Redox Liquid Research Articles

Electrocatalytic Reactions Mediated by N,N,N‘,N‘-Tetraalkyl-1,4-phenylenediamine Redox Liquid Microdroplet-Modified Electrodes: Chemical and Photochemical Reactions In, and At the Surface of, Femtoliter Droplets

The electro-oxidation of electrolytically unsupported ensembles of N,N-diethyl-N',N'-dialkyl-para-phenylenediamine (DEDRPD, R = n-butyl, n-hexyl, and n-heptyl) redox liquid femtoliter volume droplets immobilized on a basal plane pyrolytic graphite electrode is reported in the presence of aqueous electrolytes. Electron transfer at these redox liquid modified electrodes is initiated at the microdroplet-electrode-electrolyte three-phase boundary. Dependent on both the lipophilicity of the redox oil and that of the aqueous electrolyte, ion uptake into or expulsion from the organic deposits is induced electrolytically. In the case of hydrophobic electrolytes, redox-active ionic liquids are synthesized, which are shown to catalyze the oxidation of l-ascorbic acid over the surface of the droplets. In contrast, the photoelectrochemical reduction of the anaesthetic reagent halothane proceeds within the droplet deposits and is mediated by the ionic liquid precursor (the DEDRPD oil).

Voltammetry of undiluted redox systems backed by in-situ Raman spectroscopy. Evidence for strong accumulation of ions in the diffusion layer at microelectrode surface

Voltammetry of undiluted redox liquids may result in the formation of thin layers of ionic liquids at the microelectrode surface. Such a situation was postulated for several liquids (solvents) including methanol, however, no direct evidence supporting this expectation was obtained. By using in-situ Raman spectroscopy in combination with voltammetric experiments, we have shown, that the concentration of the counterion, ClO 4 −, is really much enhanced in the electrode vicinity compared to the methanol bulk.

Ceramic Carbon Electrode Modified with Butyloferrocene

AbstractCeramic carbon electrode modified with butyloferrocene was prepared and studied. The electrode consists of graphite powder, homogeneously dispersed in hydrophobic silica matrix. The latter was obtained from organically modified silicate – methyltrimethoxysilane by sol‐gel process. After gelation and drying it was filled with redox liquid – butyloferrocene by immersion. The properties of the electrode were investigated by cyclic voltammetry and chronoamperometry in aqueous salt solution of different concentration. The cyclic polarization of the electrode in aqueous electrolyte results in peak shaped voltammogram, stable during continuous potential cycling. The modification of the electrode with butyloferrocene diluted by hydrophobic non‐electroactive solvent like hexadecane leads to a decrease of the current. The specific voltammogram features were attributed to the redox process of butyloferrocene. The magnitude of the current is proportional to the aqueous electrolyte concentration. It has been concluded that the electrooxidation of redox liquid within hydrophobic silicate matrix is followed by anion transfer from aqueous phase. The electrode process occurs at the three phase junction formed by graphite particles and liquid organic‐aqueous interface supported by hydrophobic silicate.

Probing Thermodynamic Aspects of Electrochemically Driven Ion-Transfer Processes Across Liquid|Liquid Interfaces: Pure versus Diluted Redox Liquids

The voltammetry of the water-insoluble redox liquids para-N,N,N‘,N‘-tetraalkylphenylenediamines (TAPDs; A = butyl, hexyl, heptyl, octyl, and nonyl) at the TAPD|electrode|electrolyte interface is studied. The ion-transfer processes between the aqueous and the organic phaseconsequent of the necessity to maintain charge neutrality during electrochemical oxidation and reduction reactions in the organic phaseare elucidated as a function of the Gibbs free energy of transfer of the electrolyte anions, of the concentration of TAPD in the organic phase, and of the length of the alkyl chain of the phenylenediamines. Two conditions are considered: the voltammetry of microdroplets of the pure redox liquids and the voltammetry of the TAPDs dissolved in nitrobenzene, either deposited as microdroplets at an electrode surface. The oxidation of the neutral redox liquids, accompanied by the uptake of anions from the adjacent aqueous solution leads to the formation of ionic liquid phases, TAPD+X- and TAPD2+X-2 (X- = monovalent electrolyte anions). Despite the high concentration of redox centers in the microdroplets, the redox potentials of the TAPDs are exclusively determined by the Gibbs free energy of the electrochemically driven transfer of anions between the aqueous and the redox liquid phase. There is no evidence for ion pair formation of the TAPD+ and TAPD2+ with the transferred anions X- either in the pure redox liquid phase or in the nitrobenzene matrix. Depending on the Gibbs energy of transfer of the electrolyte anions two different reaction mechanisms are observed: At low ΔG values, a reversible transfer of electrolyte anions across the liquid/liquid interface as a consequence of the redox process takes place. However, anions with high transfer energies do not enter the organic phase. Instead, upon oxidation TAPD mono- and dications are transferred from the organic into the aqueous phase, which leads to the diminution of the voltammetric signals due to the loss of material from the droplets. From the voltammetric data, the Gibbs energy of transfer of the different TAPD cations and of the hexafluoroarsenate anion were derived for the water|nitrobenzene interface. Whereas the oxidation of the TAPD in the nitrobenzene droplets due to its low concentration does not measurably affect the properties of the nitrobenzene|water interface of the droplet, the oxidation of the pure redox liquids leads to a considerable change of the liquid|liquid interface upon ionic liquid formation. This change depends on the nature of the inserted anion: the more hydrophilic the inserted anion is, the less hydrophobic the ionic liquid becomes.

Phosphate and arsenate electro-insertion processes into a N, N, N′, N′-tetraoctylphenylenediamine redox liquid

The electro-insertion of ions is a well-known phenomenon, which allows the transfer of anions or cations across phase boundaries to be monitored and driven electro-chemically. Extremely hydrophilic anions, such as phosphate and arsenate, are not usually observed to undergo electro-insertion. It is shown here that at organic redox liquid|water|electrode triple interfaces these anions can be forced electro-chemically to transfer into organic media. The transfer process of phosphate anions from aqueous buffer solutions into organic microdroplets of the redox liquid N, N, N ′, N ′-tetraoctylphenylenediamine (TOPD) is pH and concentration sensitive. It is shown that phosphate is transferred in the form of PO 4HK − in the presence of phosphate buffer. Two distinct potential regions are identified and attributed to (i) interfacial redox processes at the liquid|liquid interface associated with deprotonation and (ii) bulk redox processes associated with anion transfer from the aqueous to the organic phase. The comparison of phosphate and arsenate electro-insertion processes suggests that arsenate is less hydrophilic and transferred into the organic phase preferentially.

Carbon ceramic electrode modified with redox liquid.

A carbon ceramic electrode modified with a redox liquid, butylferrocene, exhibiting in aqueous salt solution electrochemical behaviour resulting from the redox process of the modifier and ion transfer across the liquid-liquid interface has been prepared.

Chromate and Dichromate Electro-Insertion Processes into a N,N,N′,N′-Tetraoctylphenylenediamine Redox Liquid

Ion transfer processes across aqueous vertical bar organic phase boundaries are of considerable importance in biological and analytical chemistry. When driven by a potential applied to the liquid vertical bar liquid interface the ion transfer may be detected as voltammetric response, Here, a potential applied to the electrode vertical bar aqueous electrolyte vertical bar organic oil (N,N,N',N'-tetraoctylphenylenediamine, TOPD) triple interface is employed to drive anion transfer. Upon oxidation of TOPD deposited in the form of microdroplets onto the electrode and immersed in aqueous media, the transfer of monovalent anions (perchlorate) and divalent anions (sulfate, chromate, dichromate) occurs to maintain charge neutrality in the organic oil phase. Sulfate (0.1 M) as a very hydrophilic anion undergoes transfer into the organic phase at a potential positive of 0.4 V (vs. Ag/AgCl) and may be employed as an 'inert' background electrolyte, Chromate is shown to undergo electro-insertion at low concentrations down to 10 muM. Within the oil phase chromate is undergoing condensation to the more stable dichromate.

Properties of Microlayers of Ionic Liquids Generated at Microelectrode Surface in Undiluted Redox Liquids. Part II

For several simple alcohols such as methanol, ethanol, 1-propanol, 1-butanol, and 1-pentanol, physical properties of the microlayers of the new-type ionic liquids generated during the electrode processes of these undiluted redox liquids were studied. In all cases, well-reproducible, transport-controlled, anodic voltammograms were obtained at Pt microelectrodes. It has been found that the transport properties of the undiluted alcohols, such as activation energy of diffusion and diffusion coefficients, are strongly affected by the enormously large concentration of ions in the region adjacent to the electrode surface. The results obtained for alcohols were compared to those of diluted 1,1‘-ferrocenedimethanol, which compound may be treated as a model redox system. This comparison revealed, in contrast to the Stokes−Einstein relation, that the molecules of each alcohol, while having much smaller size than the molecules of 1,1‘-ferrocenedimethanol, diffuse to the microelectrode surface at a similar rate (metha...

Electrochemically Driven Ion Insertion Processes across Liquid|Liquid Boundaries: Neutral versus Ionic Redox Liquids

The oxidation and rereduction of the redox liquids para-N,N,N‘,N‘-tetrahexylphenylenediamine (THPD) and para-N,N,N‘-trihexylphenylenediamine (TriHPD) associated with anion and proton insertion and expulsion are studied as a function of the proton concentration in aqueous NaClO4 electrolyte. Voltammetric, in situ UV/vis-spectroelectrochemical, and quartz crystal microbalance techniques are employed. The biphasic acid−base equilibria of the redox liquids involving protonation and simultaneous anion transfer from the aqueous phase are shown to exhibit only small deviation from ideal behavior and well-defined biphasic dissociation constants, pKA,biphasic have been determined. However, the protonation of the bulk redox liquids is shown to be dominated by intermolecular rather than intramolecular factors. In particular, the ability of THPD to undergo bulk protonation by HClO4 is higher (pKA2,biphasic = 5.1) compared to that of TriHPD (pKA2,biphasic = 3.9); this is opposite to the behavior predicted on the basis of the estimated values for the aqueous protonation equilibrium constants, pKA2 = 7.5 ± 0.5 and pKA2 = 8.8 ± 0.5 for THPD and TriHPD, respectively. Further, the electrochemically driven deprotonation occurs irrespective of protonation constants at essentially the same potential for both materials. The three-phase junction electrode|redox liquid|aqueous electrolyte for the initiation of the anion and proton insertion−electrochemical reactions is shown to be the key to processes observed for neutral redox liquids, whereas ionic redox liquids show reactivity independent of the three-phase junction due to sufficient ionic bulk conductivity.

Voltammetry in solutions of low ionic strength. Electrochemical and analytical aspects

Recent progress in microelectrode voltammetry in solutions without or with low concentrations of supporting electrolyte is reviewed. The following points are addressed: mathematical treatment of transport, experimental setup, steady state and non steady state transport, migration coupled with homogeneous equilibrium, voltammetry in undiluted redox liquids, studies on the mechanism of the electrode processes, transport of ions in solutions of polyelectrolytes and colloids, and analytical applications.

Physicochemical Consequences of Generating a Thin Layer of Ionic Liquid at Microelectrode Surface in Undiluted Redox Liquid

Very-thin layers of ionic liquids are formed at microelectrode surfaces during electrolysis of undiluted redox liquids containing only supporting electrolyte at a low concentration. The layers consist of ionic product and counterion coming from supporting electrolyte. Formation of ionic−liquid layer leads to increased viscosity, changes in activity coefficients, and thus to changes in diffusion coefficients of all species involved in the electrode process. This engenders also a specific type of convection. Exchange of one supporting electrolyte for another may change dramatically the physicochemical properties of the generated ionic layer and, in consequence, the magnitude of the electrochemical response of an undiluted substance. A computational model for predicting the phenomena mentioned above is presented. An important step in the model is extension of the concentration dependence of diffusivities to concentrations corresponding to ionic liquids. The calculations done have shown a substantial influence of counterion volume on the electrochemical behavior of undiluted redox liquids even if the counterions are present at a very-low concentration. The consequences of changing supporting electrolyte concentration are also included in the model. The theoretical predictions were compared with experimental data obtained for undiluted nitrobenzene and methanol. The model can be used for both transient and steady-state responses.

Electrochemistry of Organic Redox Liquids at Elevated Pressures

The effect of hydrostatic pressure on the 1-e- reductions of acetophenone (AP) and nitrobenzene (NB) in the absence of an inert solvent is reported. Steady-state voltammetric responses for these re...

Electrochemistry of organic redox liquids. Reduction of 4-cyanopyridine

Electrochemistry of organic redox liquids. Reduction of 4-cyanopyridine