Lucien Gierst

Electrogeneration and some properties of the superoxide ion in aqueous solutions

Summary In the presence of surfactants such as quinoline which can form compact hydrophobic films, the inhibition of the polarographic reduction of oxygen selectively blocks the second electron transfer, and yields as main product the superoxide ion O 2 − . Under polarographic conditions, the reaction O 2 +e⇌O 2 − nearly achieves reversibility. By comparing the data obtained by pulse polarography and reoxidation chronocoulometry, the standard potential of the system has been determined ( E o =−0.270 V/NHE). From this value and the p K of the acid-base reactions between O 2 − , O 2 H and O 2 H 2 + , the full potential-pH diagram of the system O 2 o −O 2 −1 −O 2 −2 has been established. It shows that thermodynamic stability of O 2 − can only be expected in molten hydroxides—in agreement with fact. Electroreduction of O 2 − is a highly irreversible process. The non-additivity of the polarographic diffusion currents due to O 2 − generation and ferricyanide reduction, indicates that these two species react chemically in the diffusion layer. The behavior of electrodes other than mercury is briefly examined, in view of the possible role of O 2 − as first intermediate.

On the behaviour of molecules of the quinoline group at the water-mercury interface: Part III. The kinetics of the isoquinoline transition in the presence of an inert electrolyte

Abstract The reorientation transition, which involves two distinct monolayers of isoquinoline molecules, has been investigated by using the single-potential step and the double-potential step methods under various experimental conditions. The cathodic transients correspond to a liquid → solid transition. Their morphology and half-times have been measured at various initial and final potentials, and for several isoquinoline concentrations close to the saturation value. At small overvoltages, the reorientation process is determined by heterogeneous (“instantaneous”) nucleation, while progressive nucleation and growth prevail at higher potentials. Recourse to the double-potential step method affords an efficient way of assessing separately the influence of the overvoltage on the rate constants for nucleation and growth. Determination of the critical nucleus size and the activation energy for each of the two processes has been based on an adaptation of the Brandes theory developed for two-dimensional crystallization from a supersaturated vapour phase. The kinetics are markedly dependent on the initial state, which can be easily controlled by the potential and the bulk concentration. When the potential is stepped from the region where there is superadsorption, progressive nucleation is particularly fast, while starting from a partially depleted layer gives much slower transients with extensive tailing due to diffusional hindrance. The anodic transients show consonant characteristics which indicate that the initial film can be considered as a two-dimensional solid, and that boundary defects are acting as nucleation centres, during the solid → liquid transition triggered by the potential step.

On the behaviour of molecules of the quinoline group at the water—Mercury interface: Part II. Electrocapillary study of isoquinoline in solutions containing only an “inert” electrolyte

Abstract The present paper is the first of a series dedicated to the interfacial behaviour of several surfactants of the quinoline group, respectively in the absence and in the presence of coadsorbed anions. The present communication deals with the adsorption of quinoline (Q) itself at the mercury—water interface, in a medium such that coadsorption can be considered as negligible. Study of equilibrium adsorption has been based on measurements of interfacial tensions, charge densities and differential capacities as a function of the potential and the Q concentration. The experimental isotherms have been analysed with the help of Esin-Markov plots. In the range of sufficiently negative potentials, a compact film of invariant structure is observed, which remains stable up to a sharply defined cathodic desorption potential. At less negative potentials, the Esin-Markov plots, as well as the capacity—concentration—potential map, indicate that three different superficial structures may prevail, according to the domain of potential and concentration: o (1) water with dilute Q molecules lying flat; (2) a mixed aqueous layer which embodies both flat and standing Q molecules; (3) a film characterized by abnormally large values of the superficial excess, reflecting the presence of randomly distributed multilayered clusters. No clear evidence has been found for a stable compact layer of molecules lying flat. As can be expected, surface—pressure plots are non-congruent, in view of the variety of orientation and lateral forces involved with potential or concentration changes. Some data are presented which will be made fully explicit at a later stage (Parts IV and V of this series), where the various surfactants will be compared in terms of their molecular structure and dipole orientation.

Mechanism of electron transfer through monomolecular films of neutral organic species adsorbed at an electrode surface

The kinetics of reduction of Co(NH3)3+6, CrO2-4, S2O2-8 and O2 at a Hg electrode coated with monolayers of quinoline, iso-quinoline and 3-methyl-iso-quinoline have been investigated. The mechanism of electron transfer at the film-covered electrode depends on the structure of the superficial film. At a liquid-like film, the reacting ions are discharged both from inside and outside the film. Within the potential range where sold-like films are observed, it has been found that: (1) the rate of ETR reactions is slowed down by 6–7 orders of magnitude; (2) the decelerating effect of the film is distributed equally between changes in the probability of the electron transition and the energy of activation of ETR; (3) the logarithm of the reaction rate decreases linearly with the film thickness; (4) Tafel plots are linear and their slopes are slightly smaller than those observed for the bare electrode surface; and (5) the electronic transitions are probably elastic in character although distinction between elastic and resonance tunnelling is difficult for thin films.

On the two modes of condensation displayed by thiourea at the mercury-water interface

Abstract In the presence of “inert” electrolytes (sodium fluoride, sulfate, carbonate, phosphate) there exists a range of temperatures and thiourea (TU) concentrations (low and high respectively) where two distinct condensed TU monolayers, totally devoid of anions, are observed. The two relevant molecular structures which are suggested rest on the analysis of the following data: (1) E-T-[TU] phase diagrams derived from capacitance measurements, (2) superficial excesses and molecular areas, (3) charge densities, (4) inhibition power, and (5) kinetics displayed by the various phase transitions involved. Anions like ClO−4, NO−3, ClO−3 are only coadsorbed within the gas-like film of TU, with the result that the pure TU condensed regions are squeezed detectably. On the other hand, anions known to interact strongly with Hg (Cl−, Br−, I−, SCN−)_do coadsorb significantly at all potentials. However, pure TU films tend to reappear if the concentration of these anions is lowered sufficiently with respect to that of the surfactant.

On the behaviour of molecules of the quinoline group at the water-mercury interface: Part IV B. The kinetics of two-dimensional phase transitions displayed by 3-methyl-isoquinoline

Abstract Adsorption of 3-methyl-isoquinoline (3-MeisoQ) at the mercury-water interface involves three distinct kinetically controlled processes: o (1) The liquid-solid (LS) transition, which is characterized by a conspicuous step in the charge-potential curves, presents a hysteresis region which, in the absence of heterogeneous nucleants, exceeds 60 mV. The true equilibrium transition potential has been determined by double-potential-step experiments devised in a way such that there is stable coexistence at the electrode of both the L and S phases. The L→S process is controlled by a nucleation-growth-collision mechanism. Methods for the independent evaluation of the rate of nucleation and that of growth have been used. The rate constants obtained in this way are in good agreement with the combined set derived from the Avrami equation for progressive nucleation. The reverse process S→L, which is considerably faster appears to originate from defect lines at the collision boundaries between the separate plates which form the solid patchwork. (2) A qualitatively similar behaviour characterizes the adsorptiondesorption (GS) process which occurs at very negative potentials and is partially influenced by mass transfer from the bulk phase. (3) The solid phase, which prevails in the middle range of the potential scale, shows indications of a limited slow pre-melting process, at the immediate proximity of the GS transition. With the exception of the latter phenomenon, which is specific to 3-MeisoQ, and provided that the concentrations are normalized with respect to the saturation value, 3-MeisoQ adsorption kinetics does not differ sharply from that of isoQ, although significant differences demonstrate that the presence of the methyl group weakens the stability of the flat and tilted molecular orientations, which are observed in the most positive range of charge densities.

Phase transition processes displayed by coumarin monolayers at the water−mercury interphase

Abstract At low temperatures and high concentrations, coumarin was found to form a quasi-solid monolayer at the merucy electrode, in addition to the two structurally different monolayers previously reported. This new condensed layer is characterized by a very low (3.5 μF cm−2) potential-independent double layer capacitance. The effects of potential, temperature, coumarin concentration and background electrolyte on the adsorption behaviour are presented and analyzed. It is shown that the condensed layer can be formed from either of the other two adjoining layers by a nucleation and growth mechanism. The related growth rates and line tensions have been calculated. The most probable structures for the three layers are proposed.

On the behaviour of molecules of the quinoline group at the water-mercury interface: Part IV A. Electrocapillary study of 3-methyl-isoquinoline

Abstract The adsorption of 3-methyl-isoquinoline at the mercury-water interface involves a number of rate-determining factors (local depletion uncompensated by mass transfer, influence of the diffuse-layer potential, slow kinetics) which has limited the present study to the most concentrated solutions. In that range, the major traits of the behaviour of 3-MeisoQ and isoQ are comparable at least qualitatively (rearrangement and desorption transitions, condensed layers, existence of a quasi-solid two-dimensional structure of invariant properties at negative potentials). Differences result from the steric hindrance of the methyl group on all adsorption modes, except that which involves the perpendicular-high orientation (with nitrogen and the methyl group facing the solution side of the interface). It appears that, as is the case for aliphatic alcohols or acids among others, meaningful comparison must be based on relative concentrations normalized with respect to the saturation value. The dependence of the rearrangement transition potential on surfactant concentration and ionic strength indicates that the inner potential difference is the most suitable independent variable.

On the nature of the factors which govern the electrochemical condensation of neutral surfactants

Abstract 2-D phase transitions involving neutral surfactants are primarily controlled by the interfacial free energy gap Δγ # which is triggered when an adequate perturbation step is imposed to the surface. By combining (i) the Gibbs differential equation for electrocapillarity, (ii) the Avrami expression for progressive nucleation and growth and (iii) the Erdey—Gruz and Volmer relation for the activation energy of nucleation, the separate effects of pressure, potential, concentration and temperature can be predicted in a quantitative way. The three latter factors are examined. It is found that polynucleation transients depend on E, c and T , in close agreement with the theory. The system investigated is isoquinoline at the mercury—water interface.

Potential step chronocoulometry of dioxygen in aqueous alkaline solutions saturated with isoquinoline

Abstract In alkaline solutions saturated with isoquinoline acting as inhibitor and for potentials negative with respect to the phase transition, dioxygen displays a drawn-out bielectronic irreversible polarization wave with a shape suggesting that the first electron transfer is entirely rate-determining. However, use of reverse pulse polarography (RPP) and other related methods demonstrates that an intermediate (the superoxide ion) is built up in appreciable amounts. The surface concentrations of O 2 , O − 2 and O 2 H − have been determind as a function of the potential and the reduction rate constants presented under the form of Tafel plots. Potential-step programmes of the RPP type are shown to be much more informative about reaction schemes and rate constant values than classical dc methods.