Spectroelectrochemical analysis of ion-transfer and adsorption of the PAMAM dendrimer at a polarized liquid|liquid interface

The interfacial behavior of the fourth generation polyamidoamine (G4 PAMAM) dendrimer at a water|1,2-dichloroethane (DCE) interface was studied by cyclic voltammetry and potential modulated fluorescence (PMF) spectroscopy. Irregular voltammetric responses were observed at positively polarized interfaces. The cyclic voltammogram was strongly dependent on pH and on the concentrations of the G4 PAMAM dendrimer and the organic supporting electrolyte. PMF spectroscopy was successfully used to analyze the interfacial mechanism of the dendrimer by adding an anionic porphyrin derivative as a fluorescent probe. The results of the PMF measurements demonstrated that the G4 PAMAM dendrimer was transferred across the interface, a process that was accompanied by an adsorption step at pH 7. In contrast, under alkaline conditions, the adsorption process did not seem to be involved in the interfacial behavior.     

Potential scale for ion transfers at the water|1,6-dichlorohexane interface

1,6-dichlorohexane (DCH) is a solvent with a low permittivity (? = 8.8) in which ion-pairing affects electrolytic solutions. This can be noticed from voltammetric experiments at a microinterface between water (W) and DCH. Because most ion-association constants are unknown, the experimental measurements cannot be adequately corrected to calculate standard values for partition parameters, so that only conditional partition potentials can be measured. A formal ion partition potential scale is presented (electrolytes: 10⁻² M Li₂SO₄ in W; 10⁻² M THATPBCl in DCH) for a few tetraalkylammonium cations and some anions (hexafluorophosphate, perchlorate, picrate).     

Cation adsorption at a distearoylphosphatidic acid layer adsorbed at a liquid/liquid interface

The interfacial behaviour of tetraethylammonium cations (TEA⁺) at a liquid-liquid interface modified with an anionic phospholipid layer, distearoyl phosphatidic acid (DSPA), is analysed, with the purpose of characterising the permeation properties of the film. The TEA⁺ concentration in aqueous solution and the amount of DSPA solution employed to generate the lipidic layer, were varied. The results indicate that the layer is tightly compact and the transfer of TEA⁺ cations by permeation does not take place. Instead of this, TEA⁺ ions adsorb at the polar head groups of DSPA, and these adsorbed cations could be acting as nucleation centres of DSPA molecules when the DSPA amount is low.     

Derivation of the explicit equation relating mass-transport-limited-current to voltage at the interface between two immiscible electrolyte solutions (ITIES)

The potential drop between two immiscible electrolyte solutions consists of the sum of that across the double layer and the diffusion barrier layer. A relation between these components has been proposed by Indenbom. We extended his approach to give a relation between the current density and the overall potential drop between the two bulk solutions. The final expression is mathematically similar to the Butler–Volmer equation for classical electrode kinetics.     

Electrochemical behaviour of haemoglobin at the liquid/liquid interface

The electrochemical behaviour of haemoglobin (Hb) at the liquid/liquid interface was investigated. Based on ion transfer cyclic voltammetry and bulk ionolysis experiments, it is proposed that Hb adsorbs at the liquid/liquid interface and forms a multi-layer deposit thereafter repetitive cycling. However Hb does not cross the interface. The transfer peak current measured increased non-linearly with the bulk concentration of Hb in the aqueous phase but it did vary linearly with the square root of the scan rate. This diffusion-controlled process is ascribed to the Hb-facilitated transfer of the anion of the organic phase electrolyte. The electrochemical signal also increases with time suggesting a conformational re-arrangement of Hb at the liquid/liquid interface. The transfer peak potential was seen to vary with the nature of the anion, in agreement with the trend in hydrophobicity of the anions examined. Furthermore, at pH values ≥pI of Hb, no transfer peak was observed, suggesting that neutral or negatively charged Hb does not facilitate the transfer process.     

Charge transfer processes on liquid/liquid interfaces: The first century

Electrochemistry dealing with interfaces between two immiscible and ionically conductive solutions (ITIES) has been documented in literature already a hundred years ago. The work has only recently attracted a wider attention, with more modern treatment founded by the late Professor Ji?i Koryta. The principle of the work, its past and new trends in experimental electrochemistry, optical studies of the interface and recently emerging theoretical work are reviewed.     

Interaction of acridine-calix[4]arene with DNA at the electrified liquid|liquid interface

The behaviour of an acridine-functionalised calix[4]arene at the interface between two immiscible electrolyte solutions (ITIES) is reported. Molecular modelling showed that the acridine-calix[4]arene has regions of significant net positive charge spread throughout the protonated acridine moieties, consistent with it being able to function as an anion ionophore. The presence of this compound in the organic phase facilitated the transfer of aqueous phase electrolyte ions. Upon addition of double stranded DNA to the aqueous phase, the transfer of electrolyte anions was diminished, due to DNA binding to the acridine moiety at the ITIES. The behaviour provides a basis for DNA hybridization detection using electrochemistry at the ITIES.     

Kinetics of facilitated proton transfer by hydrophobic aromatic amines across the water/1,2-dichloroethane interface

The kinetics of proton transfer facilitated by 4-octylaniline, 4-dodecylaniline and 4-hexadecylaniline across the water/1,2-dichloroethane interface has been investigated by cyclic voltammetry and ac impedance. It was found that their electrochemical behaviour is very similar and the three amines assist proton transfer from the aqueous to the organic phase. The formal Gibbs energy of proton transfer and the formal rate constant were evaluated. It is concluded that interfacial organisation of these molecules is an important factor in the kinetics of facilitated proton transfer and that the aromatic ring is able to penetrate the interface whilst the long hydrophobic chain contributes to the stability of the amine in the organic phase.     

Enhancement of electrochemical ion/electron-transfer reaction at solid|liquid interface by polymer coating on solid surface

Controlling the ion/electron-transfer reaction at the electrode|electrolyte (solid/liquid) interface is a necessary and vitally important point for improving the operation of various electrochemical devices. In this paper, we propose a polymer coating technique which enhances the Li⁺ ion-transfer reaction at the LiCoO₂|electrolyte interface, and confirm the effects of the polymer coating by cyclic voltammetry (CV) and AC impedance techniques. The results from the experiments indicated that the application of an F-introduced polymer on the LiCoO₂ (LCO) surface decreased the activation barrier for Li⁺ ion transfer. Besides the electrochemical studies, the present computational results indicated that the Li⁺ exchange process between two states, which are both solved with ethylene carbonate (EC) and coordinated with an F-introduced polymer, might occur due to the close energy levels of Li⁺ stability. Accordingly, we inferred that the transition state of this exchange process promotes the observed decrease in activation energy for the interfacial Li⁺-transfer reaction.     

Energy conversion at liquid/liquid interfaces: artificial photosynthetic systems

This chapter focuses on multielectron reactions in organized assemblies of molecules at the liquid/liquid interface. We describe the thermodynamic and kinetic parameters of such reactions, including the structure of the reaction centers, charge movement along the electron transfer pathways, and the role of electric double layers in artificial photosynthesis. Some examples of artificial photosynthesis at the oil/water interface are considered, including water photooxidation to the molecular oxygen, oxygen photoreduction, photosynthesis of amphiphilic compounds and proton evolution by photochemical processes.     

Promazine π-mers formation at a 1,2-dichloroethane/water interface

The behavior of promazine in 1,2-dichloroethane was studied using cyclic voltammetry at the ITIES and UV-vis spectrophotometry. The analysis of voltammograms and spectra obtained varying promazine concentration, pH, nature and concentration of organic electrolyte and applying positive polarisation at the interface allowed us to postulate a reaction scheme consisting in a first step of promazine partition to organic phase, followed by a charge transfer complex (CTC) formation with 1,2-dichloroethane. This CTC induces the oxidation of promazine to the corresponding radical cation which is stabilised in organic phase by π-mers formation. π-mer complexes form ion pairs with the anions of the organic base electrolyte. Similar results were found using methotrimeprazine, another phenothiazine derivative, with a methoxyl group attached to the ring. Chlorpromazine, triflupromazine, perphenazine and fluphenazine were stable in 1,2-dichloroethane.     

The transfer mechanisms and analytical properties of the variable-valency drug cinchonidine across the liquid/liquid interface

The electrochemical transfer of cinchonidine across the liquid/liquid (L/L) interface was investigated by linear current-scanning polarography at the ascending water electrode and chronopotentiometry at the stationary water electrode. The relation between half-wave potential and the pH value of the aqueous phase was established and is in good agreement with the experimental results. Defending on the pH value in the aqueous phase, both singly- and doubly-charged cinchonidine cations can transfer across the L/L interface. This process is controlled simultaneously by diffusion and the rate of re-establishment of the disturbed protonation equilibrium. The effects of the supporting electrolyte, buffer and organic solvent on the polarographic wave were examined, and the transfer characteristics of cinchonidine in the aqueous and organic phases were compared. Direct determinations of cinchonidine in the aqueous phase and organic phase were carried out by linear current-scanning polarography with high selectivity. The linear range for determination is 5 × 10⁻⁵−1 × 10⁻³ mol dm⁻³.     

Investigation of liquid–liquid interfacial electron transfer kinetics using multicenter ferrocenyl complexes

The redox behavior of two novel multicenter redox molecules (triferrocenylmethane and triferrocenylmethanol) has been studied in a thin film of nitrobenzene (NB) imposed between a graphite electrode and an aqueous electrolyte. The well separated three sets of redox peaks indicate strong intramolecular electronic communications between the three ferrocene centers in each molecule. They were adapted as model compounds for the study of electron transfer kinetics across the liquid/liquid interface with varied overall driving force using only one-type redox couples in the organic and aqueous phase, respectively. It has been shown that in both cases the dependence of interfacial electron transfer rate on the increased overall driving force across the nitrobenzene/water interface is not monotonic.     

Passage of a bubble through the interface between a shear-thinning heavier liquid and a Newtonian lighter liquid

Abstract

Numerical simulations are carried out to study the phenomenon of passage of a bubble through the interface between two initially quiescent liquids. The heavier liquid is a viscous shear-thinning liquid whereas the lighter liquid is a viscous Newtonian liquid. The computational model, which is based on axisymmetric computational domain and VOF (volume of fluid) method, is validated by using the data reported in literature. The validated computational model is then used to perform parametric analysis to understand the effects of bubble diameter, interfacial tension between the two liquids and flow index of the heavier liquid on the drainage time, drainage height, retention time and retention height of the bubble at the liquid–liquid interface. Finally, correlations for retention time and retention height are proposed.     

Electrodes covered with random arrays of microdroplets: heterogeneous electron transfer coupled to catalytic reaction at the liquid/liquid interface in the prism film geometry approximation

A model for the catalytic (EC’) mechanism on the three-phase electrode arrangement is developed. It is assumed that a solid electrode surface is partly covered by an insulating solution of substance A in an organic liquid which is immiscible with water and immersed into an aqueous solution of a depolarizer and a supporting electrolyte. At the liquid/liquid interface the substance A reacts with the aqueous phase product of the electrode reaction occurring at the uncovered parts of the electrode with regeneration of the depolarizer. An average cyclic voltammetry response, a distribution of the current density and a pure kinetic current are calculated using a finite difference method.     

New mechanism of gas transport at the interface solid/liquid

It was recently shown that an abnormally fast transport of CO molecules takes place at the electrode/electrolyte interface of Pt and PtRu electrodes in H₂SO₄ and HClO₄ solutions. In the present paper, this phenomenon is tested for other gases, such as hydrogen and oxygen. The fast transport is also observed at the solid/electrolyte solution interface of other electrode materials and at the glass/electrolyte interface. Several experiments are shown, demonstrating that mass transfer takes place at a velocity, which is more than one order of magnitude higher than expected for usual diffusion conditions.Assuming radial mass transfer at the interface of a Pt disc, the activation energy, E_a = 23 kJ mol⁻¹, was calculated from Arrhenius plots. The same value was measured in H₂SO₄ and HClO₄ as supporting electrolytes. The mass transport parameter, Y, at 298 K was 4.8 × 10⁻³ cm² s⁻¹ and 2.9 × 10⁻³ cm² s⁻¹ in 0.5 M H₂SO₄ and 1 M HClO₄ respectively.     

Liquid-liquid interfacial processes at hydrophobic silica carbon composite electrodes: ion transfer at water-nitrobenzene, water-o-nitrophenyloctylether, and at water-o-nitrophenylphenylether interfaces

Ion transfer processes across liquid-liquid phase boundaries of the type aqueous solution-polar organic solvent supported on a hydrophobic silica carbon composite are studied by cyclic voltammetry and differential pulse voltammetry. The organic phase consists of a redox probe (ferrocene, t-butylferrocene, or decamethylferrocene) dissolved in a polar hydrophobic solvent (nitrobenzene, o-nitrophenyloctylether, or o-nitrophenylphenylether). The organic phase was immobilised in a ceramic carbon material composed of a hydrophobic silicate prepared via a sol-gel process from a methyltrimethoxysilane based sol and carbon particles. When immersed into aqueous electrolyte, ion transfer processes can be monitored as a function of potential. The contributions of solvent, electrolyte, and redox probe to the transition from anion transfer to cation transfer are discussed. Effects due to the presence of a high surface area microporous solid matrix are considered.     

Double-layer effects on the Cs ion transfer kinetics at the water/nitrobenzene interface

Double layer effects on the kinetics of the Cs⁺ ion transfer across the water/nitrobenzene interface were studied by an ac polarographic technique. Apparent kinetic parameters agree well with those reported previously. By applying the Frumkin-type correction based on the Gouy-Chapman theory, the non-linear Tafel plots were inferred indicating some dependence of the charge transfer coefficient on the interfacial potential difference. The latter effect can weaken the contribution of the diffuse-layer potentials to the apparent kinetic parameters, thereby accounting in part for the observed negligible change of these parameters with the aqueous electrolyte concentration. The Frumkin-type correction failure at low electrolyte concentrations and high surface charge densities is due to an overestimation of the diffuse/layer potential by the Gouy-Chapman theory, rather than due to the dynamic (Levich) effect of the diffuse double layer.     

Experimental study of O2 diffusion coefficient measurement at a planar gas–liquid interface by planar laser‐induced fluorescence with inhibition

A new method for determining the molecular diffusivity of oxygen in liquids is described. The technique was applied through a flat air–liquid interface in a Hele‐Shaw cell (5 × 5 × 0.2 cm³) and was based on planar laser‐induced fluorescence (PLIF) with inhibition. A ruthenium complex (C₇₂H₄₈N₈O₆Ru) was used as the fluorescent dye sensitive to oxygen. A mathematical analysis was developed to determine the molecular diffusivity of oxygen simply by localizing the gas diffusion front. The specificity of this mathematical analysis is that it does not require the properties of the fluids (such as the saturation concentration) to be considered, which is especially relevant for complex media that are sometimes difficult to characterize properly. This technique was applied to three different fluids (viscosities ranging from 1 to 2.4 mPa·s) corresponding to binary diffusion coefficients ranging from 9.5 × 10^?10 to 2 × 10^?9 m²/s. Experimental data were found with an uncertainty of about 5% and were in good agreement with the literature. Particle image velocimetry and numerical simulations were also carried out to determine the optimal gas flow rate (0.01 L/s) to reach purely diffusive transfer, and the corresponding hydrodynamic profiles of the two phases. © 2012 American Institute of Chemical Engineers AIChE J, 59: 325–333, 2013     

Approximate theoretical solution for the Sherwood number of oscillating bubbles at different Reynolds numbers

In this paper a theoretical approach to the effect of bubble oscillations on the mass transfer rates has been carried out to get a better understanding of the effect of bubble oscillations in multiphase gas–liquid contactors. The perturbation method has been used to approximate the velocity profile surrounding the bubbles while they oscillate. The shape of the oscillating bubble was modelled taking into account the effect of the liquid viscosity on bubble oscillation frequency and amplitude. The modelled shapes match the photographs of bubbles oscillating in liquids with different viscosities. As a result, new approximate theoretical models for the Sherwood number in viscous fluids at different flow regimes have been proposed. The models extend the work already available in the literature for mass transfer rates from oscillating bubbles in inviscid fluids and provides good results in predicting the Sherwood number at high and moderate Reynolds numbers, the preferred regimes in many industrial operations where, as a result of the hydrodynamics processes involving the bubbles. Their oscillations do not completely decay.