Rolando Guidelli

Defining the transfer coefficient in electrochemistry: An assessment (IUPAC Technical Report)

Abstract The transfer coefficient α is a quantity that is commonly employed in the kinetic investigation of electrode processes. In the 3rd edition of the IUPAC Green Book, the cathodic transfer coefficient αc is defined as –(RT/nF)(dlnkc/dE), where kc is the electroreduction rate constant, E is the applied potential, and R, T, and F have their usual significance. This definition is equivalent to the other, -(RT/nF)(dln|jc|/dE), where jc is the cathodic current density corrected for any changes in the reactant concentration at the electrode surface with respect to its bulk value. The anodic transfer coefficient αa is defined similarly, by simply replacing jc with the anodic current density ja and the minus sign with the plus sign. It is shown that this definition applies only to an electrode reaction that consists of a single elementary step involving the simultaneous uptake of n electrons from the electrode in the case of αc, or their release to the electrode in the case of αa. However, an elementary step involving the simultaneous release or uptake of more than one electron is regarded as highly improbable in view of the absolute rate theory of electron transfer of Marcus; the hardly satisfiable requirements for the occurrence of such an event are examined. Moreover, the majority of electrode reactions do not consist of a single elementary step; rather, they are multistep, multi-electron processes. The uncritical application of the above definitions of αc and αa has led researchers to provide unwarranted mechanistic interpretations of electrode reactions. In fact, the only directly measurable experimental quantity is dln|j|/dE, which can be made dimensionless upon multiplication by RT/F, yielding (RT/F)(dln|j|/dE). One common source of misinterpretation consists in setting this experimental quantity equal to αn, according to the above definition of the transfer coefficient, and in trying to estimate n from αn, upon ascribing an arbitrary value to α, often close to 0.5. The resulting n value is then identified with the number of electrons involved in a hypothetical rate-determining step or with that involved in the overall electrode reaction. A few examples of these unwarranted mechanistic interpretations are reported. In view of the above considerations, it is proposed to define the cathodic and anodic transfer coefficients by the quantities αc = –(RT/F)(dln|jc|/dE) and αa = (RT/F)(dlnja/dE), which are independent of any mechanistic consideration.

Recent developments in models for the interface between a metal and an aqueous solution

Abstract After a bird’s eye view of double-layer models of interfaces between metals and aqueous solutions from their very beginning, recent developments are reviewed. The role of the metal is examined by considering calculations for metal clusters and the jellium model, both in vacuo and in contact with model solutions. Integral equation approaches to the solution side of the interfaces are reviewed and compared with Monte Carlo and molecular dynamics simulations of analogous molecular models. Computer simulations of metal–water interfaces (including Car-Parinello simulations) and of ionic solution–metal interfaces are considered. Finally, a field-theoretical approach to the double-layer and the treatment of rough electrodes are briefly reviewed.

Definition of the transfer coefficient in electrochemistry (IUPAC Recommendations 2014)

Abstract The transfer coefficient α is a quantity that is commonly employed in the kinetic investigation of electrode processes. An unambiguous definition of the transfer coefficient, independent of any mechanistic consideration and exclusively based on experimental data, is proposed. The cathodic transfer coefficient αc is defined as –(RT/F)(dln|jc|/dE), where jc is the cathodic current density corrected for any changes in the reactant concentration on the electrode surface with respect to its bulk value, E is the applied electric potential, and R, T, and F have their usual significance. The anodic transfer coefficient αa is defined similarly, by simply replacing jc with the anodic current density and the minus sign with the plus sign. This recommendation aims at clarifying and improving the definition of the transfer coefficient reported in the 3rd edition of the IUPAC Green Book.

Electrified interfaces in physics, chemistry and biology

An overview of electrified interfaces structure and electronic properties of metal surfaces physics of surfaces ab-inito molecular dynamics - selected applications to disordered systems and surfaces the problem of Schottky barrier the semiconductor/electrolyte interface stark effect on adsorbates at electrified interfaces thermodynamics of adsorption electrode potentials and energy scales phenomenological approach to metal/electrolyte interfaces the application of scanning tunnelling microscopy to electrochemistry singly crystal electrodes modelling of metal-water electrified interfaces molecular models of organic adsorption from water at charged interfaces the interface between a metal and a solution in the absence of specific adsorption adsorption at the metal/solution interface electron-transfer reactions at metal-solution interfaces - an introduction to some contemporary issues the solid-electrolyte interface as exemplified by hydrous oxides - surface chemistry and surface reactivity discrete charges on biological membranes structural rearrangements in lipid bilayer membranes evaluation of the surface potential at the membrane-solution interface of photosynthetic bacterial systems electrical currents of the light driven pump bacteriorhodopsin the role of asp 85 and asp 96 on proton translocation the electrochemical relaxation at thylakoid membranes evaluation of the electric field in a protein by dynamic measurements of proton transfer.

Adsorbed monolayer of H-bonded water molecules with and without neutral polymer molecules: A statistical mechanical treatment accounting for local order

Abstract A statistical mechanical treatment of a monolayer consisting both of H-bonded solvent molecules adsorbed in an unspecified number of orientations and of polymeric molecules of a neutral solute is provided. The different size of solvent and solute molecules is accounted for using Flory—Huggins statistics, whereas local order within the monolayer is accounted for using the quasi-chemical approximation. The above treatment is applied to a hexagonal array of adsorbed water molecules oriented in such a way as to be in a condition to be singly or double H-bonded laterally in the monolayer; a further water orientation characterized by full alignment of the dipole moment in the direction away from the electrode and simulating chemisorbed water monomers is included in the molecular model treatment. An adsorption isotherm is derived upon generalizing the molecular model at hand so as to include the presence of polymeric neutral solute molecules adsorbed in a single orientation. The model accounts satisfactorily for a number of salient features of experimental capacity curves at metal—water interphases in the absence of adsorbed solute species, as well as for the adsorption behaviour of aliphatic compounds on mercury, provided that the doubly H-bonded water molecules are excluded from the molecular model. A justification for this exclusion, based on the existence of H-bonds between the first and second layer of water molecules, is provided.

Trends in the adsorption behaviour of n-aliphatic alcohols on mercury from aqueous 0.5 M Na2SO4

Abstract The electrosorption behaviour of n-aliphatic alcohols from n-butanol to n-octanol at the mercury/water interface was determined from capacitive charge measurements by using a computerized chronocoulometric technique. The surface area A occupied by one adsorbed alcohol molecule at maximum coverage was found to increase by about 0.06 nm2 per each CH2 group, thus suggesting strongly that the alcohols are adsorbed on mercury from aqueous solutions of 0.5 M Na2SO4 in a flat orientation up to maximum coverage. This interpretation is further supported by the approximate constancy of the differential capacity at maximum coverage for all alcohols investigated, and by the gradual increase in the coefficient with an increase in chain length, where ΔGoads is the standard Gibbs energy of adsorption and σm is the charge density of maximum adsorption. This adsorption behaviour differs from that from aqueous 0.1 M NaF as reported by B.B. Damaskin, A.A. Survila and L.E. Rybalka (Elektrokhimiya, 3 (1967)146); in fact, the latter behaviour suggests a vertical orientation of the hydrocarbon chains at maximum coverage.

Pre-steady State Electrogenic Events of Ca2+/H+ Exchange and Transport by the Ca2+-ATPase

Native or recombinant SERCA (sarco(endo)plasmic reticulum Ca2+ ATPase) was adsorbed on a solid supported membrane and then activated with Ca2+ and ATP concentration jumps through rapid solution exchange. The resulting electrogenic events were recorded as electrical currents flowing along the external circuit. Current transients were observed following Ca2+ jumps in the absence of ATP and following ATP jumps in the presence of Ca2+. The related charge movements are attributed to Ca2+ reaching its binding sites in the ground state of the enzyme (E1) and to its vectorial release from the enzyme phosphorylated by ATP (E2P). The Ca2+ concentration and pH dependence as well as the time frames of the observed current transients are consistent with equilibrium and pre-steady state biochemical measurements of sequential steps within a single enzymatic cycle. Numerical integration of the current transients recorded at various pH values reveal partial charge compensation by H+ in exchange for Ca2+ at acidic (but not at alkaline) pH. Most interestingly, charge movements induced by Ca2+ and ATP vary over different pH ranges, as the protonation probability of residues involved in Ca2+/H+ exchange is lower in the E1 than in the E2P state. Our single cycle measurements demonstrate that this difference contributes directly to the reduction of Ca2+ affinity produced by ATP utilization and results in the countertransport of two Ca2+ and two H+ within each ATPase cycle at pH 7.0. The effects of site-directed mutations indicate that Glu-771 and Asp-800, within the Ca2+ binding domain, are involved in the observed Ca2+/H+ exchange.

Two-state model for adsorbed solvent with and without adsorption of a neutral polymer species: A statistical mechanical treatment accounting for local order

Abstract The implications of the zeroth (random) approximation in the statistical mechanical treatment of an adsorbed monolayer consisting of “up” and “down” solvent molecules, with and without the further presence of the polymer molecules of a neutral solute adsorbed in a single orientation, are briefly examined. Certain inconsistencies in previous work on the subject are thus evidenced. It is concluded that the role played by local order in affecting the inner-layer properties cannot be disregarded. The above model of adsorbed monolayer is therefore re-examined taking local order into account via the quasi-chemical approximation. It is thus shown that an increase in attractive lateral interactions between adsorbed solvent molecules causes the single hump in the curve of the inner-layer capacity versus the charge density on the metal to split into two distinct humps. This may explain the presence of two humps in the differential capacity curves of certain highly polar organic solvents [52,53]. Moreover, it is shown that a Frumkin isotherm behaviour over a wide range of surface coverages is to be expected, provided that solvent-solvent lateral interactions are more attractive than solvent-surfactant and surfactant-surfactant lateral interactions. Noting that the adsorption of several bulky organic molecules from aqueous solutions actually satisfies the Frumkin isotherm, an appreciable hydrogen bonding between adsorbed water molecules is postulated, and a very rough estimate of the effect of such a bonding upon the adsorption behaviour of weakly interacting surfactant molecules is provided.

Electrostatic effect of specifically adsorbed electroinactive ions upon electrode processes

Abstract The electrostatic effect exerted by specifically adsorbed electroinactive ions upon electrode processes is accounted for by introducing into the Frumkin equation the average potential at the position occupied by the reacting particle in the transition state. The value of this potential is obtained from a simple model of the double layer in the presence of specific ionic adsorption. It is thus shown that the logarithm of the rate constant k f at constant applied potential is expected to vary linearly with the charge density q i at the inner Helmholtz plane. Some examples in support to the above theoretical prediction are reported.

Kinetics of electron and proton transfer to ubiquinone-10 and from ubiquinol-10 in a self-assembled phosphatidylcholine monolayer

Upon incorporating from 0.5 to 2 mol% ubiquinone-10 (UQ) in a self-assembled monolayer of dioleoylphosphatidylcholine (DOPC) supported by mercury, the kinetics of UQ reduction to ubiquinol-10 (UQH2) as well as that of UQH2 oxidation to UQ were investigated in borate buffers over the pH range from 8 to 9.5 by cyclic voltammetry. A general kinetic approach was adopted to interpret the dependence of the applied potential upon the scan rate at constant pH and upon pH at constant scan rate, while keeping the initial reactant concentration and the faradaic charge constant. The oxidation of UQH2 to UQ in DOPC monolayers occurs via the reversible release of one electron with formation of the semiubiquinone radical cation UQH2.+, followed by its rate-determining deprotonation by hydroxyl ions with formation of the UQH. neutral radical; the latter is then instantaneously oxidized to UQ. Analogously, the rate-determining step in UQ reduction to UQH2 consists in the protonation by hydrogen ions of the semiubiquinone radical anion UQ.- resulting from the reversible uptake of one electron by UQ. However, a non-negligible fraction of UQ.- uptakes protons very slowly, and hence, retains its intermediate oxidation state during the recording of the cyclic voltammetric peak for UQ reduction.