Mass Transport Conditions Research Articles

Metal Speciation Dynamics in Colloidal Ligand Dispersions

In this work we propose a dynamic metal speciation theory for colloidal systems in which the complexing ligands are localized on the surface of the particles; i.e., there is spatial heterogeneity of binding sites within the sample volume. The differences between the complex formation and dissociation rate constants of complexes in colloidal dispersions and those in homogeneous solutions originate from the differences in kinetic and mass transport conditions. In colloidal systems, when the effective rate of dissociation of the surface complexes becomes fully diffusion controlled, its value is defined via the geometrical parameters of the particle. We assess the extent to which the conventional approach of assuming a homogeneously smeared-out ligand distribution overestimates the lability of surface complexes in colloidal ligand dispersions. The validity of the theory is illustrated by application to binding of lead and cadmium by carboxyl modified latex particles: our approach correctly predicts the formation/dissociation rate constants, which differ by several orders of magnitude from their homogeneous solution counterparts.

Microwave-enhanced electrochemical processes in micellar surfactant media

High intensity microwave radiation effects are demonstrated for electron transfer processes at 25 or 50-μm diameter platinum electrodes immersed in micellar sodium dodecylsulfate (SDS) solutions. First, a solution containing 2 mM Fe(CN)63− and 2 mM Fe(CN)64− in aqueous 0.1 M NaCl with and without SDS is employed to calibrate the electrode temperature and mass transport conditions. Addition of 0.1 M SDS has only a small effect on the microwave enhanced voltammetry for the Fe(CN)63-/4− system. Next, two highly water-insoluble redox systems are studied. A solution of 1 mM tert-butylferrocene in aqueous 0.1 M NaCl containing 0.1 M SDS is shown to give no current response in the absence of microwaves. In the presence of focused microwaves at a platinum disc electrode, a strong current for the one electron oxidation of tert-butylferrocene is detected presumably due to localized disruption of the micellar solution. Concentrations of tert-butylferrocene down to the micromolar level are detected. α-Tocopherol, a lipophilic vitamin and antioxidant, is soluble in aqueous 0.1 M SDS/0.1 M NaCl. In the presence of microwave radiation, a strong and concentration dependent anodic current response consistent with the two-electron oxidation of α-tocopherol is observed. A heptode array of seven individual 50 μm diameter platinum microelectrodes placed in ca. 720 μm distance of each other is shown to allow microwave enhanced currents to be increased sevenfold with each electrode exhibiting the same microwave effect.

Surface modification of a reactive metal or alloy by polyaniline for electrooxidation of iodide

Generally, an inert metal such as platinum is used for studying electrooxidation reactions. As a non-platinum metal or alloy undergoes corrosion and oxidation, it is not useful for this purpose. In the present study, surface modification of non-platinum metals by coating electronically conducting polymers for electrooxidation reactions was investigated. Polyaniline (PANI) was electrochemically deposited on stainless steel (SS) substrate by potentiodynamic method. The oxidation of I was studied by cyclic voltammetry and amperometry experiments. The I/I2 reaction couple was found to be quasireversible on the PANI/SS electrode. The amperometry study, conducted under fast mass transport conditions, has provided linear relationship between current and concentration of I. The data were analyzed and rate constant of the reaction was evaluated. Thus the oxidation of I, which does not occur on bare SS electrode, was shown to occur through electron transfer mediated by polyaniline.

The reduction and recovery of step velocity in crystal growth induced by convection variation under various gravities

It is demonstrated from the parabolic flight experiments on crystal growth from solution, that a short time disturbance of the crystal growth condition significantly influences the crystal growth kinetics via variation of the mass transport condition by directly observing the movement of spiral steps in situ. An obvious reduction of step velocity was observed in short microgravity periods of less than 20 s, and the step advancing rate recovered quickly under normal gravity. The suppression of impurity adsorption at the growth steps could also be detected in such a short microgravity time by measuring the recovery process of growth steps. It is also shown from the experimental results that the regular change of gravity condition will reduce the time-dependent impurity effect on crystal growth by, probably, variation of the mass transport property.

Functioning of the White Sea Ecosystem: Studying the Transformations of Organogenic Substances Using a Mathematical Model

Long-term observational data on hydrology, hydrochemistry, and hydrobiology are generalized and used for systems analysis of the biohydrochemical transformation processes of organic and biogenic substances in a marine environment. An ecological model with the systematized data is used to assess the annual dynamics of concentrations of organic and mineral N, P, and Si compounds and dissolved organic C and O2 in eight water areas within the White Sea at specified conditions of water mass transport, river runoff, and water exchange with the Barents Sea. Variations in the biomasses of the major transformers of organic and biogenic substances (heterotrophic bacteria, phyto- and zooplankton, and microphytes) and their biological production were also evaluated. These characteristics serve as indicators of the state of the water environment, the presence of nutrients in it, and their import from outside.

Electrocatalysis under Conditions of High Mass Transport: Investigation of Hydrogen Oxidation on Single Submicron Pt Particles Supported on Carbon

The mechanism and kinetics of the hydrogen oxidation reaction (hor) has been investigated using carbon-supported single particles of Pt electrocatalyst with radii as small as 40 nm. The high mass transport rates on such small particles enable us to investigate the rapid kinetics of the hor in the absence of diffusion limitations. Surface kinetic controlled polarization curves during the electrochemical oxidation of hydrogen molecules in acid solution have been obtained in the entire H UPD region, showing features obviously different from those obtained on normal micrometer electrodes or in RDE experiments. For instance, two current plateaus rather than one are seen during the steady-state polarization of the hor on electrodes made of small particles. Upon decreasing the size of the Pt particles, the two current plateaus show greater separation and become better defined. A theoretical model for the steady-state polarization of the hor has been developed in which UPD H atoms of various states are considered as the reactive intermediates and the Frumkin adsorption mode is assumed for the atomic H on Pt electrodes. It is shown that the first current plateau represents the limiting reaction rate under adsorption or combined adsorption−diffusion control while the second plateau current corresponds to the limiting diffusion-controlled reaction rate. It is pointed out that Tafel plots that have been frequently used for kinetics analysis in the hor are meaningless, especially in the potential region below 0.05V vs RHE. The polarization curves are fitted with a general polarization equation derived according to our model. The fitting shows that the hor on Pt proceeds most likely via the Tafel−Volmer reaction mechanism rather than the Heyrovsky−Volmer mechanism. These results have significant implications on the understanding and modeling of the reactions in solid polymer electrolyte fuel cells.

Electrocatalysis under Conditions of High Mass Transport Rate: Oxygen Reduction on Single Submicrometer-Sized Pt Particles Supported on Carbon

The effect of the mass transport coefficient of reactant and product species during the oxygen reduction reaction, orr, on platinum in an acidic electrolyte has been experimentally examined and kinetically modeled. By using carbon electrodes having electroactive radii on the nanometer scale it is possible to produce single Pt particles having effective radii ranging from several micrometers to several tens of nanometers. As the mass transport coefficient is directly related to the size of these platinum particles, it is possible to examine the effect of mass transport on the orr in regions inaccessible to other experimental techniques. At the smallest of these Pt particles, mass transport coefficients equivalent to a rotating disk electrode at rotation rates (ω) of greater than 108 rpm are obtainable. Under low mass transport conditions equivalent to those obtainable using the normal rotating disk technique (i.e, ω < 10 000 rpm), oxygen reduction is seen to proceed via a four-electron reduction to water a...

A novel dual thin-layer flow cell double-disk electrode design for kinetic studies on supported catalysts under controlled mass-transport conditions

We present a novel dual thin-layer flow cell double-disk electrode design, which due to its high collection efficiency in combination with a reasonable time resolution allows highly sensitive measurements of electro-active species generated during electrochemical reactions on the working electrode. Most important, it allows for rapid changes of the electrolyte during the measurements, which in combination with a thin-film electrode, where an inert glassy carbon substrate is covered by a thin film of carbon-supported noble metal catalyst, makes it particularly useful for kinetic studies on electrocatalytic fuel cell reactions under realistic conditions (continuous reaction, continuous electrolyte flow) and on realistic materials (supported catalysts). The performance of the set-up and its suitability for kinetic studies, in particular for transient experiments involving rapid electrolyte exchange, are demonstrated in potentiodynamic and transient potentiostatic measurements for the oxygen reduction reaction on carbon-supported Pt catalysts and, for comparison, on Pt-free Vulcan carbon supports.

Sonovoltammetric Elucidation of Electron Transfer Rates: The Oxidation of Dimethyl‐p‐phenylenediamine in Aqueous Solution

AbstractThe electrochemical oxidation of dimethyl‐p‐phenylenediamine (DMPD) in aqueous solution (pH 7 phosphate buffer) has been studied under conventional hydrodynamic and microelectrode voltammetric conditions and found to undergo a two‐electron electrochemically reversible oxidation. Upon the application of ultrasound to the system an observed shoulder emerges in the oxidation wave. This effect has been attributed to the resolution of the two‐electron transfer processes occurring: the first a relatively fast electron transfer (0.1 cm s−1) followed by a second slower (10−3 cm s−1) electron transfer: under the very high mass transport rates induced by insonation an overpotential develops for the second electron transfer so leading to the observed voltammetric resolution. The range of mass transport conditions accessible via sonication allows the estimation of the two rate constants reported.

On some electrochemical oscillators at the mercury ∣ water interface

The principles underlying electrochemical oscillators on mercury have been known for three decades. They are (1) the existence of a negative charge transfer resistance, (2) the general coupling between interfacial kinetics and mass transport, and (3) the inclusion of the effects of additional impedances, such as the double layer capacitance and any series (film, solution, and external) resistances. The charge transfer resistance needs to be defined in general rather than within the confines of a specific model, such as that of Erdey-Grúz and Volmer. The effects of the double layer capacitance and of series resistance can be incorporated by using general stability criteria for electrical circuits. The above effects suffice for a quantitative representation of several types of oscillations observed at the mercury ∣ water interface. Electrochemical oscillations are typically non-linear, and closed-form analytical solutions are not available. Models in which diffusion is approximated in terms of Nernst diffusion layers are useful for experiments enforcing stationary mass transport conditions, as with rotating disk electrodes, but do not apply to measurements on hanging or dropping mercury electrodes. Digital simulations do not require such steady-state approximations.

A mesh microcontactor for 2-phase reactions.

The design, fabrication and use of mesh microcontactor structures is described. These allow contact between immiscible fluid phases (liquid/liquid or gas/liquid) enabling mass transfer and reaction between and within the phases. The phases are not mixed and are removed separately for subsequent use or analysis. The structures were designed for kinetic studies on biphasic reactions with the ability to handle sequential samples without excessive sample dispersion. Mass transport conditions are defined by fluid layer depths of 100 microm and the volume for each phase contacting in the reaction region was selected at 100 microl as sufficient for a range of analytical techniques. Micromeshes with pore diameter, depth, and spacing each of approximately 5 microm were formed by electrodeposition of nickel onto substrates with defined photoresist layers, and released by etching a copper sub-layer. Reactor enclosures defining chambers on each side of the mesh were formed from milled glass and metal components. The assembled structures have been used in flow-through and static fluid modes. System function has been demonstrated for both liquid/liquid and gas/liquid reactions. Test chemistries selected for ease of optical monitoring were hydrolysis of colourless fluorescein diacetate in toluene with transfer as fluorescein anion to aqueous alkali, and oxygen absorption into aqueous alkaline pyrogallol solution generating a coloured product, purpurogallin.

Influence of Mass Transport on Formation of Si-Nanostructures

Nanowire-like, condyloid-like and flakes of Si-nanostructures weresynthesized by thermal evaporation under different mass transportconditions by changing the ambient pressure. The structural analysisshows that a higher mass transport rate isnot favourite for the formation of fine single crystalline nanowireswhen the substrate placed closely to the thermal vapour source. Thehigher mass transport rate can induce a lower Si partial pressure nearthe source and hence results in a lower supersaturation near thesubstrate. Experimental results reveal that the formation ofSi-nanowires is not controlled by mass transport but by surfaceprocess. The driving force on the surface is the key factor for theformation of well-crystallized nanowires.

Determination of active concentrations and association and dissociation rate constants of interacting biomolecules: an analytical solution to the theory for kinetic and mass transport limitations in biosensor technology and its experimental verification.

Accurate determination of kinetic rate constants for interacting biomolecules requires knowledge of the active concentrations of the participating molecules. Also, in other biomedical and clinical applications, sensitive, precise and accurate methods are needed to determine the concentration of biologically active molecules, which frequently constitute only a fraction of the total molecular pool. Here we report a novel development of the approach to determining active concentrations based on surface plasmon resonance (SPR) technology. The method relies on changes in binding rates with varying flow rates under conditions of partial mass transport, and does not require standards of known concentrations, given that the molecular mass of the molecule of interest is known. We introduce an analytical solution to the differential equations describing the formation of a 1:1 bimolecular complex, taking into account both the association and dissociation reactions, under partial mass transport limitations. This solution can be used in global fitting to binding curves obtained at different flow rates. The accuracy, precision, and sensitivity of this approach were determined in experiments involving binding of tyrosine-phosphorylated recombinant proteins to anti-phosphotyrosine antibodies, where the active concentration could be determined independently by in vitro phosphorylation with (33)P. There was an excellent agreement between the active concentrations determined by the analytical SPR-based method and by determination of the level of radioactivity of the phosphorylated protein. The SPR-based method allows determination of protein concentrations at picomolar levels. A procedure for accurate determinations of association and dissociation rate constants, based on the analytical solution of the mass transport and binding theory, is outlined.

Numerical simulation of convective and diffusive transport effects on a high‐pressure‐induced inactivation process

High-pressure-induced conversions, such as the inactivation of enzymes or of microorganisms, are dependent on the applied pressure and the temperature of the process. The former can be considered to be a spatially homogeneous quantity, while the latter, being a transport quantity, varies over space and time. Here we question whether the uniformity of a high-pressure conversion can be disturbed by convective and conductive heat and mass transport conditions. Enzyme inactivation is taken as a model process for a high-pressure conversion. To cover a broad range of parameters and to consider scale-up effects, the investigation is based on mathematical modeling and numerical simulation for different sizes of the pressure chamber and different solvent viscosities. Apart from viscosity, the physical properties of the enzyme solutions are assumed to be identical in all cases. Therefore, matrix effects other than that of viscosity are excluded. Moreover, the authors postulate that viscosity solely acts on the continuum mechanical scale of momentum exchange but not on the molecular scale on the inactivation kinetics. It has been found that nonuniform thermal conditions can strongly influence the result of a high-pressure process. A variation of the activity retention between 28% and 48% can be observed after 20 minutes for a 0.8-L high-pressure chamber and a matrix fluid with a viscosity comparable to that of edible oils. The same process carried out in a 6.3-L device leads to an activity retention that varies between 16% and 40%. From the analysis of the time scales for the inactivation and for hydrodynamic and thermal compensation, it can be deduced that a nonuniform activity retention has to be expected if the inactivation time scale is larger than the hydrodynamic time scale and smaller than the thermal compensation time scale.

Consequences of Forced Convection for the Constraints on Size and Shape in Embryos

Previously, predictions of the maximum size of biological objects based on oxygen availability have been made for both zero and infinite water velocity around the object. In reality, however, water velocity is always intermediate between zero and infinity. We predicted maximum size and optimal shape of biological objects, pending the velocity of water around them. We assumed oxygen inside the object to be transported by diffusion and outside the object by diffusion and convection. Fick's first law of diffusion describes the inner transport. For the outer transport, we relied on semi-empirical relations between mass transport and flow conditions (Friedlander's equations). To keep mathematical complexity acceptable, we restricted ourselves to the analysis of a sphere and a cylinder in cross flow. If water velocity is low, a spherical shape is most favourable for gas exchange. If water velocity is high, an elongated and flattened shape is more favourable. A size-dependent intermediate velocity exists where shape does not matter (10−4ms−1for teleost embryos). Teleost embryos are typically exposed to flow velocities equal to or larger than 10−4ms−1, making an elongated shape more favourable than a spherical one. Although teleost eggs are typically spherical, the oxygen-consuming embryos inside are indeed elongated.

Electrooxidation of CO and H2/CO mixtures on a carbon-supported Pt catalyst—a kinetic and mechanistic study by differential electrochemical mass spectrometry

We report the results of a differential electrochemical mass spectrometry study on the electrooxidation of CO and H2/CO mixtures on a high surface area carbon-supported Pt/Vulcan catalyst. The measurements were performed in a thin-layer flow-cell, allowing the quantitative determination of the individual reaction rates under controlled mass transport conditions as a function of the electrode potential, parallel to the Faraday current. COad monolayer oxidation (stripping) experiments in a 0.5 M H2SO4 solution saturated with H2 or with 2% CO in H2 show that (i) the hydrogen oxidation reaction (HOR) in CO-free, H2-saturated solution starts already at potentials right at the onset of CO oxidation (0.15 VRHE), as evidenced by the simultaneous increase in Faraday current, the increase in the CO2 signal, and the decrease in H2 content in the solution, and that (ii) in CO containing solution the onset of the HOR is shifted to more positive potentials due to continuous CO readsorption. In the former case 90% of the mass transport limited current is reached already at a potential where only ∽5% of the saturated CO adlayer is oxidized, in the latter case the limiting HOR current is reached only in the potential regime of the main CO stripping peak. Furthermore, in the second case CO oxidation in the pre-wave regime is enhanced, while the HOR is suppressed in this potential regime compared to CO stripping in CO-free H2-saturated electrolyte. As a result the ignition of the HOR in the H2/CO(2%) saturated electrolyte occurs only together with the main CO oxidation peak. The data are direct proof that the HOR proceeds in an almost closed CO adlayer, most likely in small (fluctuating) holes of the adlayer and has to compete with CO readsorption. Kinetic measurements at constant electrode potential (0.04 V) in H2/CO(2%)-saturated solution show that CO adsorption from an H2/CO mixture follows a precursor-type adsorption kinetics. For the HOR the data show substantial deviations from both a (1 − θCO) and (1 − θCO)2 dependence of the HOR rate on the CO coverage, pointing to a more complex mechanism than a simple site-blocking mechanism.

Low-temperature sonoelectrochemical processes: Part 2: Generation of solvated electrons and Birch reduction processes under high mass transport conditions in liquid ammonia

Solvated electrons in liquid ammonia at −60°C (0.03 M LiCl) can be electrogenerated in the presence of power ultrasound and are shown to be essentially inert under these conditions in the presence of 3-methylanisole (3-MA). The fast mass transport and mixing induced by power ultrasound is then used to monitor the homogeneous Birch reduction process facilitated by the addition of ethanol as a proton source. Pseudo first-order kinetics for both the reduction of 3-MA and the unwanted formation of hydrogen is observed and analysed quantitatively. The rate laws proposed by Greenfield and Schindewolf (A. Greenfield, U. Schindewolf, Ber. Bunsenges. Phys. Chem. 102 (1998) 1808) for the reduction of protons, −(d[e −]/d t)= k H[CH 3CH 2OH][e −][Li +], and for the Birch reduction of 3-MA, −(d[e −]/d t)= k A[CH 3CH 2OH][e −][Li +][3-MA], are confirmed by varying the concentrations [CH 3CH 2OH] and [3-MA]. The rate constants k H=2±0.2 M −2 s −1 and k A=700±100 M −3 s −1 at −60°C are determined and employed for the optimisation of the product yield and current efficiency. Fast mass transport and electrode depassivation induced by power ultrasound are shown to maximise the current efficiency observed for the electrochemical Birch reduction process and to allow the process to be conducted in the presence of a high ethanol concentration and therefore with high overall rate.

EIS study of hydrogen insertion under restricted diffusion conditions

The theoretical expressions for the Faradaic impedance and electrode impedance are derived for indirect (two-step) insertion of hydrogen in metals and alloys, irrespective of the mass transport conditions in the electrode. Next, impermeable boundary (restricted diffusion) conditions are considered. The determination of the hydrogen concentration, diffusion coefficient and kinetic parameters from EIS data is discussed. The thermodynamic and kinetic conditions for which it is possible to distinguish between the direct (one-step) insertion mechanism and the indirect (two-step) mechanism on the basis of EIS data are analysed. Previous results in the electrochemical literature are discussed in relation to the theoretical derivation in this paper.

Pulse plating of cobalt-iron-copper alloys

Pulse plating of cobalt–iron–copper (CoFeCu) alloys was studied. A simple theoretical model with an analytical solution developed for binary alloys is applied to predict the copper content of the pulse plated ternary alloys. Studied compositions are in the range of Co90-xFe10Cux with x varying between 5 to 20 wt%. These compositions are of interest as soft magnetic materials with high saturation magnetization. The deposits were produced from a boric acid and sodium acetate electrolyte with low concentrations of copper and iron. All experiments were carried out under well-controlled mass transport conditions and current distribution using a recessed rotating cylinder electrode (rRCE) or an inverted rotating disc electrode (IrRDE). With the latter design alloys can be plated on flat substrates with or without application of a magnetic field to induce uniaxial magnetic anisotropy. Results show that by changing pulse parameters one can increase and decrease in opposite ways the copper and the iron content in the deposits. To test the influence of pulse parameters on the coercive field strength, a microstructure dependent property, theoretical predictions were used to produce films of identical composition with different pulse parameters. Within the range of pulse parameters studied the coercive field strength of this alloy does not vary. Transmission electron microscopy confirms that the deposits have the same nano-size grain structure.

Removal of tin from dilute solutions

The fluidised bed cell of inert glass beads is an electrolytic reactor which is designed to provide higher ion-transfer conditions during electrolysis, thus enabling metals to be removed efficiently from dilute solutions. The effectiveness of the method as a means of removing metals from effluent to meet discharge consent levels is studied for the in situ removal of tin from dilute solution (concentration range 0.25–1.00 gdm−3). The results show that the combination of high mass transport conditions and a moderately high electrode surface area per unit electrode volume provides a system for continuous removal of metal from dilute solutions. The effects of acid concentration, tin concentration, current density, fluidised bed agitation, electrode spacing, type of electrode and lead impurities on the removal of tin are reported and expressed in terms of the percentage removal of tin (αSn), the efficiency of tin deposition (ϕSn), and the energy consumption (WSn) for 1 kg of tin deposited. The results show that tin can, under optimised conditions, be removed from dilute solutions to a residual concentration of 0.001 gdm−3. © 2001 Society of Chemical Industry