Interaction of ions with phosphatidylcholine and phosphatidylethanolamine at the n-heptane/water interface : II. Hydrophobic ions

Electrocapillary phenomena at the n-heptane/water interface in the presence of phosphatidylcholine or phosphatidylethanolamine in alkane were studied by the drop-volume technique. The regulating role of the electric field was established in phospholipid interaction with inorganic ions: changes in interfacial pressure at the n-heptane/water interface with addition of electrolytes were observed only when an electric field was applied to the system of two immiscible liquids. It is suggested that ions can penetrate and bond by phospholipid functional groups at the phase boundary only after reorientation of the latter in the electric field. As in the case of ionic surfactants, change in interfacial pressure was directly proportional to the cube root of the concentration of electrolyte. It is assumed that as a result of preferential binding by phospholipid functional groups of ions of one sign, lipid molecules are charged and at the phase boundary behave themselves like ionic surfactants. Experiments with salts of multicharged ions, La³⁺, [Fe(CN)₆]³⁻, and [Fe(CN)₆]⁴⁻, have shown that mainly phospholipid functional groups bind cations.     

Co-catalysis by transition metal ions in the hydrogen ion catalysed oxidation of iodide ions. A kinetic study

The reaction between KI and [Fe(CN)₆]^3– ion, catalysed by hydrogen ions, was found to be catalysed further by PdCl₂. Separate reactions under similar conditions, studied in the absence as well as in the presence of PdCl₂ catalyst, were found to follow first order kinetics w.r. to [Fe(CN)₆]^3– and [H⁺], while the order was two w.r. to [I^–]. [Fe(CN)₆]^4– ions were found to have a negative effect while changes in ionic strength of the medium do not effect the reaction velocity. Reaction in the presence of PdCl₂ showed direct proportionality w.r. to [PdCl₂]. The rate and extent of the reaction, which takes place even at zero [PdCl₂] in the co-catalysed reaction, was calculated and was found to be in accordance with the rate values of the separately studied reaction at similar concentrations without adding PdCl₂.     

Cocatalysis by ruthenium(III) in hydrogen ions catalyzed oxidation of iodide ions: A kinetic study

RuCl₃ further catalyzes the oxidation of iodide ion by K₃Fe(CN)₆, already catalyzed by hydrogen ions. The rate of reaction, when catalyzed only by hydrogen ions, was separated graphically from the rate when both Ru(III) and H⁺ ions catalyzed the reaction. Reactions studied separately in the presence as well as absence of RuCl₃ under similar conditions were found to follow second‐order kinetics with respect to [I^?], while the rate showed direct proportionality with respect to [Fe(CN)₆]^3?, [RuCl₃], and [H⁺]. External addition of [Fe(CN)₆]^4? ions retards the reaction velocity, while changing the ionic strength of the medium has no effect on the rate. With the help of the intercept of the catalyst graph, the extent of the reaction that takes place without adding Ru(III) was calculated and it was in accordance with the values obtained from the reaction in which only H⁺ ions catalyzed the reaction. It is proposed that ruthenium forms a complex, which slowly disproportionates into the rate‐determining step. Arrhenius parameters at four different temperatures were also calculated. © 2004 Wiley Periodicals, Inc. Int J Chem Kinet 36: 545–553, 2004     

Lithium Ferrocyanide Catholyte for High-Energy and Low-cost Aqueous Redox Flow Batteries**

Aqueous redox flow batteries (ARFBs) are a promising technology for grid-scale energy storage, however, their commercial success relies on redox-active materials (RAM) with high electron storage capacity and cost competitiveness. Herein, a redox-active material lithium ferrocyanide (Li₄[Fe(CN)₆]) is designed. Li⁺ ions not only greatly boost the solubility of [Fe(CN)₆]⁴⁻ to 2.32 M at room temperature due to weak intermolecular interactions, but also improves the electrochemical performance of [Fe(CN)₆]^4−/3−. By coupling with Zn, ZIRFBs were built, and the capacity of the batteries was as high as 61.64 Ah L⁻¹ (pH-neutral) and 56.28 Ah L⁻¹ (alkaline) at a [Fe(CN)₆]⁴⁻ concentration of 2.30 M and 2.10 M. These represent unprecedentedly high [Fe(CN)₆]⁴⁻ concentrations and battery energy densities reported to date. Moreover, benefiting from the low cost of Li₄[Fe(CN)₆], the overall chemical cost of alkaline ZIRFB is as low as $11 per kWh, which is one-twentieth that of the state-of-the-art VFB ($211.54 per kWh). This work breaks through the limitations of traditional electrolyte composition optimization and will strongly promote the development of economical [Fe(CN)₆]^4−/3−-based RFBs in the future.     

Electrochemical and thermodynamic properties of hexacyanoferrate(II)/(III) redox system on multi-walled carbon nanotubes

Novel films consist of multi-walled carbon nanotubes (MWCNT) were fabricated by means of catalytic chemical vapor deposition (CVD) technique with decomposition of either acetonitrile (ACN) or benzene (BZ) using ferrocene (FeCp₂) as catalyst. The electrochemical and thermodynamic behavior of the ferrocyanide/ferricyanide, [Fe(CN)₆]^3−/4− redox couple on synthesized MWCNT-based films was investigated by means of cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) techniques at T = (278.15, 283.15, 293.15, and 303.15) K. The redox couple [Fe(CN)₆]^3−/4− behaves quasi-reversibly on fabricated MWCNT-based films and its reversibility is enhanced upon increasing temperature. Namely, the findings establish that with the rise in temperature the barrier for interfacial electron transfer decreases, leading, consequently, to an enhancement of the kinetics of the charge transfer process. According to thermodynamics the equilibrium of the redox process is shifted towards the formation of [Fe(CN)₆]³⁻ at elevated temperatures.     

Sorption of [Fe(CN)6]3− and [Fe(CN)6]4− ions on the surface of Fe(III), Cr(III), and Zr(IV) oxyhydroxide hydrogels from aqueous solutions

The sorption of [Fe(CN)₆]^3? and [Fe(CN)₆]^4? anions on the surface of Fe(III), Cr(III), and Zr(IV) oxyhydroxide hydrogels at various pH values of hydrogel precipitation from solutions without a support electrolyte and from NaCl and Na₂SO₄ solutions with an ionic strength of 0.5 was studied. It was found that isotherms of sorption of [Fe(CN)₆]^3? and [Fe(CN)₆]^4? anions from solutions without a support electrolyte and from NaCl solutions and those of sorption of [Fe(CN)₆]^4? from Na₂SO₄ solutions are described by the Langmuir equation. It was established that the sulfate background suppresses the sorption of [Fe(CN)₆]^3? on Fe(III) and Zr(IV) oxyhydroxides. Both anions are sorbed only when the surface of the oxyhydroxides is charged positively; the Langmuir equation parameters A _max and K tend to decrease to the point of zero charge as the pH value of oxyhydroxide precipitation increases. An electrostatic mechanism of the sorption of [Fe(CN)₆]^3? and [Fe(CN)₆]^4? anions was suggested.     

Self‐Assembly Mechanism of Nanoparticles of Ni‐Based Prussian Blue Analogues at the Air/Liquid Interface: A Synchrotron X‐ray Reflectivity Study

Prussian Blue analogue (PBA) nanoparticles can be self‐assembled at air/liquid interfaces to build novel materials with interesting magnetic features. Herein, we study the influence of the size of PBA Cs_0.4Ni[Cr(CN)₆]_0.9 and K_0.25Ni[Fe(CN)₆]_0.75 nanoparticles on the self‐assembly behavior by synchrotron X‐ray reflectivity. Both nanoparticles show similar Z‐potential values. The phospholipid dipalmitoylphosphatidylcholine and the amino surfactant dimethyldioctadecylammonium have been used as Langmuir monolayers to anchor the PBA nanoparticles and study the interplay of forces directing the self‐assembly of the nanoparticles at the surfactant/liquid interface. Whereas Cs_0.4Ni[Cr(CN)₆]_0.9 nanoparticles with a diameter of 8 nm form an incomplete layer at the surfactant/water interface, the larger K_0.25Ni[Fe(CN)₆]_0.75 nanoparticles with a diameter of 20 nm generate complete layers that can be stacked to one another. The size of the PBA nanoparticles is the main parameter determining the final arrangement at the air/liquid interface, due to the different extent of interparticle interaction. This study aims at the rationale design of PBA nanoparticles for an effective interfacial self‐assembly, ultimately leading to functional materials.     

Heptanuclear high-spin complex compounds based on S-donors

The precursors [Fe(III)(^SYL)Cl] (^SYLH₂) = N,N′-bis(1-hydroxy-Y-2-benzyliden)-1,6-diamino-3-thiohexane, (Y = H, 3EtO, 5Me) are high-spin (S = 5/2) complexes. The precursors are combined with [Fe(II)(CN)₆]⁴⁻ and [Co(III)(CN)₆]³⁻ to yield star-shaped heptanuclear clusters, [Fe(II)(CN–Fe(III)^SYL)₆]Cl₂ and [Co(III)(CN–Fe(III)^SYL)₆]Cl₃. The star-shaped compounds are high-spin (HS) systems at room temperature. On cooling to 20 K some of the iron(III) centers perform some HS–HS transition.     

Iridium(III) catalyzed oxidation of iodide ions in aqueous acidic medium

Oxidation of iodide ions by K₃Fe(CN)₆, catalyzed by hydrogen ions obtained from hydrochloric acid was found to be further catalyzed by iridium(III) chloride. Rate, when the reaction is catalyzed only by the hydrogen ions, was separated from the rate when iridium(III) and H⁺ions both, catalyze the reaction. Reactions studied separately in the presence as well as in the absence of IrCl₃ under similar conditions were found to follow second order kinetics with respect to [I⁻]. While the rate showed direct proportionality with respect to [K₃Fe(CN)₆] and [IrCl₃]. At low concentrations the reaction shows direct proportionality with respect to [H⁺] which tends to become proportional to the square of hydrogen ions at higher concentrations. Strong retarding affect of externally added hexacyanoferrate(II) ions was observed in the beginning but further addition affects the rate to a little extent. Changes in [Cl⁻] and also ionic strength of the medium have no effect on the rate. With the help of the intercept of catalyst graph, the extent of the reaction, which takes place without adding iridium(III), was calculated and was found to be in accordance with the values obtained from the separately studied reactions in which only H⁺ ions catalyze the reaction. It is proposed that iridium forms a complex, which slowly disproportionates into the rate-determining step. Thermodynamic parameters at four different temperatures were calculated.     

The Effect of Adsorption of Ions of the Hexacyanoferrate(II)/(III) Redox Pair on Self-Assembly of Octanethiol at Its Adsorption from Aqueous Solutions on Gold Electrode

The effect of components of the redox pair K₃[Fe(CN)₆]/K₄[Fe(CN)₆] on the dynamics of formation of octanethiol (OT) monolayers from aqueous thiol-containing solutions of 0.1 М NaClO₄ is studied by cyclic voltammetry (CVA). The formation of OT monolayers is shown to depend on the presence of ions of hexacyanoferrate(II)/(III) in solution. Being added to solution, the components of the [Fe(CN)₆]^3–/4– redox pair sharply increase the time of formation of the insulating monolayer OT films and make them less stable. The destabilizing and inhibiting action of [Fe(CN)₆]^3–/4– ions becomes stronger as their concentration in solution increases. The adsorption activity of individual components of the redox pair is assessed. The strong and approximately equal adsorption activity of ions [Fe(CN)₆]^3– and [Fe(CN)₆]^4– on gold in the presence of octanethiol is observed. At the same time, OT and the hexacyanoferrate(II)/(III) ions can be placed in the following row: OT > [Fe(CN)6]^3– ≈ [Fe(CN)₆]^4–. Recommendations are given on how to eliminate the interfering action of the K₃[Fe(CN)₆]/K₄[Fe(CN)₆] redox-pair ions when studying the insulating properties of thiol monolayers on gold.     

Evanescent wave broadband cavity enhanced absorption spectroscopy using supercontinuum radiation: A new probe of electrochemical processes

An evanescent wave variant of broadband cavity enhanced absorption spectroscopy using a supercontinuum light source has been used to detect electrogenerated species at the silica-water interface. In proof-of-concept experiments [IrCl₆]²⁻ was produced by electro-oxidation of [IrCl₆]³⁻ in a thin layer electrochemical cell. Diffusion of the Ir(IV) across the cell to a silica interface was monitored yielding real-time concentrations within an evanescent field region at the interface. The optical response was compared with the electrochemical response during chronoamperometric step and cyclic voltammetric experiments and both were simulated by finite element modeling. The experiment is highly sensitive to interfacial processes and its wide spectral width and fast time resolution make it a potentially powerful tool for in situ spectroscopic monitoring of processes and intermediates in dynamical electrochemistry.     

Cu(II) promoted oxidation of L-valine by hexacyanoferrate(III) in cationic micellar medium

The kinetics of Cu(II) accelerated L-valine (Val) oxidation by hexacyanoferrate(III) in CTAB micellar medium were investigated by measuring the decline in absorbance at 420 nm. By adjusting one variable at a time, the progression of the reaction has been inspected as a function of [OH⁻], ionic strength, [CTAB], [Cu(II)], [Val], [Fe(CN)₆³⁻], and temperature using the pseudo-first-order condition. The results show that [CTAB] is the critical parameter with a discernible influence on reaction rate. [Fe(CN)₆]^3- interacts with Val in a 2:1 ratio, and this reaction exhibits first-order dependency with regard to [Fe(CN)₆³⁻]. In the investigated concentration ranges of Cu(II), [OH⁻], and [Val], the reaction demonstrates fractional-first-order kinetics. The linear increase in reaction rate with added electrolyte is indicative of a positive salt effect. CTAB significantly catalyzes the process, and once at a maximum, the rate remains almost constant as [CTAB] increases. Reduced repulsion between surfactant molecules' positive charge heads brought on by the negatively charged [Fe(CN)₆]^3-, OH⁻, and [Cu(OH)₄]^2- molecules may be responsible for the observed drop in CMC of CTAB.     

Layer-by-layer thin film-coated electrodes for electrocatalytic determination of ascorbic acid

Multilayer thin films composed of poly(allylamine hydrochloride) (PAH) and carboxymethyl cellulose (CMC) have been prepared on the surface of a gold (Au) disk electrode by a layer-by-layer deposition of PAH and CMC and ferricyanide ions ([Fe(CN)₆]³⁻) were confined in the film. [Fe(CN)₆]³⁻ ions can be successfully confined in the films from weakly acidic or neutral [Fe(CN)₆]³⁻ solutions, while, in basic solution, [Fe(CN)₆]³⁻ ion was not confined. The [Fe(CN)₆]³⁻ ion-confined Au electrode showed clear redox peaks in the cyclic voltammogram around 0.35 V versus Ag/AgCl. The amounts of [Fe(CN)₆]³⁻ ions confined in the films depended on the thickness of the films or the number of layers in the LbL films. The [Fe(CN)₆]³⁻ ion-confined Au electrode was used for electrocatalytic determination of ascorbic acid in the concentration range of 1-50 mM.     

A 2-D bimetallic assembly with bridging cyanide ions

A novel cyanide-bridged bimetallic assembly, [Cu(1,3-Pn)₂]₂[Fe^III(CN)₆]ClO₄ · 2H₂O (1,3-Pn = 1,3-diaminopropane), derived from [Fe(CN)₆]^3– building blocks and four-coordinated bisdiamine metal(II) ions [Cu(1,3-Pn)₂]²⁺ is described and characterized by X-ray crystal analysis. The compound contains a two-dimensional network structure extended through Fe^III—CN—Cu linkages. Mössbauer experimental results indicate that the iron is ferric (Fe³⁺) in the complex. Cryomagnetic measurements reveal an antiferromagnetic exchange interaction between the nearest paramagnetic metal ions in the compound. The exchange mechanism was also discussed.     

Study of the ligand substitution reaction Fe (CN)5H2O3− + pyrazine in micellar solutions

The ligand substitution reaction Fe(CN)₅H₂O³⁻ + pyrazine → Fe(CN)₅ pyrazine³⁻ + H₂O has been studied in sodium dodecyl sulfate SDS, hexadecyltrimethylammonium bromide, CTAB, and salt aqueous solutions at 298.2 K. Kinetics were studied in dilute and concentrated salt solutions and in SDS and CTAB solutions at surfactant concentrations below and above the critical micelle concentration. Experimental results show that salt effects can be explained by considering the interaction between the cations present in the working media which come from the background electrolyte, and the Fe(CN)₅H₂O³⁻ species in the vicinity of the cyanide ligands. This interaction makes the release of the aqua ligand from the inner-coordination shell of the iron(II) complex to the bulk more difficult resulting in a decrease of the reaction rate when the electrolyte concentration increases. Kinetic data in surfactant solutions show that not only micellized surfactants are operative kinetically, but also nonmicellized surfactants are influencing the reactivity. © 1997 John Wiley & Sons, Inc. Int J Chem Kinet 29: 377–384, 1997     

A novel three-dimensional mixed bridging–ligand complex with an unexpected 1,3-diaminopropane bridge

Reaction of K₃[Fe(CN)₆] with [Cu(tn)₂](ClO₄)₂ (tn=1,3-diaminopropane) leads to a novel mixed cyano and tn bridged three-dimensional (3D) bimetallic assembly (1), in which each [Fe(CN)₆]⁴⁻ anion connects six copper(II) cations via six CN⁻ groups, whereas each copper(II) cation is linked to three [Fe(CN)₆]⁴⁻ ions and two other copper(II) ions through Cu–NC–Fe and Cu–tn–Cu linkages, respectively. Magnetic studies reveal weak antiferromagnetic interactions between the nearest Cu^II (S=1/2) ions through the diamagnetic [Fe(CN)₆]⁴⁻ anion.     

Stereo-electrochemistry by a self-assembled monolayer of sulfur-bridged calixthiophene on gold

Sulfur-bridged calixthiophene formed a self-assembled mono-molecular layer on polycrystalline gold, and it regulated an electrochemical electron transfer by the host–guest interaction between the cavity and reactants. 1,7,13,19,25-Tetrathia[1.₅](2,5)thiophenophane (thiacalix[5]thiophene) perfectly passivated the gold electrode for relatively large reversible metal complexes: [Fe(CN)₆]^4−/3− and [IrCl₆]^3−/2−. However, for mono-atomic ions, such as silver and some of the halogen ions, the electrode behaved reversibly. For copper reduction, a large activation overpotential was observed to induce an initial copper reduction in the cavity.     

Preparation and characterization on the carbon nanotube chemically modified electrode grown in situ

Carbon nanotube chemically modified electrode (CNT-CME) was prepared by growing carbon nanotube (CNT) in situ on the pretreated graphite electrode (GE) via the catalytic chemical vapor deposition. The pretreated GE was prepared by ultrasonic immersion method using Ni(NO₃)₂ as the catalyst. The CNT growing on the CNT-CME was characterized by transmission electron microscope and scanning electron microscope. The obtained electrode electrochemical performance was characterized by cyclic voltammetry with the Na₂SO₄ solution and [Fe(CN)₆]³⁻/[Fe(CN)₆]⁴⁻ solution. The results showed that the obtained electrode has good current responsive sensitivity and good testing result accuracy, indicating that the obtained electrode may have great application in electrochemical testing field.     

Ruthenium(III) catalysis in the reaction of hexacyanoferrate(III) and iodide ions in perchloric acid medium

RuCl₃ can further catalyze the reaction between hexacyanoferrate(III) and iodide ions, which is already catalyzed by the hydrogen ions obtained from perchloric acid. Rate, when the reaction is catalyzed only by the hydrogen ions, was separated graphically from the rate when ruthenium(III) and H⁺ ions both catalyze the reaction. Reactions studied separately in the presence as well as in the absence of RuCl₃ under similar conditions were found to follow second order kinetics w.r.t. [I⁻]. While the rate showed direct proportionality w.r.t. [Fe(CN)₆]³⁻ and [RuCl₃]. At low concentrations the reaction shows direct proportionality with respect to [H⁺] which tends to become proportional to the square of hydrogen ion concentrations. External addition of [Fe(CN)₆]⁴⁻ ions retards the reaction velocity while change in ionic strength of the medium has no effect on the rate. With the help of the intercept of the catalyst graph, extent of the reaction, which takes place without adding ruthenium(III) was calculated and it was in accordance with the values obtained from the separately studied reaction in which only H⁺ ions catalyze the reaction. It is proposed that ruthenium forms a complex, which slowly disproportionates into the rate-determining step. Arrhenius parameters at four different temperatures were also calculated.      

The formation of an antitubercular complex [Fe(CN)5(INH)]3− through mercury(II)‐catalyzed ligand substitution reaction: A kinetic and mechanistic study

The kinetics and mechanism of the formation of an antitubercular complex [Fe(CN)₅(INH)]^3? based on the substitution reaction between K₄[Fe(CN)₆] and isoniazid (INH), i.e., isonicotinohydrazide, catalyzed by Hg²⁺ in aqueous medium was studied spectrophotometrically at 435 nm (the λ_max of the golden‐yellow‐colored complex [Fe(CN)₅(INH)]^3?) as a function of pH, ionic strength, temperature, and the concentration of the reactants and the catalyst. The replacement of coordinated CN^? in [Fe(CN)₆]^4? was facilitated by incoming ligand INH under the optimized reaction conditions: pH 3.5 ± 0.02, temperature = 30.0 ± 0.1°C, and ionic strength I = 0.05 M (KNO₃). The stoichiometry of the reaction and the stability constant of the complex ([Fe(CN)₅(INH)]^3?) have been established as 1:1 and 2.10 × 10³ M, respectively. The rate of catalyzed reaction was found to be slow at low pH values, to increase with increasing pH, to attain a maximum value at 3.50 ± 0.02, and finally to decrease after pH > 3.5 due to less availability of H⁺ ions needed to regenerate the catalytic species. The initial rates were evaluated for each variation from the absorbance versus time curves. The reaction was found to be pseudo‐first order with respect to [INH] and first order with respect to [Fe(CN)₆^4?] at lower concentration, whereas it was found to be fractional order at higher [INH] and [Fe(CN)₆^4?]. The ionic strength dependence study showed a negative salt effect on the rate of the reaction. Based on experimental results, a mechanism for the studied reaction is proposed. The rate equation derived from this mechanism explains all the experimental observations. The evaluated values of activation parameters for the catalyzed reaction suggest an interchange dissociative (I_d) mechanism. © 2012 Wiley Periodicals, Inc. Int J Chem Kinet 44: 398–406, 2012