Electron-conductor separating oil–water (ECSOW) system: a new strategy for characterizing electron-transfer processes at the oil/water interface

A new electrochemical method for studying the electron transfer (ET) at the oil (O)/water (W) interface (or the liquid/liquid) interface has been devised, in which the O- and W-phases are separated by an electron conductor (EC; e.g. Pt). For the EC separating O–W (ECSOW) system, the ET across the EC phase can be observed voltammetrically in a similar manner to the O/W interface, however, no ion-transfer (IT) process can be taken place. Although the ECSOW system is thermodynamically equivalent to the corresponding O/W interface, they may be different from a kinetic viewpoint. In practice, the cyclic voltammograms obtained with the nitrobenzene NB/W interface and the ECSOW system in the presence of ferrocene in NB and hexacyanoferrate in W have shown quite different features, when the concentrations of both redox species are lower. The voltammograms for the NB/W interface have strongly supported the IT mechanism which involves an interfacial transfer of ferricenium ion. Also, the ECSOW system has been shown to be promising for clarification of complicated charge-transfer processes involving biological compounds such as l-ascorbic acid.     

Voltammetric study of the transfer of fluoride ion at the nitrobenzene / water interface assisted by tetraphenylantimony.

The transfer of F- ion assisted by an organometallic complex cation tetraphenylantimony (TPhSb+) across the polarized nitrobenzene / water (NB / W) interface has been studied by means of ion-transfer voltammetry. A well-defined voltammetric wave was observed within the potential window at the NB / W interface when tetraphenylantimony tetrakis(4-chlorophenyl) borate and F- ion were present in NB and W, respectively. The voltammogram can be interpreted as being due to the reversible transfer of F- ion assisted by the formation of the TPhSbF complex through the coordination of F- to Sb atom in NB. The formal formation constant of TPhSbF in NB has been determined to be 10(1.95 +/- 0.2 M(-1). No voltammetric wave due to the TPhSb(+)-assisted transfer of other anions such as Cl-, Br, I-, NO3-, CH3COO- and H2PO4(-) ions has been observed within the potential window.     

Ion transfer of bromophenol blue across liquid-liquid interfaces

The ion transfer of the acidic dye bromophenol blue (BPB) at the interfaces of water/nitrobenzene (W/NB), water/1,2-dichloroethane (W/1,2-DCE) and water/(nitrobenzene+chlorobenzene) (W/(NB +CB)) was studied in detail by cyclic voltammetry (CV), chronopotentiometry with linear current scanning (CLC), controlled potential electrolysis and UV spectroscopic methods. Using controlled potential electrolysis, we observed successfully the transfer process of BPB across the W/NB interface from the colour changes of BPB in two different phases. The proposed transfer mechanism for BPB is proved to be reasonable using UV spectroscopy of the product of the electrolysis. The standard potential differences Δ^o_w^o and the standard Gibbs energies of the BPB transfer from water to some organic solvents were calculated. The dissociation constants of BPB obtained were quite close to the literature values.     

Ion-transfer voltammetry of local anesthetics at an organic solvent/water interface and pharmacological activity vs. ion partition coefficient relationship.

The ion-transfer reaction of local anesthetics at an organic solvent/water interface has been studied using cyclic voltammetry (CV) with a stationary nitrobenzene (NB)/water (W) interface. Procaine and seven other local anesthetics gave reversible or quasi-reversible voltammograms at the NB/W interface in the pH range between 0.9 and 9.6. These drugs are present in aqueous solution in either neutral or ionic form, or both forms. The half-wave potential, as determined by the midpoint potential in CV, vs. pH curves, were determined and analyzed to determine the partition coefficients of both neutral and ionic forms of the drugs between NB and W. The partition coefficients of the ionic forms were derived from their formal potential of transfer at an NB/W interface. The dissociation constants of ionic forms of the drugs in NB were also deduced. A high correlation between the pharmacological activity and the partition coefficient of the ionic form of amide-linked local anesthetics has been shown.     

A comparative study of the anion transfer kinetics across a water/nitrobenzene interface by means of electrochemical impedance spectroscopy and square-wave voltammetry at thin organic film-modified electrodes

The kinetics of the transfer of a series of hydrophilic monovalent anions across the water/nitrobenzene (W/NB) interface has been studied by means of thin organic film-modified electrodes in combination with electrochemical impedance spectroscopy and square-wave voltammetry. The studied ions are Cl-, Br-, I-, ClO4-, NO3-, SCN-, and CH3COO-. The electrode assembly comprises a graphite electrode (GE) covered with a thin NB film containing a neutral strongly hydrophobic redox probe (decamethylferrocene or lutetium bis(tetra-tert-butylphthalocyaninato)) and an organic supporting electrolyte. The modified electrode is immersed in an aqueous solution containing a supporting electrolyte and transferring ions, and used in a conventional three-electrode configuration. Upon oxidation of the redox probe, the overall electrochemical process proceeds as an electron-ion charge-transfer reaction coupling the electron transfer at the GE/NB interface and compensates ion transfer across the W/NB interface. The rate of the ion transfer across the W/NB interface is the limiting step in the kinetics of the overall coupled electron-ion transfer reaction. Moreover, the transferring ion that is initially present in the aqueous phase only at a concentration lower than the redox probe, controls the mass transfer regime in the overall reaction. A rate equation describing the kinetics of the ion transfer that is valid for the conditions at thin organic film-modified electrodes is derived. Kinetic data measured with two electrochemical techniques are in very good agreement.     

A true electron-transfer reaction between 5,10,15,20-tetraphenylporphyrinato cadmium(II) and the hexacyanoferrate couple at the nitrobenzene/water interface.

The ability of some metal complexes of 5,10,15,20-tetraphenylporphyrin (TPP) to give a voltammetric wave due to the heterogeneous electron transfer (ET) at a nitrobenzene (NB)/water (W) interface has been examined. The previously-proposed, electron-conductor separating oil-water (ECSOW) system has been successfully employed to find that the TPP complex with cadmium(II) added to NB gives a well-defined, reversible wave for the heterogeneous (i.e., "true") ET with the hexacyanoferrate couple in W. A digital simulation analysis has entirely excluded the possibility of the ion-transfer mechanism due to the homogeneous ET in W. The a.c. impedance method has then been used to determine the kinetic parameters including the standard rate constant k0 (= 0.10 cm M(-1) s(-1)) and the transfer coefficient alpha (= 0.53 at the half-wave potential). These values are in good agreement with those predicted from the Marcus theory with the assumption that the heterogeneous ET due to molecular collision occurs at the "sharp" NB/W interface.     

Potential-dependent adsorption and transfer of poly(diallyldialkylammonium) ions at the nitrobenzene|water interface

Electrochemically driven adsorption and partition of a series of poly(diallyldialkylammonium) ions (PDADAA(+): alkyl = methyl, ethyl, propyl, and butyl) at the nitrobenzene (NB)|water (W) interface have been studied using voltammetry and electrocapillary measurements. When the phase-boundary potential, Δφ, that is, the inner potential of the W phase referred to that of the NB phase, is negative, poly(diallyldimethylammonium) (PDADMA(+)) shows little surface activity. The scanning of Δφ in the positive direction induces, first, the adsorption of PDADMA(+) at the interface and, then, the desorption of adsorbed PDADMA(+) ions into the NB phase, followed by the diffusion-limited transfer of PDADMA(+) from W to NB. The elongation of the dialkyl chains gives the stronger surface activity of PDADAA(+) even when Δφ < 0. The PDADAA(+) polyions studied are only slightly more hydrophilic than the corresponding monomers. However, the polycationic character of PDADAA(+) renders the adsorption, desorption, and ion transfer strongly dependent on Δφ and gives rise to unusual, M-shaped electrocapillary curves. The interplay of adsorption-desorption and ion transfer of PDADAA(+) ions induces the electrochemical instability of the interface and the emulsion formation on the NB side of the interface.     

Cyclic Voltammetry of Ion Transfer for Phenylpropanolamine Hydrochloride at Water|Nitrobenzene Interface

The transfer on phenylpropanolamine ion, PPAH⁺, has been studied at the Interface between Two Immiscible Solutions (ITIES). The polarizable potential range was determined by cyclic voltammetry at the interface between an aqueous solution of lithium chloride (LiCl) and a nitrobenzene (NB) solution of electrolyte tetrabutylammonium tetraphenylborate (TBATPB). The half‐wave potential of ion transfer for phenylpropanolamine accross the water|NB interface was found 465.3 mV. The peak separation, the diffusion coefficient, and the standard ion transfer potential of PPAH⁺ were observed to be 59.1 mV, 1.7 × 10^?6 cm²/s, and 104.6 mV, respectively. The temperature of experiment was kept constantly at 25 ± 1 °C using water flow thermostate.     

Evaluation of Gibbs free energy for the transfer of a highly hydrophilic ion from an acidic aqueous solution to an organic solution based on ion pair extraction

A method to determine the standard Gibbs free energy for the transfer, ΔG°_tr, of a highly hydrophilic metal ion from an aqueous solution, W, in the presence of high concentration of H⁺ to an organic solution, O, was proposed based on the theoretical consideration of the distribution process of ions between W and O. The usefulness of the proposed method was verified experimentally by comparing ΔG°_tr of Mg²⁺ determined by the method with that obtained by voltammetry for the ion transfer at the W|O interface. The O examined were nitrobenzene (NB) and 1,2-dichloroethane (DCE). By applying the proposed method, ΔG°_tr of NpO₂⁺, UO₂²⁺, NpO₂²⁺ and PuO₂²⁺ from an acidic W to NB were determined.     

Determination of the standard Gibbs energies of transfer of cations across the nitrobenzene|water interface utilizing the reduction of iodine in an immobilized nitrobenzene droplet

When a nitrobenzene (NB) droplet containing iodine is attached to a graphite electrode and immersed into a chloride containing aqueous (AQ) solution, the electrochemical reduction of iodine is accompanied by a transfer of chloride ions from NB to water. These chloride ions enter the NB phase in a preceding partition between the AQ and the NB phases, supported by formation of I₂Cl⁻ ions in NB and accompanied by the transfer of stoichiometric amounts of cations. The overall electrode reaction is of CE_rev type, where C refers to the preceding chemical step forming I₂Cl⁻, and E_rev refers to the reversible reduction of iodine at the graphite|NB interface and the simultaneous transfer of chloride from NB to water. If the chloride concentration in NB is insufficient to compensate by leaving the NB the amount of electrochemically produced iodide, a second voltammetric signal occurs at more negative potentials due to the transfer of iodide from NB to water. The kinetics and thermodynamics of the preceding chemical step C, determine the voltammetric behaviour of the system in such way that the ratio of peak currents of the first and second signals depends linearly on the Gibbs energy of transfer of the co-partitioned cations. The method was validated for cations of known Gibbs energies of transfer, and it was applied to cations of amino acids.     

液/液界面新溶剂体系的电化学研究

用循环伏安法测定了离子在水-异硫氰酸烯丙脂(AIT)体系中的标准转移Gibbs能△_o~w G_(tr,i)~0。对含有AIT的混合溶剂的研究, 发现了一系列电位窗比较宽的水/有机溶剂体系, 讨论了溶剂效应对△_o~w G_(tr,i)~0的影响。     

Probing non-polarizable liquid/liquid interfaces using scanning ion conductance microscopy

The study of microscopic structure of a liquid/liquid interface is of fundamental importance due to its close relation to the thermodynamics and kinetics of interfacial charge transfer reactions.In this article,the microscopic structure of a non-polarizable water/nitrobenzene(W/NB) interface was evaluated by scanning ion conductance microscope(SICM).Using SICM with a nanometer-sized quartz pipette filled with an electrolyte solution as the probe,the thickness of this type of W/NB interface could be measured at sub-nanometer scale,based on the continuous change of ionic current from one phase to another one.The effects for thicknesses of the non-polarizable W/NB interfaces with different electrolyte concentrations,the Galvani potentials at the interface and the applied potentials on the probe were measured and systematically analyzed.Both experimental setups,that is an organic phase up and an aqueous down,and a reverse version,were employed to acquire the approach curves.These data were compared with those of an ideal polarizable interface under the similar experimental conditions,and several characteristics of non-polarizable interfaces were found.The thickness of a non-polarizable interface increases with the decrease of electrolyte concentration and the increase of applied potential,which is similar to the situation of a polarizable liquid/liquid interface.We also find that the Galvani potential across a non-polarizable interface can also influence the interfacial thickness,this phenomenon is difficult to observe when using polarizable interface.Most importantly,by the comparison of two kinds of liquid/liquid interfaces,we experimentally proved that much more excess ions are gathered in the space charge layer of non-polarizable interfaces than in that of polarizable interfaces.These results are consistent with the predictions of molecular dynamic simulations and X-ray reflectivity measurements.     

Avalanche transfer of charged particles across the electrochemical liquid|liquid interface

Emulsion particles formed in the vicinity of the interface between nitrobenzene (NB) and water (W) pass through the interface concurrently, resulting in current spikes on current versus potential and current versus time curves. The fact that the spikes appear only in the limited range of the phase boundary potential suggests the importance of the electrostatic interaction between charged particles and the polarized NB∣W interface. Even in the potential range where the transfer of emulsions is favored, the transfer occurs only intermittently. Each current spike reflects the transfer of an accumulation of emulsion particles that were dammed up in the vicinity of the interface, which resembles an avalanche.     

铬天青S的液/液界面离子转移过程

本文研究了酸性染料显色剂铬天青S的液/液界面离子转移行为, 用循环伏安法和电流扫描计时电位法研究了铬天青S在水/硝基苯和水/1,2-二氯乙烷两种界面上的离子转移过程, 根据铬在青S在溶液中的离解平衡和电化学性质, 讨论了界面离子转移机理,研究了基础电解质和溶剂对铬天青S转移性能的影响, 在Britton-Robinson缓冲溶液中测得半波电位PH曲线与理论公式相一致, 由本法所得离解常数与文献值接近, 计算了转移离子的标准转移电位和标准吉布斯转移能。     

Simple electrochemical method for deposition and voltammetric inspection of silver particles at the liquid-liquid interface of a thin-film electrode

A novel experimental methodology for depositing and voltammetric study of Ag nanoparticles at the water-nitrobenzene (W-NB) interface is proposed by means of thin-film electrodes. The electrode assembly consists of a graphite electrode modified with a thin NB film containing decamethylferrocene (DMFC) as a redox probe. In contact with an aqueous electrolyte containing Ag(+) ions, a heterogeneous electron-transfer reaction between DMFC((NB)) and Ag(+)((W)) takes place to form DMFC(+)((NB)) and Ag deposit at the W-NB interface. Based on this interfacial reaction, two different deposition strategies have been applied. In the uncontrolled potential deposition protocol, the electrode is immersed into an AgNO(3) aqueous solution for a certain period under open circuit conditions. Following the deposition step, the Ag-modified thin-film electrode is transferred into an aqueous electrolyte free of Ag(+) ions and voltammetrically inspected. In the second protocol the deposition was carried out under controlled potential conditions, i.e., in an aqueous electrolyte solution containing Ag(+) ions by permanent cycling of the electrode potential. In this procedure, DMFC((NB)) is electrochemically regenerated at the electrode surface, hence enabling continuation and voltammetric control of the Ag deposition. Hence, the overall electrochemical process can be regarded as an electrochemical reduction of Ag(+)((W)) at the W-NB interface, where the redox couple DMFC(+)/DMFC acts as a mediator for shuttling electrons from the electrode to the W-NB interface. Ag-particles deposited at the W-NB interface affect the ion transfer across the interface, which provides the basis for voltammetric inspection of the metal deposit at the liquid-liquid interface with thin-film electrodes. Voltammetric properties of thin-film electrodes are particularly sensitive to the deposition procedure, reflecting differences in the properties of the Ag deposit. Moreover, this methodology is particularly suited to inspect catalytic activities of metal particles deposited at the liquid-liquid interface toward heterogeneous electron-transfer reactions occurring at the at the liquid-liquid interface.     

Direct quantification of creatinine in human urine by using isotope dilution extractive electrospray ionization tandem mass spectrometry

Urinary creatinine (CRE) is an important biomarker of renal function. Fast and accurate quantification of CRE in human urine is required by clinical research. By using isotope dilution extractive electrospray ionization tandem mass spectrometry (EESI–MS/MS) a high throughput method for direct and accurate quantification of urinary CRE was developed in this study. Under optimized conditions, the method detection limit was lower than 50 μg L⁻¹. Over the concentration range investigated (0.05–10 mg L⁻¹), the calibration curve was obtained with satisfactory linearity (R² = 0.9861), and the relative standard deviation (RSD) values for CRE and isotope-labeled CRE (CRE-d3) were 7.1–11.8% (n = 6) and 4.1–11.3% (n = 6), respectively. The isotope dilution EESI–MS/MS method was validated by analyzing six human urine samples, and the results were comparable with the conventional spectrophotometric method (based on the Jaffe reaction). Recoveries for individual urine samples were 85–111% and less than 0.3 min was taken for each measurement, indicating that the present isotope dilution EESI–MS/MS method is a promising strategy for the fast and accurate quantification of urinary CRE in clinical laboratories.     

Quantitative analysis of the structure-hydrophobicity relationship for di- and tripeptides based on voltammetric measurements with an oil/water interface

The transfer of 18 di- and 27 tripeptides with un-ionizable amino acid side chains at a nitrobenzene/water (NB/W) interface was studied by cyclic voltammetry. The reversible half-wave potential (E(r)(1/2)), i.e., the midpoint potential could be accurately determined at pH 2 for both the facilitated and non-facilitated transfers, respectively, in the presence and absence of dibenzo-18-crown-6 (DB18C6) in NB. A multiple linear regression analysis was then performed for the E(r)(1/2) using the 'corrected' Dubois steric parameter for amino acid side chain substitutents. The result shows that the hydrophobicity of the peptides is governed not only by the intrinsic hydrophobicity of the peptide backbone and side chains, but also by the steric effects of side chain substituents. For the non-facilitated transfer of peptides, the steric effect of a bulky side chain is more significant at the N-terminus than at the C-terminus (and central for tripeptides). The more bulky the side chain at the N-terminus, the less hydrophobic the peptide becomes due to inhibition of the solvation of a terminal -NH(3)(+) group by organic solvents. For the facilitated transfer by DB18C6, however, the steric effect of a bulky side chain is the most significant at the central position of a tripeptide. A MOPAC calculation of optimized structures of DB18C6-peptide complexes has also shown that there is a notable steric hindrance between the central side chain and the benzene rings of DB18C6, which would reduce the 'apparent' hydrophobicity or transferability of the tripeptide.     

Investigation of the radical copolymerization and terpolymerization of maleic anhydride and norbornenes by an in situ 1H NMR analysis of kinetics and by the mercury method: Evidence for the lack of charge‐transfer‐complex propagation

The radical copolymerization of electron‐deficient maleic anhydride (MA) and electron‐rich norbornene (NB) derivatives with 2,2′‐azobis(isobutyronitrile) (AIBN) in dioxane‐d₈ has been monitored in situ by ¹H NMR spectroscopy with free induction decays recorded every 30 min at 60, 70, or 84 °C. The ratios of the monomer pairs were varied in some cases. The NB derivatives employed in this study included bicyclo[2.2.1]hept‐2‐ene (NB), t‐butyl 5‐norbornene‐2‐carboxylate, methyl 5‐norbornene‐2‐methyl‐2‐carboxylate, and ethyl tetracyclo[4.4.0.1^2,5.1^7,10]dodec‐3‐ene‐8‐carboxylate. Decomposition of AIBN, consumption of the monomers, feed ratios, endo/exo ratios, copolymer compositions, and copolymer yields were studied as a function of polymerization time. Furthermore, a homopolymerizable third monomer (t‐butyl methacrylate, methacrylic acid, t‐butyl acrylate, or acrylic acid) was added to the NB/MA 1/1 system, revealing that the methacrylic monomer polymerizes rapidly in the early stage and that the ratio of MA to NB in the terpolymer strongly deviates from 1/1. In contrast, however, the acrylic monomers are more uniformly incorporated into the polymer. Nevertheless, these studies indicate that MA and NB do not always behave as a pair in radical polymerization and disproves the commonly believed charge‐transfer mechanism. Electron‐deficient fumaronitrile was also included in the kinetics study. To further understand the copolymerization mechanism, MA and NB were competitively reacted with a cyclohexyl radical generated by the treatment of cyclohexylmercuric chloride with sodium borohydride (mercury method). A gas chromatographic analysis of the reaction mixtures has revealed that a cyclohexyl radical reacts with MA almost exclusively in competition and that the cyclohexyl adduct of MA essentially accounts for all the products in a mass balance experiment, eliminating a possibility of the formation of an adduct involving the MA–NB charge‐transfer complex. Thus, the participation of a charge‐transfer complex in the copolymerization of MA and NB cannot be important. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 3521–3542, 2000     

Ion-transfer voltammetry at a polarized room-temperature molten salt/water interface.

Tetraoctylammonium cation forms a room-temperature molten salt (RTMS) with 2,4,6-trinitrophenolate anion. The RTMS is immiscible with water (W) and forms a stable RTMS/W interface. It has been shown that the RTMS/W interface can be electrochemically polarized. A well-defined voltammetric wave due to the transfer of thiocyanate ion across the RTMS/W interface was observed within the potential window. This is the first example of a polarized RTMS/W interface.     

An electrochemical method for determination of the standard Gibbs energy of anion transfer between water and n-octanol

The transfer of the ions Cl⁻, Br⁻, I⁻, ClO₄⁻, SCN⁻, NO₃⁻, BF₄⁻, and (C₆H₅)₄B⁻ across the water|n-octanol (W|OC) liquid interface was studied and the standard Gibbs energies of ion transfer were determined. The ion transfer was achieved by oxidation of decamethylferrocene dissolved in a droplet of n-octanol that was attached to a graphite electrode immersed in the aqueous solutions of the respective alkali salts of the anions. The electrode reaction can be described by the equation: dmfc_(OC)+X_(W)⁻⇄dmfc⁺_(OC)+X⁻_(OC)+e⁻, where X⁻ is the transferred anion. Square-wave voltammetry at this three-phase arrangement was utilised to determine the formal potential of the decamethylferrocene/decamethylferrocenium (dmfc/dmfc⁺) couple under the condition of ion transfer across the water|n-octanol interface. For calibration the standard Gibbs energies of ion transfer have been extrapolated to octanol from the series of known data for methanol, ethanol, n-propanol, and n-butanol. All these data are consistent and the experimental dependence of the formal potentials on the standard Gibbs energies is as predicted by theory. The validity of data is further supported by calculations of Gibbs energies of ion transfer using the Born theory. Until now it was not possible to perform electrochemical measurements at the water|n-octanol interface because in the conventional four-electrode cells this interface cannot be polarised. With the new method it is now for the first time possible to determine the Gibbs energies of transfer of ions across the water|n-octanol interface. These values are of very wide use for assessing the lipophilicity of compounds in chemistry, medicine, and pharmacology.