The influence of ion pairing at the electrode/solution interface on the kinetics of heterogeneous electron transfer

A model is developed for the accelerating effect of non-reacting ions adsorbed on the inner Helmholtz plane on the kinetics of electron transfer to a reactant which is also situated on the same plane near the electrode. It is assumed that the charge density of non-reacting ions around the reacting ion can be described by a two dimensional distribution function analogous to that used by Fuoss to describe ion pairing in the bulk of the solution. Kinetic equations are presented in which the discreteness-of-charge terms are explicitly included and the magnitude of these terms is estimated on the basis of a simple model. The estimates show that the acceleration of hydrogen ion in the presence of adsorbed iodide ions can be attributed to the electrostatic effect of the surrounding ionic atmosphere.     

Computer simulation of electron transfer processes across the electrode|electrolyte interface: a treatment of solvent and electrode polarizability

A polarizable molecular dynamics model for adiabatic electron transfer across the electrode|electrolyte interface is presented. The electronic polarizability of the water and of the metal electrode is accounted for by a dynamical fluctuating charge algorithm, image charges, and the Ewald summation adapted for a conducting interface. The effects of the solvent electronic polarizability are studied by computing the diabatic and adiabatic free energy curves for both polarizable and non-polarizable water models. This represents the first effort to compute the adiabatic free energy curves from simulation for a fully polarizable electrochemical system.     

Direct measurement of ion accumulation at the electrode electrolyte interface under an oscillatory electric field

The ionic charge accumulation at the metal-electrolyte interface is directly measured by using differential interferometry as a function of magnitude and frequency (2-50 kHz) of external electric field. The technique developed probes the ion dynamics confined to the electrical double layer. The amplitude of modulation of the ions is linearly proportional to the amplitude of applied potential. The linearity is observed up to high electrode potentials and salt concentrations. The frequency response of the ion dynamics at the interface is interpreted in terms of the classical RC model.     

Lithium ion phase-transfer reaction at the interface between the lithium manganese oxide electrode and the nonaqueous electrolyte

The lithium ion phase-transfer reaction between the spinel lithium manganese oxide electrode and a nonaqueous electrolyte was investigated by the ac impedance spectroscopic method. The dependence of the impedance spectra on the electrochemical potential of the lithium ion in the electrode, the lithium salt concentration in the electrolyte, the kind of solvent, and the measured temperature were examined. Nyquist plots, obtained from the impedance measurements, consist of two semicircles for high and medium frequency and warburg impedance for low frequency, indicating that the reaction process of two main steps for high and medium frequency obey the Butler-Volmer type equation and could be related to the charge-transfer reaction process accompanied with lithium ion phase-transfer at the interface. The dependency on the solvent suggests that both steps in the lithium ion phase-transfer at the electrode/electrolyte interface include the desolvation process and have high activation barriers.     

Interfacial ion pairing at the interface between water and a room-temperature ionic liquid, N-tetradecylisoquinolinium bis(pentafluoroethylsulfonyl)imide

The specific interaction of N-tetradecylisoquinolinium (C(14)Iq+) with Cl- and Br- has been detected in the voltammetry of ion transfer and electrocapillarity at the interface between an aqueous solution (W) and a room-temperature ionic liquid (RTIL), N-tetradecylisoquinolinium bis(pentafluoroethylsulfonyl)imide ([C(14)Iq+][C(2)C(2)N-]). This specific interaction also makes the transfer of Cl- and Br- into [C(14)Iq+][C(2)C(2)N-] energetically more favorable in comparison with that of F- and SO(4)(2-). The width of the polarized potential window in ion-transfer voltammetry at the [C(14)Iq+][C(2)C(2)N-]|W interface is significantly narrower because of the transfer of anions from W to RTIL. The degree of affinity of the anion with C(14)Iq+ agrees with the Hofmeister series. Such an ion-pair formation of anions in W with cations in the RTIL is much weaker when the cation constituting the RTIL is a symmetric tetraheptylammonium ion.     

Cyclic voltammetry of ion transfer across the polarized interface between the organic molten salt and the aqueous solution

A molten salt, or ionic liquid, composed of tetrahexylammonium bis(perfluoroethylsulfonyl)imide forms with an aqueous solution a polarized interface where the phase-boundary potential can be controlled externally. The available potential window of about 300 mV at 40 °C enables us to apply various electrochemical techniques for studying the structure and charge transfer reactions at the molten salt–water interface. Cyclic voltammetry of the transfer of moderately hydrophobic ions, such as 1-octyl-3-methylimidazolium and hexafluorophosphate ions, across the interface exemplifies the potentiality of this new electrochemical interface. This new type of polarized interface would facilitates electrochemical studies of molten salt–water two-phase systems that have been studied as an environmentally benign alternative of organic solvent–water two-phase systems for liquid–liquid extraction and two-phase organic synthesis.     

Voltammetry of ion transfer across the electrochemically polarized micro liquid-liquid interface between water and a room-temperature ionic liquid, tetrahexylammonium bis(trifluoromethylsulfonyl)imide, using a glass capillary micropipette.

Ion transfer across the polarized interface between a room-temperature ionic liquid (RTIL) or room-temperature molten salt, tetrahexylammonium bis(trifluoromethylsulfonyl)imide (THAC(1)C(1)N), and water has been studied voltammetrically using a micro liquid-liquid interface formed at the orifice of a glass capillary micropipette. A small current of nanoampere level circumvents the problem of the iR drop in the viscous ionic liquid phase. Voltammograms for the transfer of moderately hydrophilic ions, such as BF(4)(-) and ClO(4)(-), from the aqueous phase in the capillary to the bulk of THAC(1)C(1)N in which the capillary is submerged, show steady-state characteristics in that the current does not depend on the scan rate up to a few hundred millivolt per second, and the plateau in the limiting current region is proportional to the bulk concentration of analyte ions. Owing to the steady-state current, which is presumably ascribed to a noncylindrical geometry of the capillary tip, the relative magnitude of the hydrophobicity, or the affnity to the RTIL, of a series of ions can be determined from the half-wave potentials of voltammograms.     

Charge transfer between two immiscible electrolyte solutions: Part VI. Polarographic and voltammetric study of picrate ion transfer across the water/nitrobenzene interface

The transfer of the picrate ion across the interface between two immiscible electrolyte solutions, 0.05 M LiCl in water and 0.05 M tetrabutylammonium tetraphenylborate in nitrobenzene was investigated by electrolysis with the electrolyte dropping electrode and by cyclic voltammetry. Under the conditions of the experiments the charge-transfer process is controlled solely by diffusion. The maximum which appears on the polarogram of the picrate ion close to the limiting current can be suppressed by the addition of a surface-active substance (gelatine). The diffusion coefficients of the picrate ion in the aqueous and nitrobenzene phase were determined from the limiting polarographic current and from the peak current on the cyclic voltammogram. The value of the formal potential of the charge-transfer reaction, which was calculated from the half-wave potential or from the peak potential, is in good agreement with that inferred from the extraction data.     

Kinetics of surface segregation of bismuth atoms at the interface of a mechanically renewable Ag-Bi alloy electrode with a surface-inactive electrolyte solution

The time effects observed on mechanically renewed electrodes of eutectic-type Ag-Bi alloys in NaF solutions in the potential range of ideal polarizability are studied by the impedance method and cyclic voltammetry. The changes in the electric double layer (EDL) capacitance observed with the increase in the time of the electrode-solution contact from the moment of electrode renewal indicate that the process of surface enrichment with Bi atoms occurs. The analysis of obtained data leads to a conclusion that the surface segregation of Bi proceeds by the mechanism of surface diffusion, which provides the anomalously high diffusion rates as compared with the processes of volume diffusion in the solid phase. The results of studies of the surface segregation of bismuth on the Ag-Bi alloy/electrolyte interface are compared with the Auger-spectroscopic data for the interface of the same alloy with vacuum. Assumptions are put forward and substantiated that allow the differences in the kinetics of surface segregation processes observed on different interfaces to be consistently interpreted.     

Structure and dynamics of the interface between a Ag single crystal electrode and an aqueous electrolyte

The aim of this work is to elucidate the initial steps of the electrochemical oxidation of Ag(111) in alkaline electrolytes. We use electrochemical as well as ex situ (XPS) and in situ (SHG) spectroscopic techniques to reconstruct the Ag(111)/electrolyte interface as a complex dynamic entity. Moving in the direction from negative to positive potentials we first observe specific adsorption of hydroxide ions, which starts at ca. -1.1 V vs. Ag/Ag2O in 0.1 M NaOH. SHG data prove that hydroxide retains its negative charge. At -0.3 V oxidation of the surface sets in with the formation of negatively charged adsorbed oxygen species and Ag+ ions, which give rise to peaks at 528.2 +/- 0.2 eV and at 367.7 eV in the O 1s and the Ag 3d(5/2) XP spectra, respectively. Around -0.1 V the adlayer is transformed into an ordered surface oxide phase which grows via a nucleation and growth mechanism. Above the reversible Ag/Ag2O potential the 2D Ag(I) oxide transforms into a 3D Ag(I) oxide. The electrochemical oxidation is compared with the previously studied gas-phase process, demonstrating both remarkable similarities as well as some differences.     

Theory of a reversible redox reaction in an ionic liquid that is coupled to an ion transfer across the aqueous electrolyte/ionic liquid interface

A theory is provided for a reversible electro-oxidation of a neutral redox probe dissolved in room-temperature ionic liquid, which is sandwiched between an electrode surface and an aqueous solution as a thin film. If the peak potentials in cyclic voltammetry depend on the bulk concentration of electrolyte in water, the oxidation is most probably coupled to the transfer of anions from water into ionic liquid; but if the peak potentials are independent of the electrolyte concentration, the transfers of anions from water into ionic liquid and cations from ionic liquid into water are equally probable. Dedicated to Professor Dr. Yakov I. Tur’yan on the occasion of his 85th birthday.     

A molecular dynamics investigation of the structural and dynamic properties of the ionic liquid 1-n-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide

Molecular dynamics simulations have been performed to investigate the structure and dynamics of the ionic liquid, 1-n-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([C(4)mim][Tf(2)N]) in the temperature range of 283-460 K. Extensive analysis was carried out to characterize a number of structural and dynamic features. Transport properties were computed using a variety of equilibrium methods that employed the Green-Kubo and Einstein formulations. Nonequilibrium techniques were also used. In general, different methods mostly yielded consistent results, although some differences were observed. Computed self-diffusivities and ionic conductivities tended to be slightly lower than experimental values, while computed viscosities were significantly higher than experiment. Computed thermal conductivities agreed reasonably well with experimental data. Despite these discrepancies, the simulations capture the experimental temperature-dependent trends for all these transport properties. Single ion dynamics were studied by examining diffusional anisotropy, the self-part of the van Hove function, non-Gaussian parameters, and incoherent intermediate scattering functions. It is found that cations diffuse faster than anions and are more dynamically heterogeneous. A clear anisotropy is revealed in cation displacement, with the motion normal to the imidazolium ring plane being the most hindered and the motion along the alkyl chain in the plane of the ring being the most facile. Cations structurally relax faster than anions but they rotationally relax slower than anions. There is a pronounced temperature dependence to the rotational anisotropy of the cations, but only a weak temperature dependence for the anions. The ionic conductivity deviates from the Nernst-Einstein relation due to the correlated motion of cations and anions. The results suggest that the dynamical behavior of this and related ionic liquids is extremely complex and consists of many different modes with widely varying timescales, making the prediction of dynamical trends extremely difficult.     

Ion-pair formation in the ionic liquid 1-ethyl-3-methylimidazolium bis(triflyl)imide as a function of temperature and concentration.

The structures and ion-pair formation in the ionic liquid (IL) 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide are studied by a combination of FTIR measurements and DFT calculations. We could clearly distinguish imidazolium cations that are completely H-bonded to anions from those that are single H-bonded in ion pairs. Ion-pair formation already occurs in the neat IL and rises with temperature. Ion-pair formation is strongly promoted by dilution of the IL in chloroform. In these weakly polar environments ion pairs H-bonded via C(2)H are strongly favored over those H-bonded via C(4,5)H. This finding is in agreement with DFT (gas phase) calculations, which show a preference for ion pairs H-bonded via C(2)H as a result of the acidic C(2)H bond.     

Studying electron transfer reaction at the Au/n-decanethiol/aqueous solution of NaNO3 interface by electrochemical impedance spectroscopy

The electrochemical behavior of the gold/electrolyte interface in aqueous 1 M NaNO₃ solutions in the presence of an organic monolayer of n-decanethiol (CH₃(CH₂)₉S) is studied by electrochemical impedance spectroscopy in the frequency range of 10–10⁵ Hz and also by cyclic voltammetry. It is experimentally shown that in the potential interval from 0 to ?0.5 V (vs. SCE), the dense monolayer film decreases the measured current density approximately 40-fold. The measured capacitance falls down to 1–2 μF/cm². Based on the analysis of impedance characteristics acquired with the use of empirical equivalent circuits comprising ideal and nonideal analogues of electric circuits, the tentative estimates of the thickness of organic monolayers formed on Au electrodes with various roughness factors are obtained. Using the complex nonlinear regression (CNLS) method and a model of microarray electrode, the porous structure of adsorbed monolayers is revealed and the transition frequency of interfaces under study is determined. The degree of inhibition of the electron transfer across the Au/n-decanethiol/solution interface is determined by comparing the rate constants for the Ru[(NH₃)₆]^3+/2+ redox process on clean and modified electrodes. The acquired results are compared with available literature data.     

Influence of wettability and surface charge on the interaction between an aqueous electrolyte solution and a solid surface

The hydrodynamic drainage force of a Newtonian aqueous electrolyte solution squeezed between two surfaces of different wettability was measured using the AFM colloidal probe technique. The surface hydrophobicity, roughness, polarity and approach velocity, and thus the shearing rate of the liquid, were controlled. A direct relationship between the mobility of the aqueous electrolyte solution close to the surfaces and the hydrophobicity of the surfaces was not established. We predict that the mobility of the liquid depends in a more complex fashion on the polarity and charge of the surfaces and on the properties of the electrolyte.     

Physical properties of ionic liquids consisting of the 1-butyl-3-methylimidazolium cation with various anions and the bis(trifluoromethylsulfonyl)imide anion with various cations

Physical properties of 4 room-temperature ionic liquids consisting of the 1-butyl-3-methylimidazolium cation with various perfluorinated anions and the bis(trifluoromethylsulfonyl)imide (Tf2N-) anion with 12 pyrrolidinium-, ammonium-, and hydroxyl-containing cations are reported. Electronic structure methods are used to calculate properties related to the size, shape, and dipole moment of individual ions. Experimental measurements of phase-transition temperatures, densities, refractive indices, surface tensions, solvatochromic polarities based on absorption of Nile Red, 19F chemical shifts of the Tf2N- anion, temperature-dependent viscosities, conductivities, and cation diffusion coefficients are reported. Correlations among the measured quantities as well as the use of surface tension and molar volume for estimating Hildebrand solubility parameters of ionic liquids are also discussed.     

Effect of the pressure on the viscosities of ionic liquids: Experimental values for 1-ethyl-3-methylimidazolium ethylsulfate and two bis(trifluoromethyl-sulfonyl)imide salts

A new falling-body viscometer has been implemented to measure viscosity of liquids in a temperature range from (313.15 to 363.15) K at pressures up to 150 MPa. The accuracy of the viscometer was verified after comparing experimental results of squalane with previous literature data finding an average absolute deviation lower than 1.5%. With this device, we have measured viscosity values for three ionic liquids: 1-ethyl-3-methylimidazolium ethylsulfate, 1-butyl-1-methylpyrrolidinium bis(trifluoro-methylsulfonyl)imide and 1-(2-methoxyethyl)-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide within the temperature and pressure ranges noted above. The experimental values were correlated as a function of temperature and pressure with four different equations. In addition, we have analysed the pressure–viscosity derived properties for these fluids and for other five ionic liquids using literature values.     

Development of rotating liquid membrane disk electrode and rotating liquid membrane ring-liquid membrane disk electrode and evaluation of characteristics of ion transfer reactions at the rotating aqueous/organic solution interface.

A liquid membrane disk electrode, LMDE, and a liquid membrane ring-liquid membrane disk electrode, LMRE-LMDE, were developed by placing a gelled polyvinyl chloride thin membrane impregnated with 2-nitrophenyl octyl ether, NPOE-LM, on the surface of a glassy carbon, GC, disk or ring electrode. The voltammogram for the ion transfer at the interface between an aqueous solution, W, and NPOE-LM was recorded by setting the developed electrode in W and rotating at a rate, omega, between 0 and 4000 rpm. The sensitivity of the ion-transfer current at the WINPOE-LM interface, I, was enhanced to be more than 100 times better than that at the WINPOE (solution) interface when LMDE was rotated at omega higher than 200 rpm. The reversibility of the ion transfer reaction could be evaluated based on the dependence of I on omega of LMDE, and the reaction product at LMDE could be identified at LMRE when the rotating LMRE-LMDE system was adopted.