Guy Charles Quarin

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.

Electrochemical analysis of surfactants:: an overview

This work presents an overview of electrochemical techniques, namely potentiometry, amperometry, tensammetry, electrocapillary measurements and biosensors, recently applied for the determination of surfactants.

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.

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.

Voltammetric and chronoamperometric studies of tellurium electrodeposition of glassy carbon and gold electrodes

Abstract Electrodeposition of tellurium (IV) from acidic solutions was carried out on glassy carbon and gold electrodes by means of voltammetry and chronoamperometry. In both cases the anomalous recorded current-potential and current-time curves indicated the influence of adsorbed tellurium(0). Some considerations of the nucleation are discussed to explain the transient responses observed when double potential steps are applied. The semiconductor properties of tellurium when deposited as a thin layer may also be taken into account to explain the observed differences with the electroplating of a pure metal.

Electrochemical behaviour of drugs at lipid modified carbon paste electrodes

Abstract Mixing carbon paste with a charged lipid, in an appropriate ratio, permitted us to prepare a modified electrode whose surface exhibits interesting characteristics of the constituting lipid. Two negatively charged phospholipids have been selected, namely: asolectin and cardiolipin. By investigating the electrooxidation of several drugs which are known to interact strongly with the lipid component of cellular membranes (adriamycin, epirubicin and promethazine) it was possible to demonstrate markedly different behaviour at the lipid modified carbon paste electrode (LMPCE) depending on the pH of the solution. By analysing the results of linear scan, ac and adsorptive voltammetry and by taking into account literature data on drug—membrane interactions, it was possible to postulate a model of the LMCPE interface with thin layers of lipids covering the graphite particles. The differential capacity measurements as a function of time (fixed potential) and in the presence of increasing amounts of drugs enabled us to point out profound increases in the capacity of the LMCPE in the presence of adriamycin and epirubicin. In addition, the more negatively charged the lipids were, the higher was the capacity increase.

On the kinetics of adsorption and desorption of perchlorate ions in the presence of 3-methyl-isoquinoline at the mercury-aqueous solution interface

Abstract Coadsorption of perchlorate ions and 3-methyl-isoquinoline molecules at the mercury—aqueous solution interface results in the formation of a condensed film. The process of ClO − 4 incorporation (or removal) is very slow. Characteristic relaxation times (about 10 −2 s) are several orders of magnitude larger than values typical of ion adsorption at a mercury—solution interface which is free from organic surfactant. The relationships between the rate of the process, the electrode potential and the amount of adsorbed ClO − 4 ions are determined and discussed. The results indicate that the rate of the overall process is controlled by a slow adsorption (or desorption) step whose kinetics are consonant with an isotherm of the Temkin type.

Electrochemical behavior of zopiclone

The electrochemical properties of zopiclone, an anxiolytic and hypnotic drug, have been investigated by different techniques. The compound is reduced in two 2-electron steps in the pH range 0-12. The first step, which corresponds to the reduction of the pyrazine ring, is reversible in acidic and neutral solutions. Strong adsorption phenomena accompany the reduction process in acidic and neutral media. Zopiclone can be quantitatively measured over the entire pH range using DC polarography. However, the use of differential pulse and square-wave modes for quantitative measurements is more limited due to a slope modification in the current-concentration relationship. Adsorptive stripping voltammetry can be applied to the determination of low levels of the drug at pH 9, but only short deposition times may be used because large amounts of material accumulated under stirring conditions due to fast adsorption kinetics are rapidly released from the electrode surface. Detection limits are 1 x 10(-7)M and 2 x 10(-10)M for polarography and adsorptive stripping voltammetry, respectively. Only the first wave is of analytical interest for both techniques.

Phenothiazine drugs as redox mediators in horseradish Peroxidase bioelectrocatalysis

Abstract The homogeneous bioelectrocatalytic response between the enzyme Horseradish Peroxidase (HRP) and two structurally related phenothiazine drugs, promazine (PMZ), and promethazine (PMTZ), has been studied in acetate buffer pH 4.5, in the presence of hydrogen peroxide. Experiments were realized by cyclic voltammetry (CV) and chronoamperometry (CA) at a solid paraffin based carbon paste electrode. Distinct CV and CA patterns were observed for those structurally related isomers. The molecule PMZ behaved classically, as most phenothiazine drugs, with formation of the corresponding sulfoxide via an intermediate, relatively stable and electrochemically reversible, PMZ cation radical. Catalytic currents were obtained during the electroreduction of the latter due to the presence of the HRP/H2O2 system in solution which regenerated the cation radical. PMTZ electro- and enzymatic oxidation gave no corresponding stable cation radical but the molecule suffered a lateral chain breakdown with liberation of the phenothiazine nucleus (PHZ). The latter behaved reversibly at the electrode and catalytic currents were observed during the phenothiazine cation radical electroreduction. The overall rate constant between the phenothiazine drugs and HRP/H2O2 was determined by CA giving 3.8 × 105 and 2.9 × 105 M−1s−1 for PMZ and PMTZ(PHZ), respectively.

On the interaction between anions and organic surfactants in the inner part of the double layer at the mercury-water interphase

Abstract At the mercury-water interphase, synergic coadsorption is observed between organic surfactants of the azine group (quinoline, isoquinoline and some of their monomethylderivates) and bulky anions (like perchlorate and hexafluorophosphate, amongst others) which are not intrinsically very tensioactive. The characteristics of the composite inner layer have been investigated on the basis of the following data: 1. the Gibbs individual superficial excesses, 2. the Esin-Markov potential shifts at various constant charge densities, 3. the factors determining the differential capacity of the inner layer, 4. the extent of inhibition affecting electrode reactions of various degrees of complication (single electron exchange without mass transfer through the interphase, reduction coupled with amalgamation, or with chemical steps like protonation or decomplexation). As a rule, coadsorption leads to the formation of highly compact multilayers, with no definite stoechiometry. Structure stabilization appears to be essentially governed by the common hydrophobic behaviour shared by the anion, the surfactant and the positively charged mercury. Recourse to potential-step techniques indicates that the anion adsorption-desorption process is controlled by a slow relaxation step giving falling i-t transients, with time constants relatively independent from the values of the initial and final potentials. In the particular case of 3-methyl-isoquinoline, the region where coadsorption occurs is sharply limited by two potentials corresponding to phrase transitions with an abrupt charge density variation. Analysis of the related current-time transients indicates that the surface reorganization proceeds from nucleation centers, in a way reasonably described by the Avrami theory.