My research focuses on the development and application of mathematical techniques for system and process modelling, design and optimisation in process systems engineering, physical chemistry and biochemistry. I was trained as an Applied Mathematician at Kharkov National University of Radio Electronics (Ukraine) where I obtained my BSc (2000) and MSc (2001) degrees before doing a D.Phil. in Computational Electrochemistry from the University of Oxford (2004) and a PhD in Computational Mathematics and Numerical Methods from Kharkov National University of Radioelectronics (2006). Since then I have held a number of researcher and senior scientist positions in Ukraine, France (Ecole Normale Supérieure, Paris) and the UK (Imperial College London) before joining the Department of Chemical and Process Engineering of the University of Surrey in 2016 as a Lecturer.
Model-based design and optimisation of process systems
Mathematical modelling in the natural and life sciences
Uncertainty quantification in process models, particularly Global Sensitivity Analysis
I am currently module leader of two M-level modules:
- ENGM071 Process and Energy Integration (MSc only)
- ENGM214 Process Modelling and Simulation (MSc and Year 4 MEng)
and contribute the Numerical Methods component of the Year 2 BEng/MEng module
- ENG2113 Reaction Engineering and Numerical Methods
In the past, I have been involved in the delivery of a number of other undergraduate modules:
- ENG1083 Transferable Skills and Laboratory Skills
- ENG2120 Engineering Systems and Dynamics
- ENG3186 Energy and Industrial Systems
I regularly supervise Year 4 MEng and MSc research projects.
Optimisation and simulation models for the design and operation of grid-connected distributed energy systems (DES) often exclude the inherent nonlinearities related to power flow and generation and storage units, to maintain an accuracy-complexity balance. Such models may provide sub-optimal or even infeasible designs and dispatch schedules. In DES, optimal power flow (OPF) is often misrepresented and treated as a standalone problem. OPF consists of highly nonlinear and nonconvex constraints related to the underlying alternating current (AC) distribution network. This aspect of the optimisation problem has often been overlooked by researchers in the process systems and optimisation area. In this review we address the disparity between OPF and DES models, highlighting the importance of including elements of OPF in DES design and operational models to ensure that the design and operation of microgrids meet the requirements of the underlying electrical grid. By analysing foundational models for both DES and OPF, we identify detailed technical power flow constraints that have been typically represented using oversimplified linear approximations in DES models. We also identify a subset of models, labelled DES-OPF, which include these detailed constraints and use innovative optimisation approaches to solve them. Results of these studies suggest that achieving feasible solutions with high-fidelity models is more important than achieving globally optimal solutions using less-detailed DES models. Recommendations for future work include the need for more comparisons between high-fidelity models and models with linear approximations, and the use of simulation tools to validate high-fidelity DES-OPF models. The review is aimed at a multidisciplinary audience of researchers and stakeholders who are interested in modelling DES to support the development of more robust and accurate optimisation models for the future.
Background: Human memory B cells play a vital role in the long-term protection of the host from pathogenic re-challenge. In recent years the importance of a number of different memory B cell subsets that can be formed in response to vaccination or infection has started to become clear. To study memory B cell responses, cells can be cultured ex vivo, allowing for an increase in cell number and activation of these quiescent cells, providing sufficient quantities of each memory subset to enable full investigation of functionality. However, despite numerous papers being published demonstrating bulk memory B cell culture, we could find no literature on optimised conditions for the study of memory B cell subsets, such as IgM+ memory B cells. Methods: Following a literature review, we carried out a large screen of memory B cell expansion conditions to identify the combination that induced the highest levels of memory B cell expansion. We subsequently used a novel Design of Experiments approach to finely tune the optimal memory B cell expansion and differentiation conditions for human memory B cell subsets. Finally, we characterised the resultant memory B cell subpopulations by IgH sequencing and flow cytometry. Results: The application of specific optimised conditions induce multiple rounds of memory B cell proliferation equally across Ig isotypes, differentiation of memory B cells to antibody secreting cells, and importantly do not alter the Ig genotype of the stimulated cells. Conclusions: Overall, our data identify a memory B cell culture system that offers a robust platform for investigating the functionality of rare memory B cell subsets to infection and/or vaccination.
A novel theoretical and numerical framework for the estimation of Sobol’ sensitivity indices for models in which inputs are confined to a non-rectangular domain (e.g., in presence of inequality constraints) is developed. Two numerical methods, namely the quadrature integration method which may be very efficient for problems of low dimensionality and the MC/QMC estimators based on the acceptance-rejection sampling method are proposed for the numerical estimation of Sobol’ sensitivity indices. Several model test functions with constraints are considered for which analytical solutions for Sobol’ sensitivity indices were found. These solutions were used as benchmarks for verifying numerical estimates. The method is shown to be general and efficient.
Optimisation models for the design of distributed energy systems (DES) often exclude inherent nonlinearities and constraints associated with alternating current (AC) power flow and the underlying distribution network. This study aims to assess this gap by comparing the performance of linear and nonlinear formulations of DES design models, connected to and trading with an AC grid. The inclusion of the optimal power flow (OPF) constraints within the DES design framework is demonstrated in the methodology. A residential case study is used to test both models and compare the designs obtained from the two formulations. The results highlight that DES designs obtained are different when constraints related to the underlying distribution network are added, particularly when electricity storage is not considered. Overall, this study highlights the need for future modelling efforts to include OPF within DES optimisation frameworks to obtain practically feasible designs, rather than considering them as standalone problems.
The manufacture of protein-based therapeutics presents unique challenges due to limited control over the biotic phase. This typically gives rise to a wide range of protein structures of varying safety and in vivo efficacy. Herein we propose a computational methodology, enabled by the application of constrained Global Sensitivity Analysis, for efficiently exploring the operating range of process inputs in silico and identifying a design space that meets output constraints. The methodology was applied to an antibody- producing Chinese hamster ovary (CHO) cell culture system: we explored > 80 0 0 feeding strategies to identify a subset of manufacturing conditions that meet constraints on antibody titre and glycan distri- bution as an attribute of product quality. Our computational findings were then verified experimentally, confirming the applicability of this approach to a challenging production system. We envisage that this methodology can significantly expedite bioprocess development and increase operational flexibility.
An improved configuration for simultaneous electrochemical ESR is described in which a tubular flow cell occupies a cylindrical TE011 ESR cavity. The minimisation of dielectric loss allowed good cavity Q-values to be achieved, resulting in a high level of sensitivity and allowing good quality spectra to be attained for the N,N,N′, N′-tetramethyl-para- phenylenediamine (TMPD) radical cation and the 1,2,4,5-tetrachlorobenzoquinone (para-chloranil) and nitrobenzene radical anions. The theory describing the convective and diffusive mass transport in the tubular arrangement, along with its influence on the steady state ESR signal intensity for these stable radicals is fully developed and shown to be in good agreement with experimental results. The transient ESR response is also considered and shown to agree well with the theory presented in the literature at flow rates for which the Lévêque approximation holds.
In this article we investigate the origin of unexpected features appearing in voltammetry or double-step chronoamperometry of EE systems when the rate of comproportionation is extremely fast and the diffusion coefficients of the reactant and products highly differ. These features were noticed during the testing of our new software KISSA intended to solve any reaction mechanisms even when acute reaction fronts develop near the electrode surface or within the solution. To validate the principle of the new adaptive algorithm [C. Amatore, O. Klymenko, I. Svir, Electrochem. Commun. doi:10.1016/j.elecom.2010.06.009] implemented in KISSA we used analytical and numerical solutions for double-step chronoamperometry. This revealed that the peculiar current jumps stem from a rapid variation of the reaction front position when the starting material is still reducible at the electrode while the product of the second electron transfer is re-oxidized. The exact convergence between the predictions by these independent methods demonstrated that KISSA is perfectly accurate even under such extreme mechanistic conditions. © 2010 Elsevier B.V.
Herein, we extend our previous approach concerning the reconstruction of profiles of pressure-driven hydrodynamic flow in microfluidic channels based on current measurements at band electrode(s) [see the preceding paper ChemPhysChem 2005, 6, 1581]. We address the central issue of optimization of geometrical parameters describing the electrode(s) assembly (a single band and two bands working in generator-collector mode) within the channel flow cell to enhance the speed and precision of the flow profile reconstruction method.
A theoretical approach for flow profile reconstruction in a rectangular microfluidic channel equipped with one or two microband electrodes working in generator-collector and generator-generator regimes was proposed by us previously (ChemPhysChem 2005, 6, 1581-1589; ChemPhysChem 2006, 7, 482-487). The purpose of the current study is to determine the ranges of dimensionless parameters corresponding to the highest sensitivity of the minimized functional to the shape of the flow profile. By application of a cubic spline to approximate the flow profile and analysis of the least-squares functional, which can then be represented as a function of one variable, we derive the area of optimal method performance. Thus, mathematical confirmation of our previous theoretical physical predictions could be obtained.
An investigation into the oxidative electrochemistry of 1,3,5-Tris[4-[(3-methylphenyl)phenylamino]phenyl]benzene (TMPB) is described. In particular, the heterogeneous electron-transfer kinetics in N,N-dimethylformamide (DMF) and dichloromethane (DCM) are studied using the high-speed microband channel electrode. Experiments to investigate the standard electrochemical rate constants, k0, transfer coefficients, α, and formal potentials, Ef0, of the observable oxidations of TMPB in both DMF and DCM solutions containing 0.10 M tetrabutylammonium perchlorate (TBAP) are reported for 12.5 μm platinum microband electrodes using a range of linear flow velocities from 12 to 25 m s-1. The measured values of k0 for the two measurable oxidations in DMF are 1.03±0.41 and 1.02±0.20 cm s-1, and for the three oxidations in DCM are 0.44±0.04, 0.17±0.03 and 0.08±0.02 cm s-1, respectively. The values of α for these oxidations in DMF are 0.48±0.06 and 0.52±0.02, and in DCM are 0.52±0.02, 0.62±0.03 and 0.53±0.07, respectively. The respective formal oxidation potentials (all measured vs Ag) are 0.533±0.002 and 0.766±0.003 V in DMF, and 0.161±0.002, 0.495±0.002 and 1.128±0.004 V in DCM. The presence of the monocation radical in DCM is confirmed by ESR measurement. Experiments are also presented to explore the voltammetry of TMPB in microdroplets of toluene and also in the solid-phase, when in contact with aqueous solutions of sodium fluoride, perchlorate, nitrate and sulphate. It was found that, when TMPB is dissolved in a toluene microdroplet, anion insertion accompanies the first oxidation for the case of perchlorate and nitrate, with anion-facilitated dissolution occurring for sulphate and fluoride. More complex reactions occur at more positive potentials. In the solid-phase, however, slow anion-facilitated dissolution still occurs for fluoride and sulphate, and rapid direct dissolution takes place in the case of perchlorate. © 2003 Elsevier B.V. All rights reserved.
We present a mathematical model for the surface-controlled dissolution of solid particles. This is applied to the dissolution of a solid having different particle size distribution functions: those of a monodispersed solid containing particles of all one size, a two-size-particle distribution, and a Gaussian distribution of the particle sizes. The dissolution of potassium bicarbonate in dimethylformamide is experimentally studied indirectly at elevated temperatures. We monitor the dissolution via the homogeneous deprotonation of 2-cyanophenol by dissolved KHCO3. The loss of 2-cyanophenol was detected electrochemically at a platinum microdisk electrode, and separately, the formation of the 2-cyanophenolate anion was monitored via UV-visible spectroscopic analysis. The results presented show that the kinetics of the loss of 2-cyanophenol behaves on one hand as a homogeneous chemical process and on the other hand as a dissolution-rate-controlled process. Initially, predissolved KHCO3 in solution deprotonates the 2-cyanophenol and homogeneous reaction dominates the observed kinetics, and at longer times, the observed kinetics is controlled by the rate of KHCO3 dissolution. Modeling of the experimental results for the surface-controlled dissolution of KHCO3 in dimethylformamide (DMF) yielded a mean value for the dissolution rate constant, k, at elevated temperatures; k was found to have a value of (1.1 +/- 0.3) x 10(-8) mol cm(-2) s(-1) at 100 degrees C, and the activation energy for the dissolution was 34.4 +/- 0.4 kJ mol(-1) over the temperature range 60-100 degrees C.
A new equation to describe the diffusion controlled nucleation of hemispherical centres under conditions of forced convection is derived, in a manner similar to that of the Scharifker and Mostany equation (J. Electroanal. Chem. 177 (1984) 13). It has been shown that the transients measured under these conditions should show a monotonically rising transient, followed by convergence to a limiting current. The accuracy and validity of the model is tested by comparison with experimental data obtained using a 5 mM Co/Co2+ system, sonicated at 66 W cm-2. Excellent fits are obtained, and the parameters derived are in agreement with those derived from independent experiments. © 2002 Elsevier Science B.V. All rights reserved.
Concentration profiles of electrochemically generated species are simulated in a channel flow cell containing two electrodes, where a species produced upstream at the generator electrode is detected downstream by the collector electrode. The model is extended for simulation of the currents produced at each electrode when a homogeneous reaction takes place involving the species produced at the generator. For specific cell dimensions and homogeneous mechanism there is a maximum rate constant for the homogeneous reaction that can be measured using the double electrode system, which is a function of the flow rate, the cell geometry and the relative concentrations of the species initially present. The results presented function as a guide to selecting suitable cell geometries and experimental parameters in order to measure homogeneous kinetics using the double channel electrode. © 2004 Elsevier B.V. All rights reserved.
In this paper we present a combined simulation approach for solving complex electrochemical kinetic problems at (hemi)spherical and (hemi)cylindrical electrodes based on the simultaneous application of analytical conformal coordinate transformations to accurately treat the diffusional transport and adaptive grid compression to locally increase the accuracy of the approximation of acute reaction fronts. This strategy is shown to be very efficient and leads to extremely accurate results both for steady state and transient situations and is equally powerful for both pure diffusion and that complicated by severe kinetic distortions. © Pleiades Publishing, Ltd., 2012.
Distributed Energy Systems (DES) are set to play a vital role in achieving emission targets and meeting higher global energy demand by 2050. However, implementing these systems has been challenging, particularly due to uncertainties in local energy demand and renewable energy generation, which imply uncertain operational costs. In this work we are implementing a Mixed-Integer Linear Programming (MILP) model for the operation of a DES, and analysing impacts of uncertainties in electricity demand, heating demand and solar irradiance on the main model output, the total daily operational cost, using Global Sensitivity Analysis (GSA). Representative data from a case study involving nine residential areas at the University of Surrey are used to test the model for the winter season. Distribution models for uncertain variables, obtained through statistical analysis of raw data, are presented. Design results show reduced costs and emissions, whilst GSA results show that heating demand has the largest influence on the variance of total daily operational cost. Challenges and design limitations are also discussed. Overall, the methodology can be easily applied to improve DES design and operation.
The measurement of liquid-liquid kinetics in acoustically emulsified media is shown to be conveniently achieved via the monitoring of bulk concentrations. This is exemplified with reference to the extraction of copper from an aqueous solution by the ligand 5-nonyl-salicylaldoxime in ethyl acetate. The speed of the extraction under ultrasound compares favourably with that seen under conventional stirring and the benefits are illustrated with reference to the extraction of copper from industrial effluent.
The validity of Marcus theory for outer-sphere heterogeneous electron transfer for the electro-oxidation of a range of anthracene derivatives in alkyl cyanide solvents is investigated. The precision measurement of these fast electron transfers (k(0) >or= 1 cm s(-1)) is achieved by use of the high-speed channel electrode and, where necessary, fast-scan cyclic voltammetry. First, the solvent effect on the rate of electron transfer is studied by considering the first oxidation wave of 9,10-diphenylanthracene in the alkyl cyanide solvents: acetonitrile, propionitrile, butyronitrile, and valeronitrile. Second, the variation of k(0) for a series of substituted anthracenes is investigated by analyzing the voltammetric response of the one-electron oxidations of 9-phenylanthracene, 9,10-dichloroanthracene, 9-chloroanthracene, 9,10-dicyanoanthracene, 9-cyanoanthracene, 9-nitroanthracene, 9,10-diphenylanthracene, and anthracene in acetonitrile. It is shown that the rate of electron transfer of a single compound in different alkyl cyanides is determined by the longitudinal dielectric relaxation properties of the solvent, while differences in rate between the substituted anthracenes in acetonitrile can be quantitatively rationalized by considering their relative hydrodynamic radii. This makes possible the accurate prediction of electron-transfer rates for a molecule by interpolation of rate constants known for related molecules.
The electrochemical oxidation of N,N-diethyl-p-phenylenediamine in dimethylformamide has been studied at platinum and gold microdisk electrodes of various radii between 6.7 and 66 μm. The voltammetric responses revealed two electrochemically reversible waves the second of which becomes larger at higher concentrations and bigger electrode radii. The voltammetric signals have been modelled and the electrochemical oxidation reaction is not inconsistent with an ECrevECE reaction. Kinetic parameters are reported.
The electrochemical oxidation of DEPD proceeds via an ECrevECE mechanism in dimethylformamide. It has been investigated at elevated temperatures up to 130 °C at both micro and macro platinum and glassy carbon electrodes. Kinetic and thermodynamic parameters for the reaction process have been calculated for each temperature. Further, the voltammetric response of DEPD shows enhanced limiting currents in the presence of sulfide. The analytical utility of the approach has been investigated with linear range from 50 to 850 μM sulfide concentration observed and a corresponding limit of detection of 20 μM achievable at temperatures of 70 °C.
Mass transport at cylindrical and spherical microelectrodes involving diffusion and migration is analyzed by means of numerical simulation under transient conditions. The origin of the intrinsic difficulties encountered during the numerical solution of the diffusion-migration equations using implicit finite differences are outlined, especially for the particular case when the number of electrons transferred equals the charge number of the electroactive species. The numerical results for transient conditions have been compared to the general analytical solutions for the current enhancement or diminishment due to migration under steady- and quasi-steady-state conditions at 1D geometry microelectrodes (Amatore, C.; Fosset, B.; Bartelt, J.; Deakin, M. R.; Wightman, R. M. J. Electroanal. Chem. 1988, 256, 255-268). This yields that the analytical limiting currents are applicable, within experimental error, to the analysis of transient diffusion-migration current responses at microelectrodes of cylindrical and spherical geometries except extremely short times after the application of the potential step, i.e., when current measurements are anyway already corrupted by ohmic drop when the supporting electrolyte concentration is low. Also, this confirms that the current enhancements or diminishments due to migration are identical for both electrode geometries when steady- or quasi-steady states are approached and do not drastically differ even under transient regimes.
The year 2020 has seen the emergence of a global pandemic as a result of the disease COVID-19. This report reviews knowledge of the transmission of COVID-19 indoors, examines the evidence for mitigating measures, and considers the implications for wintertime with a focus on ventilation.
The variance-based method of global sensitivity analysis based on Sobol' sensitivity indices has become very popular among practitioners due to its ease of interpretation. A novel theoretical and numerical framework for the estimation of Sobol’ sensitivity indices for models in which inputs are confined to a non-rectangular domain (e.g., in presence of inequality constraints) is developed. MC/QMC estimators based on the acceptance-rejection sampling method are proposed for the numerical estimation of Sobol’ sensitivity indices. Random Sampling - High Dimensional Model Representation metamodeling method is used to approximate models and constraints which significantly reduces the cost of evaluating Sobol’ sensitivity indices. Several model test functions with constraints are considered. The method is shown to be general and efficient.
A finite element approach for numerical simulation of linear sweep voltammetry at the wall tube electrode is presented. Working curves and surfaces are computed and reported which permit the analysis of reversible, quasi-reversible and irreversible voltammograms for all voltage scan rates. © 2002 Elsevier Science B.V. All rights reserved.
In this article, the numerical approach for flow profile reconstruction in a microfluidic channel equipped with band microelectrodes introduced previously by the authors, based on transient currents, is extended to the exclusive use of steady-state currents. It is shown that, although the currents obey steady state, the flow velocity profile in the channel may be reconstructed rapidly with a high accuracy, provided a sufficient number of electrodes performing under steady state are considered. The present theory demonstrates how the electrode widths and sizes of gaps separating them can be optimized to achieve better performance of the method. This approach has been evaluated theoretically for band microelectrode arrays embedded into one wall of a rectangular channel consisting of three, four, or five electrodes, all of which are operated in the generator mode. The results prove that the proposed approach is able to accurately recover the shape of the flow profile in a wide range of Peclet numbers and flow types ranging from the classical parabolic Poiseuille flow to constant electro-osmotic-type flow.
Understanding the mechanisms of solid-liquid systems is fundamental to the development and operation of processes for the production of agrochemicals and pharmaceuticals. The use of a strong inorganic base in an organic solvent, typically, potassium carbonate in dimethylformamide, is often used to facilitate the formation of a required anionic organic nucleophile. In this paper, the dissolution kinetics of potassium carbonate in dimethylformamide at elevated temperatures is studied in the presence of ultrasound, as revealed via monitoring of the deprotonation of 2-cyanophenol by dissolved K2CO3. Two independent experimental methods were employed; the loss of 2-cyanophenol was detected electrochemically at a platinum microdisk working electrode, and the formation of the 2-cyanophenolate anion was monitored via UV/visible spectroscopic analysis. The results were modeled by fitting the experimental data to a theoretical model for the surface-controlled dissolution of solid particles. The dissolution rate constant, k, for the dissolution of K2CO3 in DMF was found to have a value of (1.3 +/- 0.2) x 10(-7) mol cm(-2) s(-1) at 100 degrees C, and the activation energy for the dissolution was 44.2 +/- 0.4 kJ mol(-1) over the temperature range of 70-100 degrees C studied.
The frequent simple gross redox reaction notation, where A is a solution species and Bads is its adsorbed reduction product, is misleading in its simplicity. It may represent a concerted reaction, i.e., a true elementary step in which the electron transfer (ET) to the solution species A is concerted with the creation of the electrode-species bond(s) in Bads. Conversely, the reaction may be a composite one involving two sequential steps. One such two-step mechanism may be termed as a CadsEads sequence where Cads stands for a pre-adsorption elementary step and Eads for the follow-up ET. Alternatively, the two steps may proceed in the reverse order, leading to an EsolnCads mechanism in which A is reduced into B, both being solution species, followed by adsorption of B onto the electrode.The present theoretical investigations show that a different panel of voltammetric behaviours may be produced by each mechanism. Hence, rationalising voltammetric peak positions and shapes in terms of surface or potential effects is challenging and may be completely erroneous if the wrong mechanistic sequence is considered. © 2014 Taylor & Francis.
The convective-diffusion equation describing mass transport in a hydrodynamic tubular electrode is given and is solved using the alternating direction implicit method. Models which simplify the full mass transport equation are described and are compared with the approximate Levich theory for flow in a tubular electrode. Limiting currents are compared and the conditions for which the radial and axial diffusion terms are important are determined. © 2004 Elsevier B.V. All rights reserved.
We propose a theoretical method for reconstructing the shape of a hydrodynamic flow profile occurring locally within a rectangular microfluidic channel based on experimental currents measured at double microband electrodes embedded in one channel wall and operating in the generator-collector regime. The ranges of geometrical and flow parameters providing best conditions for the flow profile determination are indicated. The solution of convection-diffusion equation (direct problem) is achieved through the application of the specifically designed conformal mapping of spatial coordinates and an exponentially expanding time grid for obtaining accurate concentration and current distributions. The inverse problem (the problem of flow profile determination) is approached using a variational formulation whose solution is obtained by the Ritz’s method. The method may be extended for any number of electrodes in the channel and/or different operating regimes of the system (e.g. generator-generator). © 2007 Elsevier Ltd. All rights reserved.
The oxidative dimerisation reaction of N,N-dimethyl-p-toluidine (DMT) in acetonitrile was studied by cyclic voltammetry using a range of concentrations from 1 to 70 mM at microdisk electrodes of radii 10, 12.5, and 50 μm using scan rates from 20 mV s-1 to 6 V s-1. Theory was developed to simulate the following mechanism using the ADI method and comparison made to experimental voltammograms.The mechanism was found to be consistent with the observed data under all experimental conditions employed. The following rate constants were inferred: k1=(5.9±2.2)×105 cm3 mol-1 s-1, k-1≈0, and k2=(9.2±3.8)×108 cm3 mol-1 s-1. © 2002 Elsevier Science B.V. All rights reserved.
The complex problem of diffusion-reaction inside of bundles of nanopores assembled into microspherical particles is investigated theoretically based on the numerical solutions of the physicochemical equations that describe the kinetics and the thermodynamics of the phenomena taking place. These theoretical results enable the delineation of the main factors that control the system reactivity and examination of their thermodynamic and kinetic effects to afford quantitative predictions for the optimization of the particles’ dimensional characteristics for a targeted application. The validity and usefulness of the theoretical approach disclosed here are established by the presentation of the complete analysis of the performance of thiol-functionalized microspheres aimed for sequestration of Hg(II) ions from solutions to be remediated. This allows the comparison of the microparticles’ performance at two different pH (2 and 4) and the rationalization of the observed changes in terms of the main microscopic parameters that define the system.
The application of fast-scan linear sweep voltammetry methods to a high-speed wall-tube electrode (HWTE) is reported. Experiments are reported for the oxidation of N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPD) in propylene carbonate solution containing 0.10 M tetrabutylammonium perchlorate for a 24 μm radius platinum microdisk electrode housed within the HWTE using a range of scan rates from 200 to 3000 V s-1 and average flow jet velocities from 0.24 to 9.4 m s-1 (corresponding to volume flow rates of 0.003-0.12 cm3 s-1, and centre-line jet velocities from 0.5 to 18.9 m s-1). Linear sweep voltammograms (LSVs) are analysed for a simple electron transfer under high volume flow rates, by curve fitting. Analysis of the transient LSVs yielded values for k0, α, and Ef0 for TMPD of (5.9±2.4)×10-2 cm s-1, 0.46±0.08 and 0.217±0.019 V (vs. Ag), respectively. This is in good agreement with independent experiments conducted using the high-speed channel electrode which yielded the results: k0=(6.3±0.4)×10-2 cm s-1, α=0.52±0.01, and Ef0=0.234±0.005 V (vs. Ag). The range of applicability of this method for measuring k0 was also investigated and compared with existing channel electrode techniques. © 2003 Elsevier B.V. All rights reserved.
The electrochemically initiated reaction of p-phenylenediamines with sulfide in aqueous media is well documented. We now report the adaptation of this chemistry into nonaqueous media. This is critically appraised as a means of detecting sulfide. The electrochemically initiated reaction of N,N-diethyl-p-phenylenediamine with sulfide is shown at both macro- and platinum microdisk electrodes with quantitative detection of sulfide produced by means of the enhanced currents observed upon its addition. The linear detection range for sulfide is dependent on the concentration of N,N-diethyl-p-phenylenediamine present with a linear range from 28-3290 μM and a limit of detection of 22 μM achievable. This represents a large increase compared to that found previously in aqueous media and offers the prospect of more ready applications in high temperature systems.
The theory for homogeneous ECrevE, ECrevCE and ECECE mechanisms at macroscopically large channel electrodes is derived. The photoelectrochemical reductions of para-bromonitrobenzene and 2,4-dibromonitrobenzene in acetonitrile solutions and at macroscopic platinum channel electrodes are studied using irradiation at 330 and 470 nm, corresponding to absorption bands in the corresponding radical anions. The former compound is shown to follow a homogeneous ECrevCE pathway in acetonitrile solutions containing tetrabutylammonium-based supporting electrolytes; by changing the supporting electrolyte to a salt of the tetramethylammonium cation, the mechanism is changed qualitatively and follows an ECEE pathway. The photoelectrochemical reduction of 2,4-dibromonitrobenzene in acetonitrile solution containing supporting electrolytes derived from the tetrabutylammonium cation is shown to follow an overall ECECE mechanism, with both chemical steps being chemically-reversible, and with the loss of the ortho-bromo in the dark. In the presence of chloride supporting electrolytes, it is shown that light-induced rupture of a C-Br bond occurs reversibly with the competing formation of a C-Cl bond. Unoptimised bulk photoelectrosynthesis indicates that some halogen exchange occurs, demonstrating the viability of a novel approach to halex reactions. © 2002 Elsevier Science B.V. All rights reserved.
In this paper we present a mathematical model for the surface-controlled dissolution of cylindrical solid particles. This is employed to interpret experimental data published previously for the dissolution of potassium bicarbonate in dimethylformamide at elevated temperatures. Significant kinetic differences in assuming cylindrical rather than spherical shapes are reported with the former representing a closer approximation to the true shape of the particles as revealed by scanning electron microscopy. From the fits of experimental data to the cylindrical model for the surface-controlled dissolution, the dissolution rate constant, k, for the dissolution of KHCO(3) in DMF was found to be (9.6 +/- 1.6) x 10(-9) mol cm(-2) s(-1) at 100 degrees C, and the activation energy for the dissolution was 34.5 kJ mol(-1) over the temperature range of 60-100 degrees C. Comparison between cylindrical and spherical dissolution theory highlights the importance of considering the particle shapes for realistic modeling of surface-controlled dissolution kinetics.
The High Speed Channel Electrode is used to precisely measure the standard electrochemical rate constants under steady state conditions for the oxidation of substituted ferrocenes in alkyl cyanide solvents. First, the oxidation of ferrocene itself is kinetically analysed in the solvents: acetonitrile, propionitrile, butyronitrile and valeronitrile. Second, the oxidation kinetics of the ferrocene derivatives: ferrocene carboxylic acid, decamethyl ferrocene, bis(diphenylphosphino) ferrocene, ferrocene carboxaldehyde, α-hydroxyethyl ferrocene, t-butyl ferrocene, dimethyl aminomethyl ferrocene, dimethyl ferrocene and n-butyl ferrocene are measured in acetonitrile for comparison to ferrocene. The rate of electron transfer in the different alkyl cyanides is shown to depend on the longitudinal dielectric relaxation times of the solvents in accordance with solvent dynamic theory, whilst the electron transfer rates of the different ferrocene derivatives in acetonitrile are shown to depend on the hydrodynamic radii of the molecules. In both cases, a measured k0 = 1.0 ± 0.2 cm s-1 for ferrocene oxidation in acetonitrile shows an excellent fit to the theoretical dependences. © 2005 Elsevier B.V. All rights reserved.
The application of the high-speed microband channel electrode to the study of the heterogeneous electron transfer kinetics of the oxidation of some N-substituted phenylenediamines is described. Experiments to investigate the standard electrochemical rate constant, k0, of the oxidation of 1,4-phenylenediamine (PPD), N,N-dimethyl-1,4-phenylenediamine (DMPD), and N,N-diethyl-1,4-phenylene-diamine (DEPD) in acetonitrile solutions containing 0.10 M tetrabutylammonium perchlorate (TBAP) are reported for 2.5 and 5 μm platinum microband electrodes using a range of centre-line velocities from 12 to 25 m s-1. The measured values of k0 for PPD, DMPD, and DEPD are 0.84±0.16, 3.15±0.30 and 1.64±0.25 cm s-1, respectively. The respective formal oxidation potentials are also found to be 0.287±0.002, 0.245±0.001, and 0.208±0.003 V (all measured vs. Ag). Experiments are also presented using "fast scan" cyclic voltammetry to obtain measurements of the heterogeneous rate constants for PPD, DMPD and DEPD to compare between the steady-state channel electrode and the ‘established’ transient methodologies. Scan rates in the range 102-104 V s-1 were used to measure peak separations with the resulting k0 values of 0.51±0.05, 1.89±0.10, and 1.28±0.20 cm s-1, respectively. The use of steady-state voltammetry obviates the need for capacitative corrections, perhaps suggesting a greater reliability in the resulting data. © 2002 Elsevier Science B.V. All rights reserved.
The traditional textbook view of adsorptive features in electrochemistry, viz., involving a bell-shaped wave prior to or after the diffusional wave is certainly right but concerns a very limited series of conditions in which adsorption kinetics are too slow vs. the scan rate. Laviron examined the converse situations in which extremely rapid adsorption kinetics make the voltammetric process follow the classical diffusional behavior though the effective electrochemical reactions proceed via adsorbed species. Thanks to a new simulation approach, implemented in KISSA©, the present work examines intermediate situations which could not be investigated since they do not lead to analytical formulations. Besides allowing investigating the transition between the two above limiting behaviors, it is established that during such transitions voltammograms display CE-type behaviors in which the rates of the pseudo-antecedent chemical steps feature those of adsorption. An electroactive adsorbed species is indeed involved in a dynamic steady state between its adsorption and its consumption by electron transfer at the electrode surface so that its current is independent of the potential. This is a general situation presently overlooked in electrochemical theories. For example, the same CE-like behavior is also shown to occur during electropolymerization of redox polymers though it now is hidden under the monomer diffusion-controlled wave. © 2012 Elsevier B.V. All rights reserved.
Steady-state voltammetry is used to measure the heterogeneous electron-transfer rates for the reduction of quinones to determine the dependence of k0 on molecular size, according to Marcus theory. This dependence is then used to predict the electron-transfer rate constants of related quinones, and the predictions are compared to experimental measurements.
ECL reaction mechanisms involved in all experimental approaches are characterized by a sequence of very fast second-order reactions taking place in the diffusion layer between transient species that, according to the exact process of ECL generation, may be generated at a single electrode, a pair of electrodes (anode-cathode), or through electron transfers between activated reactants and co-reactants. These extremely fast second-order reactions generate the extremely short-lived electronically excited state species, S*, which are deactivated through a rapid first-order emissive decay giving rise to the emission of light. These crucial species are therefore always generated and consumed within a narrow layer of solution of very small size compared to diffusion layers. Moreover, they are present within this layer at vanishingly small concentrations. This creates kinetic situations termed “reaction fronts” exact numerical treatment of which is almost impossible by classical numerical approaches. This chapter presents a series of numerical approaches based on the concepts developed by the authors to circumvent these severe complications and allow a precise and fast simulation of ECL reaction mechanisms. These are illustrated taking advantage of experimental examples featuring each main method of ECL generation.
© 2003 IEEE.The mathematical package for simulation of kinetic mechanisms and fitting experimental data (KinFitSim) is presented. The KinFitSim package consists of two major parts. The first part is the kinetic simulator (KS) featuring an advanced kinetic mechanism compilation algorithm and an automatic simulation procedure itself, and the second part is the fitting simulator (FS) for automated fitting of the simulated response of a chemical system with the experimental one.
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.This theoretical work examines some basic and overlooked concepts that sustain the efficiency of electrochemiluminescence (ECL) generated by co-reactant systems such as the, nowadays classical, alkyl amine/transition metal(II) complex systems. Changes in the ECL intensities emitted by these systems are much more dependent on the relative diffusivities of the two co-reactants than on the range of thermodynamic and kinetic rate constants that are possible to explore and vary. In particular, decreasing the diffusion coefficients of the metal complexes species (e.g. by adequate redox or photochemically inert large substituents) relative to that of the amine co-reactant leads to a great enhancement in the ECL intensity of the first ECL wave, namely, that observed at the level of the amine oxidation peak. Though investigated by using simulations based on the thermodynamic and kinetic constants of the most common tri-n-propylamine/Ru(bpy)32+ system (bpy=2,2’-bipyridine), the conclusions of this work are more general.
In this manuscript, the two novel numerical methods for stiff ODEs-the Almost Runge-Kutta (ARK) and Aluffi-Pentini (AP)-are applied to the solution of two large stiff ODE systems that model the Belousov-Zhabotinskii reaction and air pollution process. The efficiency and accuracy of the two methods are compared revealing advantages of the ARK method especially for multidimensional ODE systems. © 2007 Springer Science+Business Media, LLC.
The application of steady-state and fast-scan linear sweep voltammetry to a high-speed wall-tube electrode (HWTE) is reported in different solvents to investigate the response of the HWTE over a wide range of Reynolds’ numbers (Re). Experiments are reported for the oxidation of N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPD) in propylene carbonate (PC), water, butyronitrile (BN), acetonitrile (AN), and acetonitrile-water mixture solutions containing 0.10 M supporting electrolyte for a 24 μm radius platinum microdisk electrode housed within the HWTE using a range of average flow jet velocities from 0.03 to 19.8 m s-1 (corresponding to volume flow rates of 0.003-0.25 cm3 s-1 and center-line jet velocities from 0.05 to 39.5 m s-1). Fast scan linear sweep voltammetry is presented for the oxidation of TMPD in PC and of 9,10-diphenylanthracene (DPA) in AN. Theoretical results are derived using finite element methods for both one- and two-dimensional mass transport models. It is found that, for solvents with a kinematic viscosity above ca. 7.5 × 10-3 cm2 s-1, the hydrodynamic behavior for Re < 2000 is as expected with current responses in accordance with those predicted for a laminar, parabolic inlet flow profile. In low viscosity solvents, where Re < 2000, currents are lower than expected, indicating a departure from laminar flow in practical cells even at low Re. The HWTE is compared to the channel electrode in the light of the experimental results, theoretical limits of electron-transfer rate detectable, and conclusions drawn that the channel electrode is more reliable for kinetic measurements.
In this work we describe the theory and 2D simulation of ion separation and focusing in a new concept of microfluidic separation device. The principle of the method of ion focusing is classical in the sense that it consists in opposing a hydrodynamic transport ensured by the solution flow to an electrophoretic driving force so that any ionic sample results poised within the microchannel at the point where the two forces equilibrate. The originality of the concept investigated here relies on the fact that thanks to the use of an ion-conducting membrane of variable thickness in electrical contact with the channel the electrophoretic force is varied continuously all along the channel length. Similarly, changing the geometric shape of the membrane allows a facile optimization of the device separation and focusing properties. © Vilnius University, 2012.
The electrochemical reduction of oxygen in two different room-temperature ionic liquids, 1-ethyl-3-methylimidazolium bis((trifluoromethyl)sulfonyl)imide ([EMIM][N(Tf)2]) and hexyltriethylammonium bis-((trifluoromethyl)sulfonyl)imide ([N6222][N(Tf)2]) was investigated by cyclic voltammetry at a gold microdisk electrode. Chronoamperometric measurements were made to determine the diffusion coefficient, D, and concentration, c, of the electroactive oxygen dissolved in the ionic liquid by fitting experimental transients to the Aoki model. [Aoki, K.; et al. J. Electroanal. Chem. 1981, 122, 19]. A theory and simulation designed for cyclic voltammetry at microdisk electrodes was then employed to determine the diffusion coefficient of the electrogenerated superoxide species, O2·-, as well as compute theoretical voltammograms to confirm the values of D and c for neutral oxygen obtained from the transients. As expected, the diffusion coefficient of the superoxide species was found to be smaller than that of the oxygen in both ionic liquids. The diffusion coefficients of O2 and O2·- in [N6222][N(Tf)2], however, differ by more than a factor of 30 (DO2 = 1.48 × 10-10 m2 s-1, DO2·- = 4.66 × 10-12 m2 s-1), whereas they fall within the same order of magnitude in [EMIM]-[N(Tf)2] (DO2 = 7.3 × 10-10 m2 s-1, DO2·- = 2.7 × 10-10 m2 s-1). This difference in [N6222][N(Tf)2] causes pronounced asymmetry in the concentration distributions of oxygen and superoxide, resulting in significant differences in the heights of the forward and back peaks in the cyclic voltammograms for the reduction of oxygen. This observation is most likely a result of the higher viscosity of [N6222][N(Tf)2] in comparison to [EMIM][N(Tf)2], due to the structural differences in cationic component.
A surface-controlled dissolution of cylindrical solid particles model is applied to potassium carbonate, sodium bicarbonate and sodium carbonate in dimethylformamide at elevated temperatures. Previously published data for the dissolution of potassium carbonate is interpreted assuming a cylindrical rather than a spherical shape of the particles, the former representing a closer approximation to the true shape of the particles as revealed by scanning electron microscopy. The dissolution kinetics of sodium carbonate and sodium bicarbonate in dimethylformamide at 100 degrees C were investigated via monitoring of the deprotonation of 2-cyanophenol with dissolved solid to form the 2-cyanophenolate anion that was detected with UV-visible spectroscopy. From fitting of experimental results to theory, the dissolution rate constant, k, for the dissolutions of potassium carbonate, sodium bicarbonate and sodium carbonate in dimethylformamide at 100 degrees C were found to have the values of (1.0 +/- 0.1) x 10(-7) mol cm(-2) s(-1), (5.5 +/- 0.3) x 10(-9) mol cm(-2) s(-1) and (9.7 +/- 0.8) x 10(-9) mol cm(-2) s(-1), respectively.
A general mathematical model for partially blocked electrodes (PBEs) taking into account the finite volume and height of the blocking units has been developed and solved using numerical simulation. The electrochemical behaviour of this model was studied and compared to the common ’flat block’ approach in which the blocks are assumed to have zero height to highlight the main differences and explain their origins. © 2004 Elsevier B.V. All rights reserved.
The validity and accuracy of our new numerical approach implemented in KISSA-1D software when applied to a theoretical study of different types of electrochemiluminescence (ECL) is established by comparison with existing analytical solutions and others specifically derived in this work, as well as with independent numerical solutions obtained by using commercial software. The efficiency and comprehensiveness of this approach are illustrated by using a representative series of published ECL reaction schemes taken as typical case studies when ECL is generated by a single electrode under amperometric or voltammetric conditions, even when rate constants used in the simulations far exceed any of their realistic experimental limits.
Herein, we propose a method for reconstructing any plausible macroscopic hydrodynamic flow profile occurring locally within a rectangular microfluidic channel. The method is based on experimental currents measured at single or double microband electrodes embedded in one channel wall. A perfectly adequate quasiconformal mapping of spatial coordinates introduced in our previous work [Electrochem. Commun. 2004, 6, 1123] and an exponentially expanding time grid, initially proposed [J. Electroanal. Chem. 2003, 557, 75] in conjunction with the solution of the corresponding variational problem approached by the Ritz method are used for the numerical reconstruction of flow profiles. Herein, the concept of the method is presented and developed theoretically and its validity is tested on the basis of the use of pseudoexperimental currents emulated by simulation of the diffusion-convection problem in a channel flow cell, to which a random Gaussian current noise is added. The flow profiles reconstructed by our method compare successfully with those introduced a priori into the simulations, even when these include significant distortions compared with either classical Poiseuille or electro-osmotic flows.
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.
Breaking of symmetry is often required in biology in order to produce a specific function. In this work we address the problem of protein diffusion over a spherical vesicle surface towards one pole of the vesicle in order to produce ultimately an active protein cluster performing a specific biological function. Such a process is, for example, prerequisite for the assembling of proteins which then cooperatively catalyze the polymerization of actin monomers to sustain the growth of actin tails as occurs in natural vesicles such as those contained in Xenopus eggs. By this process such vesicles may propel themselves within the cell by the principle of action-reaction. In this work the physicochemical treatment of diffusion of large biomolecules within a cellular membrane is extended to encompass the case when proteins may be transiently poised by corral-like structures partitioning the membrane as has been recently documented in the literature. In such case the exchange of proteins between adjacent corrals occurs by energy-gated transitions instead of classical Brownian motion, yet the present analysis shows that long-range movements of the biomolecules may still be described by a classical diffusion law though the diffusion coefficient has then a different physical meaning. Such a model explains why otherwise classical diffusion of proteins may give rise to too small diffusion coefficients compared to predictions based on the protein dimension. This model is implemented to examine the rate of proteins clustering at one pole of a spherical vesicle and its outcome is discussed in relevance to the mechanism of actin comet tails growth.
The cyclic voltammetric response of electrodes modified with catalytically reactive blocks is simulated using finite difference methods. The responses of three different models using various block geometries are studied. The results are used to determine kinetic parameters of coupled liquid|liquid interfacial reactions. First, we examine the liquid-liquid reaction between aqueous vitamin B12S and pure trans-dibromocyclohexane (DBCH) microdroplets immobilized on a basal plane pyrolytic graphite (bppg) surface, immersed in an aqueous solution of vitamin B12. Second, cyclic voltammetry on electrodes modified with microdroplets of DBCH diluted in dodecane is employed to determine the apparent bimolecular interfacial rate constant for the initial step in the DBCH(oil)/B12S(aq) reaction. The results are compared with a previous SECM/ITIES study of a similar reaction. © 2005 Elsevier B.V. All rights reserved.
Modern renewable energy sources have a great disadvantage of being intermittent. Harvesting solar energy directly using photovoltaic panels is one of the most promising renewable energy technologies. While this allows electricity generation during daytime when the sky is clear, at night there is no production at all and it is greatly diminished in cloudy or rainy conditions. Recently a concept of all-weather solar cells was proposed by Q. Tang et al. (Angew. Chem. Int. Ed. 55(17) (2016) 5243-5246) in which a solar panel was covered with a layer of graphene. This allows collecting energy from falling raindrops containing dissolved salts through charging and discharging of an electrical double layer at the water-graphene interface, which acts as a pseudocapacitor. Although this setup allows harvesting both direct solar radiation and some of the kinetic energy of falling rain drops, the output is low for realistic salt concentrations while the graphene layer diminishes the solar-to-electric conversion rate. In this work, we propose a different approach to the same problem. Instead of relying on a sufficient concentration of salts in rain water, we propose to convert the mechanical energy delivered by drop impacts directly into electrical energy by supporting a thin-layer solar panel with an array of piezo crystals. The advantage of this setup is that the solar-to-electric performance of such a panel is not affected by the added piezoelectric support. However, only a fraction of the kinetic energy of the falling rain drops can be converted due to the energy dissipation within the material of the thin-layer panel. We have conducted detailed modelling of kinetic energy harvesting process from the drop impact and spreading to the dissipation of mechanical strain through the panel to the generation of piezoelectric potential. The results illustrate the viability of this concept, but they are still to be confirmed experimentally and require an economic feasibility analysis to be performed.
A general mathematical model for electrosorption under planar and microdisk diffusion conditions has been developed and solved using numerical simulation. This model allowed us to explore the electrochemical behaviour of such systems and to highlight some of their main properties. © 2004 Elsevier B.V. All rights reserved.
Laser flash photolysis (LFP, 308 nm) of endo-10-halo-10’-N,N-dimethylcarboxamidetricyclo[18.104.22.168]-deca-2,4-diene (1Cl and 1F) releases indan and halocarbene amide (2Cl and 2F). Although the carbenes are not UV-vis active, they react rapidly with pyridine to form ylides (4Cl, 4F), which are readily detected in LFP experiments (lambda(max) = 450 nm). Dioxane decreases the observed rate of carbene reaction with pyridine in CF(2)ClCFCl(2). Small amounts of THF decrease the observed rate of reaction of carbene 2F with pyridine but increase the rate of reaction of carbene 2Cl with pyridine. LFP (266 nm) of dienes 1Cl and 1F in CF(2)ClCFCl(2) with IR detection produces carbenes 2Cl and 2F with carbonyl vibrations at 1635 and 1650 cm(-1), respectively. In dioxane or THF solvent, LFP produces the corresponding ether ylides (5Cl, 5F) by capture of carbenes 2Cl and 2F. The ylides have broad carbonyl vibrations between 1560 and 1610 cm(-1). The addition of a small amount of dioxane in CFCl(2)CF(2)Cl extends the lifetime of the carbene. This observation, together with the ether-induced retardation of the rates of carbene capture by tetramethylethylene and pyridine, is evidence for solvation of the carbene by dioxane.
An expression which allowed Tafel analysis of electrochemically reversible systems at hydrodynamic electrodes such as the rotating disk was investigated. It was found that under hydrodynamic conditions the mass transport of species occured within a diffusion layer of thickness δD adjacent to the surface of the electrode. Newton’s method was used for solution of generally non-linear algebric equations arising from discretization. The results show that Tafel analysis of the simulated waveshapes reveals the deviation from the prediced values.
A new simulation algorithm is presented for describing the dynamics of diffusion reactions at the most common microelectrode 1D (planar, cylindrical, spherical) and 2D geometries (band, disk) for electrochemical mechanisms of any complexity and involving fast homogeneous reactions of any kind. A series of typical electrochemical mechanisms that create the most severe simulation difficulties is used to establish the exceptional performance and accuracy of this algorithm, which stem from the combination of (quasi)conformal transformation of space and a new method for auto-adaptive grid compression.
The reduction of oxygen in the presence of carbon dioxide has been investigated by cyclic voltammetry at a gold microdisk electrode in the two room-temperature ionic liquids 1-ethyl-3-methylimidazolium bis- (trifluoromethylsulfonyl)imide ([EMIM][N(Tf)2]) and hexyltriethylammonium bis(trifluoromethylsulfonyl)imide ([N6222] [N(Tf)2]). With increasing levels of CO2, cyclic voltammetry shows an increase in the reductive wave and diminishing of the oxidative wave, indicating that the generated superoxide readily reacts with carbon dioxide, The kinetics of this reaction are investigated in both ionic liquids. The reaction was found to proceed via a DISP1 type mechanism in [EMIM][N(Tf)2], with an overall second-order rate constant of 1.4 ± 0.4 × 103 M-1 s-1. An ECE or DISP1 mechanism was determined to be the most likely pathway for the reaction in [N6222][N(Tf)2], with an overall second-order rate constant of 1.72 ± 0.45 × 103 M-1 s -1.
In order to successfully model an electrochemical reaction mechanism one must ensure that all the equations, including initial conditions, satisfy the pertinent thermodynamic and kinetic relationships. Failure to do so may lead to invalid results even if they are mathematically correct. This fact has been previously emphasized (Luo, W.; Feldberg, S. W.; Rudolph, M. J. Electroanal. Chem. 1994, 368, 109 - 113; Rudolph, M. Digital Simulation in Electrochemistry. In Physical Electrochemistry; Rubenstein, I., Ed.; Marcel Dekker: New York, 1995; Chapter 3) and existing computer software for electrochemical simulations, such as DigiSim (Rudolph, M.; Reddy, D. P.; Feldberg, S. W. Anal. Chem. 1994, 66, 589A; http://www.basinc.com/products/ec/digisim/), offer the option of enforcing the so-called "pre-equilibration" which evaluates thermodynamic concentrations of all species prior to beginning a voltammetric scan. Although this approach allows setting consistent thermodynamic values it may result in a nonrealistic initial concentrations set because it corresponds to the whole solution status at infinite time for infinite kinetic constants. However, the perturbation created by the working electrode poised at its rest potential is necessarily limited by the size of the electrode, reaction kinetics, and duration of the rest period. Furthermore, natural convection limits even more the importance of the perturbation. This is analyzed theoretically through comparison of simulation results by DigiSim and KISSA-1D software for certain common electrochemical mechanisms in order to illustrate the importance of correct prediction of initial concentrations.
The physicochemical process of nitric oxide (NO degrees ) release from an active neuron is modelled based on the results obtained experimentally in independent series of experiments reported elsewhere in which the NO degrees release elicited by patch-clamping a single neuron (stellate neuron from cerebellum area) is monitored by an ultramicroelectrode introduced into a slice of living rat’s brain. This process is believed to be central to brain behaviour by coupling neuronal activity with the blood supply to active areas of the living brain through precise control of NO degrees -mediated dilatation of blood capillary vessels. This work, based on the conformal mapping approach initially proposed in a previous work, aims to model the overall physicochemical and diffusional processes giving rise to the release of NO degrees by a neuron and during its collection at an electrode sensor. Fitting simulated currents to experimental ones published previously yields indeed the gross kinetic information which represents the overall neuron activation and defines the instant value of the concentration of NO degrees at the neuron surface. This allows reconstructing the NO degrees fluxes around the neuron body as they would have been in the absence of the electrode sensor. This permits one to appreciate how far NO degrees is released by the neuron at concentrations which greatly exceed their basal values. The success of this procedure is exemplified using a set of three experimental data reported elsewhere.
A new software package, ’KINFITSIM’, for fitting and simulating kinetic data is presented. The main goals of the KINFITSIM package are to obtain the best-fit parameters-rate constants, amplitudes and others-to a user specified chemical mechanism, plots of the calculated and experimental absorbance versus time, and a report to the user with the results. The KINFITSIM package can be used in either chemical research or for educational purposes.
This article extends our previous works (Amatore, C.; Oleinick, A.; Svir, I. Anal. Chem. 2008, 80, 7947-7956; 7957-7963.) about the effects of resistive and capacitive distortions in voltammetry at disk microelectrodes. The particular case of voltammetry of a self-assembled monolayer carrying one redox site per molecule is investigated here. In addition, the effect of an uneven distribution of the effective electrochemical potential on the possibility of electron hopping (EH) contributions is examined. An original model of EH has been developed considering both diffusion-type (i.e., related to concentration gradients) and migration-type (i.e., imposed by an uneven distribution of the electrical potential due to an ohmic drop and capacitance charging) contributions. This predicts that as soon as the system performs out of thermodynamic equilibrium and provided that the EH rate constants are not too small the system tends to re-establish its out-of-equilibrium state through EH. Hence, EH somewhat tries to compensate the voltammetric distortions that would be enforced by the uneven distribution of the electrochemical driving force incurred by the system due to an ohmic drop and capacitive charging. However, this rigorous analysis established that, though EH may be effective under specific circumstances particularly near the electrode edge, its overall influence on voltammetric waves remains negligible for any realistic experimental situation.
In this work, a novel mathematical model for the description of the osmotic behavior during the cryopreservation of human Mesenchymal Stem Cells (hMSCs) from Umbilical Cord Blood (UCB) is proposed. In cryopreservation, the two-parameter formalism of perfect osmometer behavior is typically adopted and preferred due to its simplicity: cell volume osmotic excursions are described as due only to the passive trans-membrane transport of water and permeant solutes such as cryoprotectant agents (CPAs); intracellular solutes, responsible of the isotonic osmolality, are assumed to be impermeant. The application of the two-parameter model fails to capture the osmotic response of hMSCs whenever a swelling phase is involved, as demonstrated by the authors. To overcome this limitation, the imperfect osmometer behavior of hMSCs is successfully modelled herein by improving the two-parameter formalism through the coupling of osmosis with cell mechanics and cell membrane Surface Area Regulation (SAR): now the transmembrane permeation of solutes (ions/salt) during the swelling phase through the temporary opening of mechanosensitive channels is allowed. This way cells can reach an equilibrium volume different from the initial isotonic one, when isotonic conditions are re-established after contact with impermeant or permeant solutes, such as sucrose or dimethyl-sulfoxide (DMSO), respectively. The sequential best-fit procedure adopted to adjust model parameters is reported herein along with model validation through full predictions.
In this work, we illustrate two approaches to the simulation of surface diffusion over a sphere coupled with the formation of a cluster by reactive particles as a paradigm of a wide variety of problems occurring in many areas of nanosciences and biology. The problem is treated using a Brownian motion approach and a numerical solution of the corresponding continuous Fick’s laws of diffusion. While being computationally more expensive, the Brownian motion approach allows one to consider a wider range of situations, particularly those corresponding to relatively high concentrations of diffusing particles and the ensuing problem of particle overlap when they are ascribed finite sizes.
A realistic theoretical model describing the outcome of confocal microscopic imaging of electrochemiluminescence (ECL) light emission is derived for a two parallel band microelectrodes assembly operated under steady state. The model takes into account the experimental distortions ensuing from a) the specific finite shape of the sampling volume in confocal microscopy, b) the light arising directly from out-of-focus area but transmitted through the microscope diaphragm or c) transmitted after reflection from the polished platinum band electrodes. The model is based on a detailed optical, physico-mathematical and numerical analysis of the problem at hand, and on simulations of the concentration distribution of the species giving rise to the ECL generation. Its outcome allows the reconstruction of the real spatial distribution of ECL light emission based on the confocal microscopy measurements upon correcting for the effect of experimental distortions using numerical fitting procedure.