Surface alloying at the Cd ∣ Au(100) interface in the upd region. Electrochemical studies and in situ EC-AFM observation

The surface alloy formation during Cd underpotential deposition (upd) on Au(100) in sulfuric acid solution was investigated by means of long-time polarization experiments and in situ electrochemical atomic force microscopy (EC-AFM). The dynamics of the surface alloying were found to depend on polarization conditions and to compete with surface diffusion and further adsorption processes. A turnover mechanism is proposed for the surface alloying process that was observed to take place mainly on the terraces. For long polarization times, the extent of 2D alloy formation into the surface is governed by solid-state diffusion of either Cd or Au through the alloyed phase. The time dependence of the surface phase stability and composition, which was formed at a certain potential in the upd region, is discussed in thermodynamic terms derived for the surface.     

A model for electrochemical oscillations at the Si ∣ electrolyte contact: Part I. Theoretical development

We present a mathematical model that describes current oscillations of silicon in fluoride-containing solutions. The model is based on the relationship between a set of thickness oscillators (which represent the local thickness of the SiO₂ layer) and the resulting current oscillations. We introduce a probability distribution for the periods of the thickness oscillators. During a cycle, a so-called synchronization state describes the temporal drift of the instant at which the thickness oscillators pass the smallest thickness. A convolution of the synchronization state with the probability distribution for the periods determines the synchronization state of the next cycle. A convolution of the superposition of all synchronization states with an elementary current peak leads to the current.     

A non-primitive model for the electrode ∣ electrolyte interface based on the Percus–Yevick theory. Analysis of the different molecular sizes,ion valences and electrolyte concentrations

The electrode ∣ electrolyte interface is modelled by a mixture of charged and dipolar hard spheres against a planar, charged hard wall. A mean field theory is used to describe the coulombic interactions while steric effects are given by the Percus–Yevick theory. The underlying Percus–Yevick theory for three uncharged species against a planar wall is derived by using the standard method developed by Henderson et al. (D. Henderson, F.F. Abraham, J.A. Barker, Mol. Phys., 31 (1976) 1291) and compared with Monte-Carlo simulations. Although the Percus–Yevick theory has shortcomings, the theory provides an estimate of how the high density of the solvent influences the structural and thermodynamic properties. Consideration of the solvent molecules introduces oscillations in the density distribution of the ions and solvent while the different molecular sizes and ion valences lead to an asymmetry in the differential capacitance.     

A model for electrochemical oscillations at the Si ∣ electrolyte contact: Part II. Simulations and experimental results

Specifications of the theoretical model (article I) on electrochemical oscillations at silicon electrodes are made. The specifications concern: (i) the first synchronization state p₁(t) which is derived using the experimental observation that oxide growth on the initially bare silicon surface occurs by nucleation of islands with finite size; (ii) the development of the probability distribution q_i,s(t?) for the periods of the thickness oscillators involves oxide growth and an etching mechanism based on crack and pore formation due to lattice mismatch; (iii) the shape of the elementary current peak E_i(t?) is determined by simultaneous passivation and etching during the growth phase of a thickness oscillator. The model describes damped as well as sustained oscillations. Damped oscillations are obtained for an almost constant shape of q_i,s(t?). This corresponds to an almost constant morphological defect density during a cycle. For sustained oscillations, a feedback mechanism is introduced modeled by the contraction of q_i,s(t?) during a cycle. This reflects the increasing defect density towards the end of a cycle where oxides formed later are etched faster. Hence synchronization is due to the local etching behavior. It is shown that the model is able to reproduce various experimental data. Besides the simulation of damped and sustained current oscillations, rather complicated cases such as the setting behavior and results on semi-immersed electrodes can also be reproduced very well.     

Polarization phenomena at the water ∣ o-nitrophenyl octyl ether interface: Part III. Kinetics of tetraethylammonium ion transfer across the polymer supported interfaces

Kinetics of the tetraethylammonium (TEA⁺) ion transfer across the interface between an aqueous electrolyte solution (W) and the o-nitrophenyl octyl ether (o-NPOE) plasticized polyvinylchloride or the polyvinylidene difluoride supported membrane is examined by impedance spectroscopy at the equilibrium cell potential difference. Impedance data are analyzed by using the Randles-type equivalent circuit. As compared with the unsupported W ∣ o-NPOE interface, the polymer supported interfaces do not exhibit the enhanced differential capacity in the presence of the TEA⁺ ion, while they exhibit a lower charge transfer resistance (i.e. a higher rate constant of ion transfer). It is shown that these differences can be due to mechano–electric oscillations, which are related to capillary waves at the liquid ∣ liquid interface. An equivalent electrical circuit involving these oscillations comprises an internal ac voltage source in series with the interfacial capacity having the same frequency but the opposite sign to the applied voltage. At a polymer supported interface, the mechano–electric oscillations are likely to be suppressed, and the Randles-type equivalent circuit becomes a realistic representation. A revision of the previous impedance analysis of the TEA⁺ ion transfer across the unsupported W ∣ o-NPOE interface yields kinetic data which are consistent with the present results. The ion transfer rate constant still shows a correlation with the viscosity of the organic phase.     

Finite element simulation of ion transfer reactions at a single micro-liquid ∣ liquid interface supported on a thin polymer film

The electrochemical response of an ion transfer reaction at a polarised micro liquid ∣ liquid interface has been studied by a finite element method. A new penalty method developed for this application has been first validated by comparison with experimental and analytical results obtained for a symmetric micro interface. Then, asymmetric geometries have been studied and the influence on the voltammetric response of parameters such as micro hole depth, interface position within the hole and diffusion coefficient ratio is investigated.     

Transfer of β-diketone and 4-acylpyrazolone anions across the electrified water ∣ nitrobenzene interface

Dependent on the pH of the aqueous phase, the transfer of substituted β-diketone and 4-acyl-5-pyrazolone anions as well as the transfer of protons facilitated by the ligand anions across a microhole supported water ∣ nitrobenzene microinterface has been studied by cyclic voltammetry. Above a certain pH the half-wave potential of the observed wave is independent of the pH and represents the simple transfer of the ligand anion between the two phases. This critical pH-value is dependent on the basicity and the hydrophobicity of the compound. Below this pH-value, the half wave potential is shifted linearly by ca. 52 mV with the pH and reflects the protonation/deprotonation process at the interface. The protonation constants of the ligands in the organic phase, the distribution coefficients, the diffusion coefficients and the formal potentials of the ion transfer were determined. With increasing chain lengths of the acyl-substituent, the formal potential of the transfer of the ligand anions across the interface shifts to more positive values which means that the standard Gibbs energy of the ligand anion partition between water and nitrobenzene decreases. The formal potentials for the 4-acyl-5-pyrazolone anion were found between 50 and ?120 mV and the standard Gibbs energies of ion partition between ?5 and 12 kJ mol^?1. The protonation constants of the ligands in the organic phase amount to 16.5±1.0.     

Simple extension of the Gouy–Chapman theory including hard sphere effects.: Diffuse layer contribution to the differential capacity curves for the electrode ∣ electrolyte interface

An extension of the Gouy–Chapman theory based on a field theoretical approach including the hard sphere effect is presented. This approach focuses on the influence of ions on the properties of the electrode ∣ electrolyte interface. The Hamiltonian consists of the electrostatic energy and the Helmholtz free energy function for the hard sphere mixtures. The calculated differential capacity curves depend on the ionic sizes and concentration. In dilute electrolytes, close to the potential of zero charge we observe the Gouy–Chapman minimum which becomes a maximum when the concentration of electrolyte increases. This minimum is surrounded by two maxima. The capacity at these maxima depends on the size of ions and their concentration in the solution. At high charge densities the ionic density becomes constant and the capacity depends only on the size of the counterion which counterbalances the charge on the electrode, being independent of its concentration. These features are observed on the capacity curves obtained for various electrolytes on various metals.     

In vivo effects of geranylgeraniol on the development of bisphosphonate-related osteonecrosis of the jaws

Background

Bisphosphonate-related osteonecrosis of the jaws (BRONJ) is a complication of the bisphosphonate (BP) treatment and its pathopysiology is still not fully understood. The existing preventive and treatment options require updates and more attention. Geranylgeraniol (GGOH) so far demonstrated an increased activity and viability of the cells previously treated with zoledronic acid (ZA). The aim of this study was to evaluate the in vivo effects of GGOH on the development of BRONJ.

Voltammetric analysis of lipophilicity of benzodiazepine derivatives at the water ∣ 1,2-dichloroethane interface

The transfer of benzodiazepine derivatives (diazepam, bromazepam, alprazolam, oxazepam, nitrazepam, clonazepam, chlordiazepoxide, flunitrazepam, midazolam and lorazepam) across the water ∣ 1,2-dichloroethane interface was studied using cyclic voltammetry. The partition coefficients of ionic species of benzodiazepines, log P_XH⁺, were calculated from the transfer potentials measured at pH<pK_a. These values were compared with the partition coefficient of neutral species, log P_X. The difference between log P_XH⁺ and log P_X was related to the degree of charge delocalization, which depends markedly on the presence of electron acceptor substituents in the molecule. These electron acceptor groups, in turn, affect the biological activity of these drugs. The results indicated that the difference Δlog P_XH⁺=log P_XH⁺?log P_X can be used in structure–activity correlations as it takes into account the effects of substituents on the main positions within the molecule.     

A sodium-specific condensed film of α-cyclodextrin at the mercury ∣ water interface

α-Cyclodextrin adsorption at the mercury ∣ water interface exhibits several condensed phases, depending on the anions and cations present. At low charge densities the interface can be covered by a condensed film of α-cyclodextrin. In the presence of sodium ions, a condensed film containing both α-cyclodextrin and Na⁺ can be observed at a negatively charged electrode. This highly sodium-specific film contains three α-cyclodextrin molecules for every two Na⁺ cations, and can be characterized by the interfacial solubility product [αCD]³[Na⁺]²=1.65×10^–6 M⁵ at 25°C. This film exhibits isomer-specific inhibition of reduction processes; its formation can be stochastic.     

Capacitance and ionic association at the electrified oil ∣ water interface: the effect of the oil phase composition

The capacitance of the polarisable aqueous ∣ organic interface was measured with the impedance method, varying both the solvent and the electrolyte of the organic phase. The results were analysed using the idea of interfacial ion-pairing presented earlier (J. Chem. Soc. Faraday Trans. 87 (1991) 107). It appeared that this model was capable of explaining the measured capacitances quite well for 2-heptanone, 2-octanone and 1,3-dibromopropane as the organic solvent, while for 1,2-dichloroethane — which is perhaps the most common solvent used in liquid ∣ liquid electrochemistry — the model needed to be modified, assuming a mixed layer between the bulk solvent phases as suggested earlier in the literature.     

Fabrication of agar-gel microelectrodes and their application in the study of ion transfer across the agar–water ∣ 1,2-dichloroethane interface

In this paper, we describe a simple procedure to make agar-gel microelectrodes by filling micropipettes. These microelectrodes were used to study K⁺ transfer across the agar–water ∣ 1,2-dichloroethane interface facilitated by dibenzo-18-crown-6 (DB18C6), and the transfer of tetraethylammonium (TEA⁺). The results observed were similar to those obtained at micro-liquid ∣ liquid interfaces. The effect of various amounts of agar in the aqueous phase was optimized and 3% agar was chosen based on the potential window and solidification time. The different shapes of micro-agar-gel electrodes were prepared in a similar way. The fabricated agar-gel microelectrodes obey the classical micro-disk steady-state current equation, which is different from the behavior of a normal micropipette filled with aqueous solution without silanization.     

In vitro evaluation of the polishing effect and optical properties of monolithic zirconia

Statement of problem

The relationship between surface roughness and the optical characteristics of zirconia prostheses may be affected by the type of shaded block.

In situ surface Raman study of the phosphorus incorporation mechanism during electrodeposition of Ni–P alloys

Codeposition of phosphorus with nickel on an Ni–Ag alloy electrode has been investigated by means of in situ surface enhanced Raman spectroscopy (SERS) to obtain information about the phosphorus incorporation mechanism during electrodeposition of Ni–P alloys. The experimental results showed that in the solution without NiSO₄, hypophosphite was reduced only to a Ni–phosphine compound, while in the case where NiSO₄ coexisted in the solution, the Ni–phosphine compound, as an intermediate, was oxidized by Ni²⁺ to elemental phosphorus in alloys with nickel acting as the catalyst.     

Frequency dependent electrolyte electroreflectance at semiconductor ∣ electrolyte interfaces

Electrolyte electroreflectance (EER) measurements at different modulation frequencies are carried out for the characterization of semiconductor ∣ electrolyte (SC ∣ EL) junctions. It is shown that the frequency dependence of the intensity and lineshape of EER signals is governed by the electrical impedance of the junction. Examples are given for n-RuS₂ and n-GaAs electrodes in contact with aqueous electrolyte. At the n-RuS₂ ∣ H₂O interface, electroreflectance spectra reveal the presence of an interfacial layer, most probably of RuO_x composition, which determines the behavior of this oxygen evolving electrode. Modulation of the electric field in this layer, and thus observation of its electroreflectance signal, requires the trapping of photogenerated holes at the RuS₂ ∣ RuO_x interface followed by recombination with conduction band electrons. At the n-GaAs ∣ H₂O interface, the oscillatory reduction of H₂O₂ is followed through EER measurements. Oscillation in baseline, lineshape and peak-to-peak intensity of the signals is observed. Baseline oscillation can only be observed at low modulation frequency and is attributed to a surface layer, probably As⁰. The lineshape reflects an oscillating inhomogeneity in the scl electric-field attributed to H⁺ intercalation in the near-surface of the GaAs electrode.     

Comparative analysis of alkali and alkaline-earth cation transfer assisted by monensin across the water ∣ 1,2-dichloroethane interface

In the first part of this paper the electrochemical transfer of alkali cations (M⁺) assisted by monensin (HX) across the water ∣ 1,2-dichloroethane (DCE) interface at pH<5, combined with a chemical exchange reaction at 5<pH<9, is proposed as the only mechanism responsible for the transfer of these cations. At pH>9 the current is voltammetrically negligible. An equation for the dependence of Δ_o^wφ_1/2 on pH and Na⁺ concentration is developed. In the second part of the paper the transfer of alkaline earth cations assisted by the same ionophore is studied. The electrochemical reactions (Me_(w)²⁺+HX_(o)?MeHX²⁺_(o)) and (Me_(w)²⁺+X^?_(o)?MeX⁺_(o)) are responsible for the peaks observed at pH<5.0 and at pH>9.0, respectively. As expected, in both cases ΔE_p=0.030 V while at intermediate pH the electrochemical exchange reaction (Me²⁺_(w)+HX_(o)?MeX⁺_(o)+H⁺_(w)) is proposed. The net charge transfer of +1 at the interface accounts for ΔE_p=0.060 V for the peak observed ΔE_p in agreement with the hyper-Nernstian slope of 60 mV found for Ca²⁺ and Ba²⁺ ISE based on antibiotics with carboxylic groups at intermediate pH values. The higher selectivity for Ba²⁺ and the tendency in selectivity at alkaline pH found in the ISEs are also observed and thus explained according to the mechanism proposed.     

Phospholipid monolayers studied by a combination of cyclic voltammetry and Langmuir techniques at the water ∣ 1,2-dichloroethane interface

Monolayers of distearoyl phosphatidylcholine spread on the water ∣ 1,2-dichloroethane interface have been studied in a novel type of Langmuir trough. Isotherms of the lipid have been measured at controlled potential differences across the interface, and cyclic voltammetry has been applied at a controlled surface pressure. Electrocapillary curves derived from the isotherms agree with the ones measured at true thermodynamic equilibrium. At positive potentials the adsorption of the lipid appeared to be weak while at negative potentials rather stable monolayers were obtained.     

In vivo study of the effectiveness of quantitative percussion diagnostics as an indicator of the level of structural pathology of teeth after restoration

Statement of problem

Conventional diagnostic aids based upon imagery and patient symptoms do not indicate whether restorative treatments have eliminated structural pathology.