinetics of Methylene Blue (MB) on the HNO3-etched silver surface. The adsorption kinetic parameters were deduced from different vibrational modes at a low concentration of 3.5×10-6 mol/L, and it showed that MB adsorbed uniformly (monolayerly) on silver surface. However, the adsorptive behavior turned anomalous at relatively higher concentrations and a possible explanation was suggested. In addition, the influence of Cl- ions on the adsorption states of MB was investigated, and it was shown that MB molecules, adsorbed on the silver surface, tended to transform from the "lying-down" state to the "end- on"4 state after Cl- ions were added.     

Adsorption of Cl− ions on bismuth from ethanol

The specific adsorption of Cl^? ions at the bismuth-ethanol interface has been studied both in the solutions of mixed electrolytes with constant ionic strength and in the binary LiCl solutions by the method of measuring the potential dependence of differential capacity of bismuth. The charge due to specifically adsorbed anions was calculated from the experimental capacity data. It was found that in the case of specific adsorption of Cl^? ions at the bismuth—ethanol interface the conditions of undercharged as well as recharded surface of electrode could be observed experimentally. The analysis of the results obtained by fitting the charge of specifically adsorbed ions to the modified virial isotherm including the diffuse layer correction term suggests that in the conditions of recharge of the bismuth surface cations enter the inner part of the double layer and a considerable ionic association occurs in the inner layer. A procedure has been proposed for calculating the charge due to cations in the inner layer, for determining the actual value of the outer Helmholtz plane potential and for evaluating the real parameters of the adsorption isotherm. The reliability of the results of calculations was verified by comparing the data obtained by the method of mixed electrolytes both considering and neglecting the ionic association in the inner layer with the data of the method of binary electrolyte.     

Effect of halides and alkali cations on the Zn(Hg)/Zn(II) electrode reactions

The title subject has been studied by galvanostatic single-pulse, chronopotentiometric and equilibrium measurements on the Zn(Hg)/Zn(II) electrode in x M KI+(1?x) M KCl (x from 0 to 1), 1 M KBr and 1 M MeCl (Me=Li, Na, K and Cs) solutions of pH 3 at 25°C. Quantitative information about the effect of specifically adsorbed halides on the rates of the Zn(II)/Zn(I) and the Zn(I)/Zn(Hg) steps is obtained separately (for the latter step mainly at potentials near ?1.0 V(SCE)), and the latter step seems to be more influenced than the former by the adsorption. An attempt is made to correlate the adsorption effect on the rate of the Zn(II)/Zn(I) step to double-layer parameters according to recent models for such effects. The extra current observed at potentials where the halides are adsorbed, seems to vary with the surface activity of the specifically adsorbed ion. The lack of any observed kinetic effect of Cs⁺, which is specifically adsorbed at these potentials, is possibly due to the Cs⁺ specific adsorption enhancing the Cl^? specific adsorption and vice versa, so that the decelerating and accelerating effects by these ions may cancel each other.     

Quantum chemical studies of the chemisorption of water and of unhydrated and hydrated halide ions on mercury

Local structures on electrode interfaces can be explored by quantum chemical investigation of medium-sized systems consisting of a cluster of substrate (metal) atoms, one or several solvent molecules, and/or at least one ion to be adsorbed at the interface. For the study of water adsorption and halide ion adsorption (unhydrated as well as hydrated) on a mercury surface, we have used the standard CNDO method together with geometrical optimization of the atom positions.In this paper, the following topics have been treated: (a) adsorption of a single water molecule in different positions on a close-packed plane cluster of seven mercury atoms; (b) adsorption of unhydrated halide ions (Cl^?, Br^?, I^?) in the “on-top” or hollow position on the mercury surface; (c) adsorption of monohydrated halides on the mercury surface. Further studies including solvation by six water molecules are discussed.The calculations provide information about minimum-energy geometries, energetic data, and local charges. Furthermore, they allow some conclusions about water mobility and reorientation on a close-packed metal surface, water orientation under the combined influence of an adsorbed ion and the metal surface, and trends of charge distribution in the halide series to be drawn. Calculations are critically discussed in the light of experimental and other quantum chemical data.     

Estimating of surface activity of Cl− and Br− ions in the (In-Ga)/[N-methylformamide + mc KCl + (1 − m)c KClO4] and (In-Ga)/[N-methylformamide + mc KBr + (1 − m)c KClO4] systems by different methods

Differential capacitance curves in the (In-Ga)/[N-methylformamide + mc KCl + (1 ? m)c KClO₄] and (In-Ga)/[N-methylformamide + mc KBr + (1 ? m)c KClO₄] systems are measured using an ac bridge for the following molar portions m of the surface-active anion: 0, 0.01, 0.02, 0.05, 0.1, 0.2, 0.5, and 1. The Cl^? and Br^? anions specific adsorption in the systems can be described quantitatively by the Frumkin isotherm. The principal parameters of Cl^? and Br^? anions adsorption at the (In-Ga)/N-methylformamide interface are determined by different methods. Unlike Ga/N-methylformamide interface, where the adsorption energy increased in the sequence I^? ≈ Br^? < Cl^?, at the (In-Ga)/N-methylformamide interface it increased in the reverse sequence: Cl^? < Br^? < I^?. The adsorption parameters at the charge density q = 0, obtained by three different methods, are close to each other. However, the parameters α₁ and α₂, which characterize the charge effect on the adsorption energy, when determined by the analyzing of dependences of adsorption potential drop E _ads on ln(mc), differ from those determined by two other methods. The error may be caused by the assuming that the adsorption potential drop is proportional to the coverage of dense layer with the specifically adsorbed ions.     

Adsorption of Cl<Superscript>−</Superscript> ions on γ-Fe<Subscript>2</Subscript>O<Subscript>3</Subscript> oxide from nitrate solutions

The adsorption of chloride ions on γ-Fe₂O₃ oxide (maggemite) from nitrate solution is studied using the method of potentiometric titration and an ion-selective electrode. The specific character of adsorption is determined. It is shown that the maggemite surface coverage with Cl^? ions increases with increasing concentration of ions in the solution, decreasing pH value, and increasing potential. The adsorbability of ions changes drastically in the pH range about pH₀ (γ-Fe₂O₃)6.2. It is found that the adsorption of chloride ions from neutral nitrate solution exponentially increases in the potential range from 0.1 to 1.0 V. The type of adsorption isotherm and the adsorption parameters are determined. It is found that, in the absence of external polarization, the concentration dependences of adsorption of Cl^? ions are complex-shaped, and their initial portions are described by the Langmuir isotherm. Further increase of adsorption is explained by the penetration of Cl^? ions inwards the oxide.     

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^?.     

A potentiostatic study of hydrophobic ion transfer across lipid bilayer membranes: Part II. Transport of tetraphenylborate ions at high concentration in the aqueous solution

The transfer of tetraphenylborate (TPB^?) ions across black lipid membranes in aqueous solutions ranging from 10^?5M to 10^?3M, has been studied under potentiostatic conditions. A theoretical treatment of the problem accounts for the two time constants given and for the diffusion current for prolonged periods of time if one assumes that the adsorption-desorption transfer currents are large. This is tantamount to considering that these two processes are highly reversible and that the dipolar nature of the drop in potential between the closest approach and adsorption planes determines the adsorption. The resistances corresponding to the translocation and adsorption-desorption processes have thus also been evaluated. Using these results and those given by surface potential measurements on monolayers of glycerylmono-oleate, the number of adsorbed ions has been calculated. This has led to an explanation of the well known conductance maximum encountered when the concentration of the tetraphenylborate ions varies.     

Kinetics and mechanism of the Cu(I)/Cu(Hg) electrode reactions in DMSO solutions of halide ions

The electrode reaction Cu(I)/Cu(Hg) in complex chloride, bromide and iodide solutions with DMSO as solvent has been studied at the equilibrium potential by the faradiac impedance method and a cyclic current-step method. The kinetic data refer to the ionic strength 1 M with ammonium perchlorate as supporting electrolyte and to the temperature 25°C. Double-layer data have been obtained from electrocapillary measurements. From the results for the chloride system at [Cl^?]>15 mM it is concluded that the charge transfer is catalysed by ligand bridging at the amalgam and the following parallel reactions predominate: Cl_ads^?-Cu⁺+e^?(am)Cl_ads^?+Cu(am) Cl_ads^?-Cu₂Cl_j^2?j+e^?(am)Cl_ads^?+Cu(am)+CuCl_j^1?j At lower [Cl^?] and in the whole ligand concentration range available in the bromide and iodide systems the impedance measurements indicate a rate-controlling adsorption step. It is suggested that uncharged complex CuL (L^?=halide ion) then forms an adsorbed two-dimensional network on the amalgam surface.     

New data from impedance measurements concerning the anodic dissolution of iron in acidic sulphuric media

A flow cell assembly is described featuring ultra-low electrolyte flow propulsion rates (10^?5 to 10^?3 cm³ s^?1) in combination with a working electrode compartment of thin layer dimensions (thickness < 10^?2 cm), enabling thus the exhaustive electrolysis of electroactive species at a potential-controlled detector electrode. The cell can be used to detect trace amounts (>10^?12 mol) of electroactive species delivered into or withdrawn from the streaming electrolyte, such as compounds adsorbed at or desorbed from an ideally polarizable electrode, allowing the independent determination of Δq- and Λ-isotherms of cation adsorbates. The flow cell has been used to study the adsorption of Tl on polycrystalline Ag in KCl solution. The isotherms obtained can be interpreted in terms of competitive Cl^? adsorption within the Tl⁺ adsorption range.     

Electrostatic effect of specifically adsorbed electroinactive ions upon electrode processes: Part VII. Case of a charged adsorbed reactant. Dibromoacetate electroreduction in the presence of Br−, N3−, SCN− and Tl+ ions

The logarithmof the rate constant for CHBr₂COO^? electroreduction at constant applied potential, corrected for diffuse-layer effects according to Frumkin, varies linearly with the charge density q_i due to specifically adsorbed supporting ions. The rate of decrease ofwith increasing |q_i| as observed in the presence of the adsorbed anions Br^?, N₃^?, and SCN^? is about 5 times less than the rate of increase ofwith increasing the charge q_i due to adsorbed Tl⁺ cations. This behaviour, analogous to that observed in the reduction of the CCl₃COO^? ion [4], has been explained by considering that the time of adsorption of the electroactive anion CHBr₂COO^? is long enough to cause a perturbation in the distribution of the adsorbed supporting ions in the neighbourhood of the adsorbed anionic reactant.     

Electrostatic effect of specifically adsorbed electroinactive ions upon electrode processes: Part V. Case of a charged adsorbed reactant. Trichloroacetate electroreduction in the presence of Br−, l−, SCN−, N3−, Ti+ and C(NH2)3+

The rate constant k_f for CCl₃COO^? electroreduction at constant applied potential, once corrected for diffuse-layer effects according to Frumkin, still depends on the charge density q_i due to specifically adsorbed supporting ions. Thus, in the presence of the adsorbed anions Br^?, I^?, SCN^? and N₃^?, the logarithm Φ of the rate constant corrected for diffuse-layer effects decreases linearly with |q_i|, albeit slightly. In the presence of the adsorbed cations Tl⁺ and C(NH₂)₃⁺, Φ increases with the positive charge density q_i due to the cation at a rate which is 3 to 8 times greater than the corresponding rate of decrease in the presence of the adsorbed anions. This behaviour has been interpreted on the basis of the theoretical treatment of ref. 22 by considering that the time of adsorption of the electroactive anion CCl₃COO^? is long enough to permit a perturbation in the distribution of the adsorbed supporting ions in the immediate vicinity of the adsorbed reactant.     

Studies on some radical transfer reactions by entrapping the radicals as polymer endgroups. III. Reaction of ȮH radical with halide ions

The reactions of OH radical with Cl^?, Br^?, I^?, and F^? ions have been studied by entrapping the product radicals as polymer endgroup which have been detected and estimated by the sensitive dye partition technique. The rate constants of the reactions with Br^?, Cl^?, and F^? ions have been determined to be 1.51 × 10⁹, 1.32 × 10⁹, and 0.92 × 10⁹ L mol^?1 s^?1, respectively at 25°C and pH 1.00. Oxidation of I^? ions liberates I, which inhibits the polymerization and the reaction could not be followed by polymer endgroup analysis. The observed order of reactivity Br^? > Cl^? > F^? is in accordance with the electron affinities of the halide ions. The acidity of the reaction medium has a strong influence on the rate of reaction. With Br^? ions, the rate constant of the reaction falls from 1.51 × 10⁹ to 0.75 × 10⁹ L mol^?1 s^?1 at 25°C as the pH is raised from 1.0 to 2.8. The method is simple and accurate and can be applied to study very reactive radicals.     

Insights from the Adsorption of Halide Ions on Graphene Materials

Graphene has recently found applications in a wide range of fields. Density functional calculations show that halide ions can be adsorbed on pristine graphene, but only F^? has an appreciable binding energy (?97.0 kJ mol^?1). Graphene materials, which are mainly electron donors, can be made strong electron acceptors by edge functionalization with F atoms. The binding strengths of halide ions are greatly enhanced by edge functionalization and show direct proportionality with the degree of functionalization Θ and increased charge transfer. In contrast, the adsorption strengths of metal ions on pristine graphene are clearly superior to those of halide ions but decline substantially with increasing degree of edge functionalization, and for Θ=100 %, the binding energies of ?95.7, ?44.8, and ?25.9 kJ mol^?1 that are calculated for Li⁺, Na⁺, and K⁺, respectively, are obviously inferior to that of F^? (?186.3 kJ mol^?1). Thus, the electronic properties of graphene are fundamentally regulated by edge functionalization, and the preferential adsorption of certain metal ions or anions can be facilely realized by choice of an appropriate degree of functionalization. Adsorbed metal ions and anions behave differently on gradual addition of water molecules, and their binding strengths remain substantial when graphene materials are in the pristine and highly edge functionalized states, respectively.     

519—Reduction of hydrophobic porphyrin at the octane/water interface controlled by the specific adsorption of halogen anions

The redox reaction between the ferric complex of hydrophobic porphyrin and sodium dithionite in two different phases occurring at the interface between two immiscible liquids has been investigated by Volta potential measurements and spectroscopy. The reduction of the ferric complex of hydrophobic porphyrin adsorbed at the interface was found to be accompanied by a potential shift in the negative direction and to depend significantly on the nature of the anion and the ionic strength of the supporting electrolyte. Specifically adsorbed halogen anions inhibited the redox reaction in the sequence: Cl^?, Br^?, I^?. Depending on pH, the ferric complex of hydrophobic porphyrin exists in the uncharged (FeP-O-PFe) form or in the cation FeP⁺ form. The interaction between the ferric complex hydrophobic porphyrin and water has also been investigated.     

Effect of Cs+ ions on the electrochemical nanogravimetric response of platinum electrode in acid media

Platinum electrodes have been investigated in sulfuric acid solutions in the presence and absence of Cs⁺ ions by electrochemical quartz crystal nanobalance (EQCN). An unusual potential dependence of the quartz crystal frequency response has been observed in the presence of Cs⁺ ions. The frequency decrease is more pronounced in the region of the underpotential deposition of hydrogen, and the frequency decrease in the double layer region diminishes as the concentration ratio of Cs⁺ and H⁺ ions increases. After immersion in Cs₂SO₄ solutions the frequency change was higher than that expected taking into account the density and viscosity. The effects observed can be explained by the specific adsorption of Cs⁺ ions on the Pt surface, which competes with the hydrogen adsorption. At more positive potentials than the potential of zero charge (pzc) a desorption of the Cs⁺ ions starts. In this potential region both Cs⁺ and HSO₄^? ions are adsorbed at the platinum surface. In the double layer region the mass change caused by the desorption of Cs⁺ ions and the starting adsorption of sulfate ions compensates each other.     

The effect of iodide on the electrochemical reduction of thick oxide films formed on platinum electrodes in sulfuric acid solution

The reduction of thick oxide films formed on Pt under severe anodic conditions was studied in the presence of adsorbed I^?. The Pt electrode covered with a thick oxide film does not adsorb I^?. However, when a superficial monolayer oxide on the thick oxide has been reduced, I^? is irreversibly adsorbed. Iodide adsorbed on its surface blocks the adsorption of hydrogen and retards markedly the cathodic reduction of the inner thick oxide remaining. It was found that the reduction rate of the inner oxide depends only on the coverage by hydrogen, which coexists with adsorbed I^?. These results support the proton-electron theory which has previously been proposed for the explanation of the characteristic reduction of the thick oxide films.     

The electrocatalytical influence of underpotential lead and thallium adsorbates on the cathodic reduction of oxygen on (111), (100) and (110) silver single-crystal surfaces

The influence of underpotential Pb and Tl adsorbates on the electrochemical reduction of oxygen on rotating-disc Ag(111), (100), and (110) single-crystal surfaces has been studied in aerated 0.5 M HClO₄ solutions at various concentrations of Cl^?. On the bare silver substrates oxygen is reduced completely to H₂O. Depending on the degree of coverage and the structural arrangements of Pb and Tl adsorbates on the different crystal planes, a partial inhibition of the oxygen reduction is obtained predominatly leading to the formation of the stable intermediate, H₂O₂. In the presence of Cl^? ions in solution, the overvoltage for charge-transfer controlled oxygen reduction increases according to (?E/? logc_Cl^?)_{i_}=?60 mV, due to a specific adsorption of chloride on the silver substrate. In 0.5 M HCl solutions a stimulating effect on the oxygen reduction induced by the underpotential deposition of Pb has been found, which can be interpreted in terms of a competitive adsorption-desorption mechanism involving a replacement of chloride by lead.     

Effect of inorganic ions on the photocatalytic degradation of humic acid

The article studies on the effects of six inorganic ions (Ca²⁺, Mg²⁺, Cl^?, SO ₄ ^2? , H₂PO ₄ ^? , and HCO ₃ ^? ) on titanium dioxide (TiO₂)-based photocatalytic degradation of humic acid (HA). We focus on the effects of the inorganic ions on HA characters, adsorption of HA on TiO₂ and photocatalytic degradation of HA. The results indicate that Ca²⁺ and Mg²⁺ with HA can form complexes which can decrease the solubility of HA, and then increase the HA adsorbed on TiO₂. However, the complex is more difficult to be degraded than HA. The effects of Cl^? and SO ₄ ^2? are closely related to their influences on HA solubility. The solubility changes of HA to some extent can enhance the adsorption of HA on TiO₂, and promote the photocatalytic degradation. Nevertheless, great solubility decreasing of HA can weaken the photocatalytic degradation. HCO ₃ ^? and H₂PO ₄ ^? can inhibit the photocatalytic degradation process seriously, because HCO ₃ ^? and H₂PO ₄ ^? are the strong scavengers of hydroxyl radicals, and can weaken the adsorption of HA on TiO₂ due to adsorption competition.