Adsorption and association of xanthine in absence and presence of some divalent metal ions at the mercury/solution interface

The adsorption and interfacial orientations of xanthine were studied by out-of-phase ac voltammetry at a hanging mercury drop electrode. The adsorption equilibrium and its attainment have been investigated as a function of various parameters such as pH, adsorption potential, temperature, adsorption time, the nature of anions of the supporting electrolyte and the bulk concentration of xanthine. The changes in the stacking interactions of xanthine molecules in the presence of Cd(II), Cu(II), Co(II), Ni(II), Ca(II) and Mg(II) were studied by ac voltammetry as changes in “pits” or “wells” on double layer capacitance curves due to adsorption and association of M(II)-xanthine complexes on the charged interface. The results indicate that the complexation of xanthine molecules enhances the stacking interactions and hence would be expected to facilitate the formation of perpendicularly stacked layer of M(II)-xanthine complex on the electrode surface. The time dependence of the electrode impedance indicates that the formation of a compact film in absence and presence of these metal ions controlled by a fixed number of nuclei and the data were analysed according to the Avrami equation. The adsorption parameters of xanthine have been computed at different pH values in the absence and the increased presence of metal ions.     

Inhibition of aluminium corrosion in chloride media: an impedance study

Inhibition of the corrosion of pure aluminium by nitrite and chromate anions in near neutral aqueous chloride solution has been studied by electrochemical impedance spectroscopy, keeping the ionic strength constant by addition of potassium sulphate electrolyte. Negative of the chloride pitting potential impedance spectra show that both anions are successful in reducing corrosion through adsorption, nitrite more than chromate. Positive of this value, nitrite becomes ineffective, whereas chromate remains adsorbed and continues to prevent pitting corrosion, probably through its ability to repair defects in the oxide film. Impedance techniques successfully show the qualitative and quantitative differences between the two inhibitors.     

Adsorption of camphor and 2,2′-bipyridine on Bi(1 1 1) electrode surface

Impedance spectroscopy and in situ STM methods have been used for investigation of the camphor and 2,2′-bipyridine (2,2′-BP) adsorption at the electrochemically polished Bi(1 1 1) electrode from weakly acidified Na₂SO₄ supporting electrolyte solution. The influence of electrode potential on the adsorption kinetics of camphor and 2,2′-BP on Bi(1 1 1) has been demonstrated. In the region of maximal adsorption, i.e. capacitance pit in the differential capacitance versus electrode potential curve, the heterogeneous adsorption and diffusion steps are the rate determining stages for camphor and 2,2′-BP adsorption at the Bi(1 1 1) electrode. It was found that for camphor | Bi(1 1 1) interface the stable adsorbate adlayer detectable by using the in situ STM method has been observed only at the positively charged electrode surface, where the weak co-adsorption of SO₄²⁻ anions and camphor molecules is possible. At the weakly negatively charged Bi(1 1 1) electrode surface there are only physically adsorbed camphor molecules forming the compact adsorption layer. The in situ STM data in a good agreement with impedance data indicate that a very well detectable 2,2′-BP adsorption layer is formed at Bi(1 1 1) electrode in the wide region of charge densities around the zero charge potential.     

A new technique for the study of the kinetics of adsorption of polymers onto surfaces

A new and simpler technique is described for the study of the kinetics of adsorption of poly(methylmethacrylale) (PPMA) on a glass surface at 30°C. The kinetics of adsorption were followed gravimetrically by knowing the amount of polymer adsorbed on glass balls at various time intervals. A simple kinetic model for rate of adsorption has been proposed, and from the scheme outlined various adsorption parameters such as adsorption coefficient, surface coverage and film thickness have been evaluated. From the available experimental data, adsorption isotherms have been drawn, and the effect of molecular weight of PMMA on adsorption behaviour has been studied.     

Kinetics of poly(acrylic acid) adsorption at the liquid–glass interface

The kinetic behavior of the adsorption of weak polyacid, namely, poly(acrylic acid) on a smooth glass surface has been studied at 30°C. For the evaluation of various adsorption parameters—the adsorption coefficient, surface coverage (θ), and thickness of the adsorbed layer—a new simple kinetic model has been proposed. The effect of the factors such as pH, electrolyte concentration, and molecular weight on adsorption behavior has been studied. © 1994 John Wiley & Sons, Inc.     

Studien zum vorgang der wasserstoffübertragung—33. Zur kenntnis des aufbaus der elektrochemischen doppelschicht. Die asymmetrische elektrochemische reduktion einiger alkyl-phenyl-ketone in gegenwart optisch aktiver leitsalze

Alkyl phenyl ketones have been reduced in the presence of optically active supporting electrolytes (e.g. derivatives of ephedrine, pseudoephedrine and desoxyephedrine) to optically active alkyl-phenylcarbinols and optically inactive pinacols. The dependence of product composition and degree of asymmetric induction on the structure of the supporting electrolyte and the depolarizer was determined. Increasing size of the alkyl group in the alkyl-phenyl-ketone favours higher carbinol yields and diminishes the degree of optical activity. The functional groups of the supporting electrolyte also exercise a noticeable influence on the d,l/meso ratio in pinacol formation. In order to explain these combined effects, and taking intermolecular interactions into account, a model has been proposed of an adduct between the supporting electrolyte and depolarizer (or depolarizer radical anion). These results can be taken, with some care, as a contribution to the understanding of the adsorption of organic molecules in the double layer.     

The effect of anion identity on the viscoelastic properties of polyaniline films during electrochemical film deposition and redox cycling

Acoustic admittance measurements at thickness shear mode resonators were used to determine shear moduli for polyaniline films during their potentiodynamic electrodeposition and subsequent redox cycling in aqueous background electrolyte. Data were acquired for films doped with perchlorate, sulphate or chloride anion. For all media, film shear moduli increased progressively with film thickness, from values consistent with a diffuse fluid-like layer to values typical of a viscoelastic material. At any given thickness, both the storage and loss moduli were largest in perchlorate medium; values in chloride and sulphate media were similar to each other, but smaller than in perchlorate. These measures of polymer dynamics are consistent with a previous classification of polyaniline film behaviour, in which perchlorate-doped films are viewed as compact while chloride- and sulphate-doped films are viewed as more open. In monomer-free background electrolyte solution, both film shear modulus components for all anions increased modestly upon film oxidation. Despite some hysteresis on the timescale of slow scan voltammetry, these variations were chemically reversible. Based on measurements involving deposition from chloride medium and transfer to sulphate medium, film shear moduli respond promptly to changes in dopant identity; this is consistent with rapid redox-driven exchange of anions with the bathing electrolyte.     

Anion adsorption and atomic friction on Au(1 1 1)

The influence of anion adsorption on friction forces in an electrochemical environment has been studied by means of lateral force microscopy on Au(1 1 1) surfaces. Sensitivity to atomic stick-slip motion allows to reveal sulphate adsorption in ordered layers under the sliding tip at potentials lower than expected from cyclic voltammetry for the open surface. No ordered adsorption is found in lateral force measurements for the weakly adsorbed perchlorate anions. Correspondingly, some increase in friction in the anion adsorption regime is observed for sulphate but none for perchlorate adsorption. Friction increases significantly at the onset of oxidation in both sulphuric and perchloric acid solutions.     

Surface-enhanced Raman spectroscopy and electrochemistry of 2,2′-bipyridine adsorbed at copper electrode

The adsorption of 2,2′-bipyridine (22BPY) on the surface of polycrystalline copper electrode was studied by means of surface-enhanced Raman spectroscopy (SERS) and cyclic voltammetry in a wide range of electrode potential. Experiments were carried out in both acidic and neutral solutions, as well as in presence of two kinds of supporting electrolyte (KCl and LiClO₄) in order to examine the influence of molecule protonation and specifically adsorbed anions. It was found that a molecule orientation with respect to the surface changes from perpendicular to more tilted in the neutral perchlorate solution followed by the conformation change from cis to trans in strongly negative potentials. This conformational transition occurs gradually, probably through a series of intermediate states. The SERS features due to trans conformers are uniquely observed in presence of perchlorate anions. In the presence of chloride the molecular plane is at least slightly inclined towards the surface in the whole potential range. In acidic solutions the SERS results indicate that 22BPY adsorbs in non-protonated form independent on the present anion with the molecular plane tilted although it becomes more perpendicular with respect to the surface at negative potentials. The cyclic voltammetry measurements revealed that the hydrogen evolution is inhibited in presence of 22BPY in both neutral and acidic solutions. It suggests strong adsorption of this molecule in a wide range of electrode potentials.     

Adsorption behaviour of dihydrogen phosphate ions on mercury from constant ionic strength solutions

The adsorption behaviour of H₂PO⁻₄ anions on mercury has previously been analysed from capacitance-potential relations at the interface of mercury with KH₂PO₄.It is not possible to determine unambiguously the adsorption characteristics of H₂PO⁻₄ ions from single salt solution experiments because of the possible existence of a layer of water molecules between the electrode and the adsorbed ions. In this work, the adsorption behaviour of H₂PO⁻₄ ions on mercury is analysed from experimental measurements of the capacitance as a function of potential at the interface of mercury with constant ionic strength solutions of compositions x M KH₂PO₄ + (1 − x) M KF. It is shown that the calculated amounts of adsorbed ions do not depend on the position of these ions relative to the solvent molecules. The double layer parameters were calculated taking into account the variations of activity coefficients of KH₂PO₄ and KF with solution composition. In constant ionic strength solutions, the plot of adsorbed charge vs electrode charge exhibits a maximum due to competition between the adsorbed H₂PO⁻₄ anions with the solvent or other ionic species. This result is shown to be important in determining the properties of the double layer characteristics in the presence of adsorbed H₂PO⁻₄ ions.     

Untersuchung über die anionenadsorption mit hilfe markierter ionen

From previous work, the adsorption of anions is regarded as an essential factor for the different corrosion behaviour of metals in solutions containing different anions. Adsorption is measured by means of ³⁶Cl⁻, ⁸²Br⁻, ¹⁸ F⁻, ³⁶ClO₄⁻, ³⁵O₄²⁻, H³⁵S⁻ and ¹⁴CN⁻ on Pt, Ni and Fe in the form of sheets and evaporated films. Besides the determination of the adsorption after dipping into the solution, a method has been developed for the measurement of adsorption in contact with the solution and for the determination of its kinetics. The method can also be applied to O₂-free metal surfaces produced under vacuum. In this case, however, very rapid adsorption is observed, whereas normally saturation is reached only after many hours. It is concluded that, in general, exchange between oxygen on the metal and the anion takes place rather than simple adsorption.

The distribution of the anions adsorbed on the metal surface has been studied by autoradiography; adsorption takes place preferentially at the grain boundaries and increases when the crystal size decreases.

These results confirm the interpretation of passivation as a competition between various processes: metal dissolution, coverage by a passivating oxide film, and displacement of oxygen by anions.     

Adsorption and desorption properties of cationic polyethylene film gels to organic anions and their regeneration

An investigation was undertaken on the adsorption and desorption properties of 2‐(dimethylamino)ethyl methacrylate grafted polyethylene (PE‐g‐PDMAEMA) films to anionic dye anions with one to three sulfonic groups in response to pH and temperature changes. The amounts of dye anions adsorbed on the PE‐g‐PDMAEMA films passed through the maximum values at about pH 3 because of an increase in the protonation of dimethylamino groups caused by a decrease in the pH value. The amounts of adsorbed dye anions decreased below pH 3 because the ionic strength increased with the addition of HCl to adjust the initial pH values of the aqueous dye solutions. The amounts of adsorbed dye anions decreased with an increase in the number of sulfonic groups in the dye molecules at the same pH value because electrostatic repulsion was generated between free sulfonic groups of the dye anions adsorbed onto the PE‐g‐PDMAEMA films and free dye anions in the medium. A large number of dye anions adsorbed were desorbed from the PE‐g‐PDMAEMA film with initial pH values above 11.0. The cyclic processes of adsorption at pH 3.0 and desorption at pH 11.0 were repeated without considerable fatigue. The PE‐g‐PDMAEMA films showed practically regenerative adsorption and desorption behavior in response to the pH changes. In addition, when the dye‐anion‐adsorbed PE‐g‐PDMAEMA films were alternately immersed in water at two different temperatures, dye anions were desorbed in water at higher temperatures without any chemical agents because of the deprotonation of dimethylamino groups and thermosensitive contraction of grafted PDMAEMA chains. These results indicate that PE‐g‐PDMAEMA films can be applied as regenerative ion‐exchange membranes for adsorption and desorption processes of anionic compounds in response to the pH and temperature. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99: 381–391, 2006     

Adsorption of poly(dimethyl siloxanes) from solution on silica: 2

The adsorption energy (Δε_s), the change in energy on breaking a solvent-adsorbent bond and forming a segment-adsorbent bond, for poly(dimethyl siloxane) at the solution/silica interface has been measured under actual adsorption conditions. The effect of the adsorption energy on the extent of mass adsorption and on the conformation of the adsorbed polymer layer was evaluated. At low surface coverage, it is primarily the adsorption energy which determines the conformation of the adsorbed polymer layer as a function of the molecular weight. This effect is opposed by an entropy lowering, resulting from a restriction of the configuration of the adsorbed polymer molecules. At high surface coverage, lateral interactions between the adsorbed polymer molecules became an important factor causing a decrease in the fraction of directly adsorbed segments per molecule with increasing coverage.     

Electrochemical ozone production: influence of the supporting electrolyte on kinetics and current efficiency

The nature of the electrolyte strongly influences the electrode kinetics of the oxygen evolution reaction (OER) and electrochemical ozone production (EOP) mainly by affecting the degree of coverage by the intermediates of both processes. The anomalous behaviour of the Tafel coefficient, b, as a function of temperature was attributed to surface adsorption of the electrolyte species, and the competition between them, as well as gas bubble adherence. Comparison of the current efficiencies of the EOP, Φ_EOP, determined for different temperatures and supporting electrolyte compositions, showed the presence of fluorinated anions increases Φ_EOP. The influence of the anion nature on Φ_EOP, when analysed in the light of the proposed electrode mechanism, reveals introduction into the electrolyte of anions having a high electronegativity changes the double layer structure resulting in an increase of surface concentration of the active centres leading to EOP. The inhibition of the OER in the high overpotential domain during EOP provoked by fluoro-anion adsorption is supported by the activation energy data. In situ surface characterisation before and after EOP investigation revealed that even under drastic conditions (high current density, low interfacial pH) β-PbO₂ can be considered an inert electrode material.     

Capacitance properties of electrochemically deposited polyazulene films

Polyazulene films formed by electrochemical oxidation of azulene have been studied as active components in electrochemical capacitors. The film shows reversible electrochemical behavior in the positive potential range and exhibits p-doping properties. The influence of film formation conditions on the films electrochemical properties has been investigated. A strong effect of solvent on the polyazulene deposition has been observed. The highest yield of film deposition was found for dichloromethane. Polyazulene films also exhibit stable voltammetric properties in aprotic solvents. The voltammetric response of the film is affected by the size of the anion of the supporting electrolyte. In solutions containing tetra(alkyl)ammonium perchlorates, tetrafluoroborates or hexafluorophosphates, reversible oxidation of polyazulene is obtained. In the presence of large tetra(phenyl)borate anions, polyazulene is irreversibly oxidized upon electrochemical oxidation. The capacitance properties of these materials have been investigated by cyclic voltammetry and electrochemical impedance spectroscopy. The polyazulene film displays a relatively high specific capacitance close to 400 F g⁻¹. Such high value of C_s locates this material among very good polymeric redox pseudo-capacitors.     

Statistical Physics Studies of Multilayer Adsorption on Solid Surface: Adsorption of Basic Blue 41 Dye onto Functionalized Posidonia Biomass

In this article, finite multilayer adsorption modeling was presented. The grand canonical formalism was used to establish a novel finite multilayer with multisite occupancy model. Expression for the physico-chemical parameters involved in the adsorption phenomena were derived based on statistical physics treatment. This model has been applied to one of the most challenging adsorption in liquid phase, i.e., Basic Bleu 41 dye adsorption onto raw and modified Posidonia biomass. The parameters involved in the analytical expression of the multilayer model such as the number of adsorbed molecules per site, the density of occupied receptor sites, and the number of adsorbed layers were determined by fitting the experimental adsorption isotherms at temperatures ranging from 303 to 353 K. Fitting results show that the dye molecules are multimolecular adsorbed onto Posidonia surface. Furthermore, the new approach leads us to quantify the mean number of adsorbed layers. The magnitudes of the calculated adsorption energy indicate that BB41 dye is physisorbed onto Posidonia adsorbent.     

Structured chiral adsorbent formed by cyclodextrin modified layered solid film

The film of carboxymethyl-β-cyclodextrin-intercalated Zn-Al layered double hydroxide (CMCD-LDH) has been prepared by in situ crystallization on a porous anodic alumina/aluminum (PAO/Al) substrate. This film can be used to enantioselectively separate racemic molecules, 1-phenyl-1,2-ethanediol (PED). The adsorption isotherms of enantioselective as well as non-enantioselective adsorption of PED by CMCD-LDH film have been investigated, and it was found that the Langmuir-Freundlich and Freundlich model can be, respectively, used to describe the two different adsorptions satisfactorily. Furthermore, the plane sheet diffusion model was successfully validated in this work. Diffusivities (D) of PED in CMCD-LDH film were determined from the Fickian diffusion model at various temperatures. As the structured CMCD-LDH film is conveniently used and exhibits high enantioselective adsorption ratio, it can be expected that this film could be feasibly applied in the field of industrial chiral adsorption and separation.     

Polypyrrole-based electrode coatings switchable electrochemically between the anion- and cation-exchanger states

Dodecylsulphate was incorporated into a polypyrrole matrix during anodic polymerization of pyrrole. The concentration was adjusted such that in the oxidized film about one half of the polypyrrole cationic groups was charge compensated by the sulphate groups from the incorporated dodecylsulphate, the second half by anions from the electrolyte. Upon electrochemical oxidation/reduction, the film is reversibly “switched” between a cation-exchanger (at negative electrode potentials), and an anion exchanger (at more positive potentials) state. The different states of the film are characterized by electrochemical, EDAX, and in particular by Volta-potential measurements. The Donnan potentials determined with the latter techniques as a function of electrode potential and electrolyte concentration were in good agreement with calculated values.     

Complex voltammetric and fractal study of adsorbed layer’s structure of pure Triton-X-100 and in mixture with o- or p-nitrophenol

Adsorption of well-known surfactant Triton-X-100 (T-X-100) and mixed systems comprising T-X-100 with o- or p-nitrophenols, at the mercury/electrolyte solution interface versus T-X-100 bulk concentration has been studied. Diversified approach comprising capacitive current measurements, desorption peak height analysis, fractal analysis, nonthermodynamic calculation of coefficient of lateral interaction, calculation of relative coverage assuming flat and perpendicular dispositions of molecules, fit with Flory-Huggins isotherms corresponding to different molecular orientations, and predictions of random sequential adsorption (RSA) and equilibrium (generalized RSA) theories, together with analysis of reduction mechanisms of nitrophenols has been used.It is found that the adsorbed layer of T-X-100 and mixed layers are fractal. Fine structural changes of adsorbed layer(s) are identified and related to corresponding changes in fractal dimension. Evidence derived from alternating current (ac) and square wave (sw) voltammetric measurements and theoretical considerations support these conclusions. For the adsorption of T-X-100 four different adsorption phases, corresponding to increasing bulk concentration, are identified and characterized. Mixed systems undergo different changes in microstructure that are reflected in observed changes of reduction mechanisms of nitrophenols. Those are in turn related to the differences in molecular interactions between T-X-100 and certain nitrophenol. The results are relevant for study of any system exhibiting fractal features and accessible to electrochemical methods such as adsorbed films, models of biological membranes, etc.     

Investigation of ion adsorption properties of sulfobetaine gel and relationship with its swelling behavior

The adsorption of cations and anions in nitrate solutions on N,N-dimethyl(acrylamidopropyl)ammonium propane sulfonate (DMAAPS) gels prepared using various cross-linker and monomer concentrations was investigated. The influence of the temperature and nitrate concentration on the adsorption properties of the gel was evaluated, demonstrating simultaneous adsorption of cations and anions. The amount of Zn²⁺ adsorbed on the gel in Zn(NO₃)₂ solution increased as the cross-linker and monomer concentrations used in the gel preparation increased. For the gel prepared using a higher cross-linker or monomer concentration, elevation of the temperature did not induce any significant change in the amount of Zn²⁺ adsorbed on the gel. Furthermore, for the gel prepared using a lower cross-linker or monomer concentration, the amount of Zn²⁺ adsorbed on the gel decreased significantly as the temperature increased. In addition, an interesting correlation between the degree of swelling of the gel and the amount of Zn²⁺ adsorbed on the gel was found. As the degree of swelling decreased, the adsorption amount increased to eventually achieve a constant value.