Transient electrochemistry: beyond simply temporal resolution:

Some physicochem. intrigues for which transient electrochem. was necessary to solve the problem are summarized in this feature article. First, we highlight the main constraints to be aware of to access to low time scales, and particularly focus on the effects of stray capacitances. Then, the electron transfer rate const. measured for redox mols. in a self-assembled monolayer configuration is compared to the conductance measured through the same systems, but at the single mol. level. This evidences strong conformational changes when mols. are trapped in the nanogap created between both electrodes. We also report about dendrimers, for which a short electrochem. perturbation induces creation of a diffusion layer within the mol., allowing the electron hopping rate to be measured and analyzed in terms of mol. motions of the redox centers. Finally, we show that transient electrochem. provides also useful information when coupled to other methodologies. For example, when an ultrasonic field drives very fast movements of a bubble situated above the electrode surface, the motion can be dectected indirectly through a modification of the diffusion flux. Another field concerns pulse radiolysis, and we describe how the reactivity (at the electrode or within the soln.) of radicals created by a radiolytic pulse can be quantified, widening the possibilities of electrochem. to operate in biol. media

Surface acoustic cavitation understood via nanosecond electrochemistry. 2. The motion of acoustic bubbles

Acoustic cavitation considerably enhances the mass transport toward a surface. When suitably fast electrochemical equipment is used, periodic peak currents can be observed. Previous observations attributed these peaks to diffusion inside a thin liquid layer present between the electrode and the bubble (Maisonhaute, E.; White, P.C.; Compton, R.G. J. Phys. Chem. B 2001, 105, 12087-12091). This paper provides a semiquantitative model for explaining the bubble behavior, leading to an estimation of the diffusion layer thickness as well as the time during which the bubble "discovers" the electrode. Layer thicknesses ranging from 25 nm for very high acoustic pressures up to ca. 60 nm for smaller ones are found. Collapse velocities are estimated to be more than hundreds meters per second. Moreover, between two collapses, a slow bubble movement apart from the surface is evidenced. The force balance responsible for the collapse is reexamined and the viscosity constraint found to be an important parameter in explaining the global behavior

Surface acoustic cavitation understood via nanosecond electrochemistry

The application of high intensity ultrasound to liquids leads to cavitation. In contrast to the homogeneous situation this is poorly understood close to a surface despite the implications for many biological, chemical, and physical applications. By using ultrafast electrochemical equipment and arrays of electrodes, we prove that the acoustic bubbles in the range of power ultrasound are hemispherical, not spherical as usually supposed, possessing a large range of possible diameters and oscillating at harmonics and sub-harmonics of the driving frequency (20 kHz). Most importantly, contrary to inferences made previously at much lower frequencies, no liquid microjet inside the bubble is observed

Surface acoustic cavitation understood via nanosecond electrochemistry. Part III: Shear stress in ultrasonic cleaning.

Acoustic cavitation is extensively used for cleaning purposes. However, little is known about the fundamental aspects of the cleaning process. Our previous electrochemical data suggested that acoustic bubbles were oscillating at a distance of only a few tens of nanometers above the surface [J. Phys. Chem. B 105 (2001) 12,087; E. Maisonhaute, B.A. Brookes, R.G. Compton, J. Phys. Chem. B 106 (2002) 3166-3172]. The flow velocities resulting from the bubble collapse lead to important drag and shear forces on the surface, responsible for cleaning and/or eroding the latter. We review here the forces acting on an adsorbed particle located on the surface, and develop arguments to explain why small adsorbates are harder to remove by sonication. Then, experimental results on particle desorption and surface effects brought about by ultrasound are presented and shown to agree with our theoretical predictions

Microelectrode study of single cavitational bubbles induced by 500 kHz ultrasound.

Insight is gained into about the processes governing cavitational activity and acoustic streaming induced by high frequency (500 kHz) ultrasound by the use of microelectrodes with short time resolution electrochemical equipment to allow monitoring of the activity of single cavitating bubbles. Current transients are interpreted as showing the flux of solution towards the electrode surface due to microstreaming. In order to explain the current amplitude, a simplified model is produced. Important parameters such as bubble size and shape on the surface as well as the boundary layer thickness for microstreaming are taken into account. This model leads to the amplitude of the oscillations of the cavitating bubble. Introducing realistic bubble sizes, this amplitude is found to be in the order of 1 micron. The conclusions arising from this work allow a further interpretation of previous observations at millimeter scale electrodes

The application of fast scan cyclic voltammetry to the high speed channel electrode

The application of fast-scan cyclic voltammetry methods to the high-speed microband channel electrode (Rees et al. J. Phys. Chem. 99 (1995) 7096) is reported. Theory is presented to simulate cyclic voltammograms for a simple electron transfer under high convective flow rates within the high-speed channel. Experiments are reported for the oxidation of 9,10-diphenylanthracene (DPA) in acetonitrile solution containing 0.10 M tetrabutylammonium perchlorate (TBAP) for both 12.5 and 40 μm platinum microband electrodes using a range of scan rates from 50 to 3000 V s-1 and centre-line flow velocities from 12 to 25 m s-1. Analysis of the voltammograms yielded values for k0 and α for DPA which were measured to be 0.80±0.27 and 0.52±0.07 cm s-1, respectively. The range of applicability of this method was also investigated. Experiments are also presented using steady-state linear sweep voltammetry to obtain accurate measurements of the heterogeneous kinetic parameters for DPA at a platinum microband electrode. The measured value of k0 for DPA was found to be 0.94±0.16 cm s-1, with α = 0.53±0.02 and a formal oxidation potential of 1.40±0.01 V (vs. Ag)

Low-temperature sonoelectrochemical processes Part 3. Electrodimerisation of 2-nitrobenzylchloride in liquid ammonia

Dinitrobibenzyls are key intermediate species in certain drug syntheses. They can be formed by the electrochemical reduction of nitrobenzyl halides (in this work, 2-nitrobenzylchloride) in various solvents. In liquid ammonia at - 60°C, the mechanism involves a one-electron reduction and de-chlorination followed by the coupling of the neutral radical intermediate species. Exhaustive voltammetric studies, including fast scan cyclic voltammetry, of the starting material are presented prior to preparative electrolysis experiments. Electrolysis under both potentiostatic and galvanostatic conditions are compared. Under conditions of severe dryness, potentiostatic reduction at platinum gauze set at a voltage of - 0.30 V (vs. Ag wire) in the presence of ultrasound yields the dimer 2,2-dinitrobibenzyl ( > 95%) and no detectable side products. Water and oxygen have been found to decrease both the current efficiency and product yield of the process. Ultrasound is beneficial by: (i) enhancing the dissolution kinetics of the starting material; (ii) mass transport from the bulk towards the electrode, and vice versa, is greatly enhanced, thus considerably reducing the reaction times and optimising the current efficiency and product yields. © 2001 Elsevier Science B.V

The application of fast scan cyclic voltammetry to the high speed channel electrode

The application of fast-scan cyclic voltammetry methods to the high-speed microband channel electrode (Rees et al. J. Phys. Chem. 99 (1995) 7096) is reported. Theory is presented to simulate cyclic voltammograms for a simple electron transfer under high convective flow rates within the high-speed channel. Experiments are reported for the oxidation of 9,10-diphenylanthracene (DPA) in acetonitrile solution containing 0.10 M tetrabutylammonium perchlorate (TBAP) for both 12.5 and 40 μm platinum microband electrodes using a range of scan rates from 50 to 3000 V s-1 and centre-line flow velocities from 12 to 25 m s-1. Analysis of the voltammograms yielded values for k0 and α for DPA which were measured to be 0.80±0.27 and 0.52±0.07 cm s-1, respectively. The range of applicability of this method was also investigated. Experiments are also presented using steady-state linear sweep voltammetry to obtain accurate measurements of the heterogeneous kinetic parameters for DPA at a platinum microband electrode. The measured value of k0 for DPA was found to be 0.94±0.16 cm s-1, with α = 0.53±0.02 and a formal oxidation potential of 1.40±0.01 V (vs. Ag). © 2002 Elsevier Science B.V. All rights reserved