Initial Reactant Concentrations Research Articles

Synthesis and characterization of α-MnO 2 nanowires: Self-assembly and phase transformation to β-MnO 2 microcrystals

Single-crystalline α-MnO 2 nanowires and β-MnO 2 microcrystals have been synthesized by a hydrothermal method. The products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM) and simultaneous thermogravimetry–mass spectrometry (TG–MS). The effects of varying the hydrothermal reaction times and the initial reactant concentrations on the morphology and crystalline structures of the final products have been investigated. It was found that at high reactant concentrations, the α-MnO 2 nanowires formed initially became self-assembled into bundles and subsequently underwent a phase transformation to β-MnO 2 microcrystals on prolonging the hydrothermal treatment. At low reactant concentrations, however, β-MnO 2 microcrystals were formed even after short hydrothermal reaction times. A possible mechanism for the oriented attachment of the α-MnO 2 nanowires and subsequent phase transformation to β-MnO 2 microcrystals has been suggested.

Kinetic modelling of non-enzymatic browning of apple juice concentrates differing in water activity under isothermal and dynamic heating conditions

The kinetics of non-enzymatic browning in apple juice concentrates were investigated. The effect of a w (in the range of 0.74–0.99) and/or reactant concentration on brown pigment formation was monitored under isothermal heat treatment at four temperatures (60, 70, 80 and 90 °C) in apple juice solutions having either the same or different concentrations of reactant solutes. The extent of the Maillard reaction was evaluated by spectrophotometric measurements at 420 nm ( A 420). The absorbance–time curves were fitted to five different kinetic models (zero and first order, weibull, logistic and the parabolic model) and estimates of browning rate constants and other model parameters were obtained. Regression analysis revealed that the logistic model was the most appropriate for describing browning in apple juice. The initial reactant concentration, but not water activity, had a significant effect on the colour change of apple juice. The processing temperature also had a strong impact on browning kinetics. Secondary models, expressing the dependence of the best fitted primary model parameters on temperature and a w, were further developed and validated by comparing the predicted model parameters with the values observed in independent isothermal experiments. Finally, the derived model was further evaluated against the observed browning responses of apple juice under dynamic heating conditions, underlining the applicability of the developed model as a practical prediction tool for the study of non-enzymatic browning.

A kinetic mechanism for predicting secondary organic aerosol formation from toluene oxidation in the presence of NO x and natural sunlight

A kinetic mechanism to predict secondary organic aerosol (SOA) formation from the photo-oxidation of toluene was developed. Aerosol phase chemistry that includes nucleation, gas–particle partitioning and particle-phase reactions as well as the gas-phase chemistry of toluene and its degradation products were represented. The mechanism was evaluated against experimental data obtained from the University of North Carolina (UNC) 270 m 3 dual outdoor aerosol smog chamber facility. The model adequately simulates the decay of toluene, the nitric oxide (NO) to nitrogen dioxide (NO 2) conversion and ozone formation. It also provides a reasonable prediction of SOA production under different conditions that range from 15 to 300 μg m −3. Speciation of simulated aerosol material shows that up to 70% of the aerosol mass comes from oligomers and polymers depending on initial reactant concentrations. The dominant particle-phase species predicted by the mechanism are glyoxal oligomers, ketene oligomers from the photolysis of the toluene OH reaction product 2-methyl-2,4-hexadienedial, organic nitrates, methyl nitro-phenol analogues, C7 organic peroxides, acylperoxy nitrates and for the low-concentration experiments, unsaturated hydroxy nitro acids.

Mechanistic approach to enhancement of the yield of a sonochemical reaction

AbstractIn this study, a mechanistic approach has been taken to enhance yield of a sonochemical reaction. Formation of highly reactive free radicals due to the transient collapse of cavitation bubbles is the primary mechanism of a sonochemical reaction. A physical (reduction in dissolved gas concentration) and a chemical (increasing the reactant concentration) technique is used for enhancing yield of a sonochemical reaction using those techniques, which influence the phenomenon of radical formation by the cavitation bubbles. A bubble dynamics model is used for explaining the sonochemical phenomena. In a degassed medium, the ultrasound wave undergoes lesser attenuation; moreover, equilibrium size of a bubble shrinks due to rectified diffusion. Because of this, a bubble undergoes more violent collapse, resulting in greater production of radicals that give higher yield. On the other hand, increasing the initial reactant concentration shows an adverse effect on the sonochemical yield. This is ascribed to reduction in water vapor flux in the bubble due to reduction of vapor pressure of the medium. This study, therefore, demonstrates as how macroscopic manifestation (the sonochemical yield) of the microscopic phenomena (transient collapse of cavitation bubble) is a complicated function of several physical processes. The results of this study shed light on the complex and multifaceted physical mechanism of a sonochemical reaction, which may be useful in maximization of yield of other sonochemical systems. © 2007 American Institute of Chemical Engineers AIChE J, 2007

Enzymatic hydrolysis of penicillin G to 6-aminopenicillanic acid in cloud point system with discrete countercurrent experiment

A set of flasks was utilized to simulate a discrete countercurrent bioreactor. An enzymatic hydrolysis of penicillin G to 6-aminopenicillanic acid with immobilized penicillin acylase in a cloud point system was carried out with the discrete countercurrent operational mode. Byproduct phenylacetic acid is effectively transported with the auxiliary phase (coacervate phase), while product 6-aminopenicillanic acid and reactant penicillin G are effectively transported with the aqueous phase, where the enzymatic hydrolysis of penicillin G takes place. The experimentally results show that the product concentration and product yield are improved from the batch operational mode of about 38 mM and 50% to the corresponding countercurrent operational mode of above 50 mM and 65%, respectively. However, experimentally examination of the effect of operational parameters, such as feed point location, volume ratio of auxiliary phase to aqueous phase and initial reactant concentration, indicates that the achievement of a high product concentration and a high product yield at the same time is difficult by a simple utilization of the continuous countercurrent operational mode. This can be contributed to the facts that the enzymatic hydrolysis favors at a relatively higher pH, but the extraction of PAA only takes place at a relatively lower pH.

Separation parameters in the reaction between volatile halides and hydrogen

Equations for calculating the separation coefficient and factor in reactions of volatile halides with hydrogen from the equilibrium constants of the corresponding reactions and initial reactant concentrations were obtained. The concept of the limiting impurity concentration was introduced. The values specified were calculated for the reduction of volatile chlorides with hydrogen.

Kinetics of struvite to newberyite transformation in the precipitation system MgCl 2–NH 4H 2PO 4–NaOH–H 2O

The influence of the initial reactant concentrations on the composition of the solid phases formed in the precipitation system MgCl 2–NH 4H 2PO 4–NaOH–H 2O was investigated. The precipitation diagram constructed shows the approximate concentration regions within which struvite, newberyite, and their mixtures exist at 25 °C and an aging time of 60 min. It was found that immediately after mixing the reactant solutions, struvite (MgNH 4PO 4·6H 2O) precipitated in nearly the whole concentration area, while newberyite (MgHPO 4·3H 2O) appeared mostly within the region of the excess of magnesium concentration. It was also found that after aging time of 60 min the precipitation domain of struvite alone is much broader than that of newberyite or the domain of their coexistence, and shows that struvite is more abundant in the systems in which the initial concentration of ammonium phosphate is higher than that of magnesium. The kinetics of struvite to newberyite transformation (conversion) was systematically studied under the conditions of different initial reactant concentrations and different initial pH in the systems in which a mixture of both phases precipitated spontaneously. The struvite to newberyite conversion period was found to be strongly related to the ratio of initial supersaturations, S N/ S S, rather than to the any particular physical quantity that can describe and predict the behavior of the precipitation system. Experimental data suggest a solution-mediated process as a most possible transformation mechanism. Along with a continuous monitoring of the changes in the liquid phase, the content of struvite in the solid phase was estimated by means of a Fourier transform infrared (FT-IR) method, developed for this particular precipitation system.

Dopaquinone redox exchange with dihydroxyindole and dihydroxyindole carboxylic acid

A pulse radiolytic investigation has been conducted to establish whether a redox reaction takes place between dopaquinone and 5,6-dihydroxyindole (DHI) and its 2-carboxylic acid (DHICA) and to measure the rate constants of the interactions. To obviate possible confounding reactions, such as nucleophilic addition, the method employed to generate dopaquinone used the dibromide radical anion acting on dopa to form the semiquinone which rapidly disproportionates to dopaquinone. In the presence of DHI the corresponding indole-5,6-quinone (and/or tautomers) was also formed directly but, by judicious selection of suitable relative concentrations of initial reactants, we were able to detect the formation of additional indolequinone from the redox exchange reaction of DHI with dopaquinone which exhibited a linear dependency on the concentration of DHI. Computer simulation of the experimental time profiles of the absorption changes showed that, under the conditions chosen, redox exchange does proceed but not quite to completion, a forward rate constant of 1.4 x 10(6)/M/s being obtained. This is in the same range as the rate constants previously established for reactions of dopaquinone with cyclodopa and cysteinyldopa. In similar experiments carried out with DHICA, the reaction more obviously does not go to completion and is much slower, k (forward) =1.6 x 10(5)/M/s. We conclude that, in the eumelanogenic pathway, DHI oxidation may take place by redox exchange with dopaquinone, although such a reaction is likely to be less efficient for DHICA.

The regiochemistry of the NO3‐promoted gas phase nitration of toluene and phenol with NO2

AbstractProduct studies, kinetic isotope effect measurements, linear free energy relationships and calculations have been used to study the NO3‐promoted nitration of toluene and phenol in presence of NO2. The competition between of a rate‐determining hydrogen abstraction of NO3 to give the benzyl radical and HNO3 and of a rate‐determining addition‐elimination pathway has been evaluated. The preference for the nitration in ortho and para position of toluene and for the ortho position of phenol is suggested to derive from the concerted loss of nitric acid from an intermediate cyclohexadiene.A kinetic analysis suggests reasons for the dependence of the ratio ortho:para‐nitration from the initial concentration of reactants. Copyright © 2006 John Wiley & Sons, Ltd.

Modelling Thrombin Generation in Human Ovarian Follicular Fluid

A mathematical model is constructed to study thrombin production in human ovarian follicular fluid. The model results show that the amount of thrombin that can be produced in ovarian follicular fluid is much lower than that in blood plasma, failing to reach the level required for fibrin formation, and thereby supporting the hypothesis that in follicular fluid thrombin functions to initiate cellular activities via intracellular signalling receptors. It is also concluded that the absence of the amplification pathway to thrombin production in follicular fluid is a major factor in restricting the amount of thrombin that can be produced. Titration of the initial concentrations of the various reactants in the model lead to predictions for the amount of tissue factor and phospholipid that is required to maintain thrombin production in the follicle, as well as to the conclusion that tissue factor pathway inhibitor has little effect on the time that thrombin generation is sustained. Numerical experiments to determine the effect of factor V, which is at a much reduced level in follicular fluid compared to plasma, and thrombomodulin, illustrate the importance for further experimental work to determine values for several parameters that have yet to be reported in the literature.

A systematic investigation of the rate laws valid in intracellular environments

Recently there has been significant interest in deducing the form of the rate laws for chemical reactions occurring in the intracellular environment. This environment is typically characterized by low-dimensionality and a high macromolecular content; this leads to a spatial heterogeneity not typical of the well stirred in vitro environments. For this reason, the classical law of mass action has been presumed to be invalid for modeling intracellular reactions. Using lattice-gas automata models, it has recently been postulated [H. Berry, Monte Carlo simulations of enzyme reactions in two dimensions: Fractal kinetics and spatial segregation, Biophys. J. 83 (2002) 1891–1901; S. Schnell, T.E. Turner, Reaction kinetics in intracellular environments with macromolecular crowding: simulations and rate laws, Prog. Biophys. Mol. Biol. 85 (2004) 235–260] that the reaction kinetics is fractal-like. In this article we systematically investigate for the first time how the rate laws describing intracellular reactions vary as a function of: the geometry and size of the intracellular surface on which the reactions occur, the mobility of the macromolecules responsible for the crowding effects, the initial reactant concentrations and the probability of reaction between two reactant molecules. We also compare the rate laws valid in heterogeneous environments in which there is an underlying spatial lattice, for example crystalline alloys, with the rate laws valid in heterogeneous environments where there is no such natural lattice, for example in intracellular environments. Our simulations indicate that: (i) in intracellular environments both fractal kinetics and mass action can be valid, the major determinant being the probability of reaction, (ii) the geometry and size of the intracellular surface on which reactions are occurring does not significantly affect the rate law, (iii) there are considerable differences between the rate laws valid in heterogeneous non-living structures such as crystals and those valid in intracellular environments. Deviations from mass action are less pronounced in intracellular environments than in a crystalline material of similar heterogeneity.

Microemulsion-assisted precipitation of particles: Experimental and model-based process analysis

The technical-scale production of nanoparticles with tailored properties is of particular interest due to the increasing demand from industry. This study provides the basis for the scale-up of the synthesis route in water-in-oil (w/o)-microemulsion droplets by a detailed experimental and theoretical analysis of the precipitation of calcium carbonate (CaCO 3) and barium sulfate (BaSO 4). The used w/o-microemulsion system consists of water, cyclohexane and the inexpensive technical surfactant Marlipal O13/40. This system was extensively characterized with and without reactants in order to identify stable microemulsion regions. Precipitation was initialized by mixing two microemulsions, each containing one of the reactants, in semi-batch operating mode. Extensive parameter studies helped to identify control parameters, which allow the tailored synthesis of nanoparticles with certain properties. For CaCO 3 a dependence of the morphology on the reaction time can be observed. For BaSO 4 it was found out that the final particle size can be controlled by varying the initial reactant concentration ratio. A deterministic population balance equation (PBE) model and a stochastic Monte-Carlo (MC) model were derived for the BaSO 4 system. The calculation time of the discretized PBE model compared with the MC model is much faster, but the MC simulations show a better agreement of experimental and simulation data, especially the particle size distribution (PSD).

Instabilized Fluid Flow at Interface of Chemical Reaction in Liquid Phase

Evolution of chemically reacting flow has been studied experimentally using Hele-Shaw Cell.Neutralizaftion reaction of HCl+NaOH with different pH indicators is adapted for the chemical reaction. Low density liquid (HCl) and high density liquid (NaOH with different pH indicator) pour into upper layer in the cell and lower layer in the cell, respectively. In HCl+NaOH with 10% Bromothymol Blue (BTB) system, although some small proturbulences form and disappear repeatedly at a reaction front, flow patterns are stable and the front moves almost parallel to a horizontal line with time elapses. The higher initial concentration of reactants hastens the chemical reaction. On the other hand, in HCl+NaOH with 10% phenolphthalein (PP) system, unstable flow patterns are visualized, the chemical reaction is higher than that in the HCl+NaOH with BTB system. By the chemical reaction, temperature increases locally in the cell and density of product is smaller than that of HCl. It seems that the local temperature difference and density difference bring on the unstable flow patterns.

D232 毛細管内の粘性の異なる二液体の混和性置換に関する反応性流れ(反応性流れの基礎と応用3)

It is known that the spike is formed in the miscible displacement in capillary tubes under a certain condition. In the present study, miscible displacement with chemical reaction in vertical capillary tubes is investigated experimentally in the condition where the spike is formed. The effect of reactant concentration on the reaction characteristics is focused on. We have found that the initial reactant concentration ratio included in the more-and less-viscous liquids normalized by the stoichiometric ratio of the chemical reaction, ψv, significantly influences the distribution of the products. We propose that the difference in the product distribution is caused by both two factors ; the idiosyncratic shape (spike) of boundary region and the difference between the diffusion coefficient of interdiffusion of miscible two liquids and that of reactants in less viscous liquid.

Variation in stoichiometry and mechanism of some redox systems

Synopsis 1. Introduction 2. Reduction of (S 2 O 2- 8 + Ag 1 ) system in acid solutions. Competence of intermediate SO - 4 to react with H 2 O/substrate/Ag I leading to a variation in the stoichiometry and mechanism. (a) Oxidation of (b) Oxidation of Ce' (c) Oxidation of hyponitrous acid (H 2 N 2 O 2 ) 3. Reactions of hydrazine in acid solutions. Competence of hydrazyl radical (N 2 H 3 ) to get dimerized and decomposed/oxidised leading to a variation in the stoichiometry and mechanism. (a) Oxidation by Ce (b) Oxidation by TI III 4. Reactions of hydroxylamine. Ratio of the initial reactant-concentrations deciding the stoichiometry and mechanism. Intermediates sensitive to light. (a) Oxidation by peroxomonophosphoric acid (H 3 PO 5 ) in acid solutions (b) Oxidation by TI III in acid solutions (c) Oxidation by Fe III in buffered solutions 5. Reactions of hyponitrous acid. Competence of intermediatee oxyhyponitrous acid (H2N 2 O 3 ) and HNO 2 to decompose/get oxidised leading to a variation in the stoichiometry and mechanism. (a) Oxidation by TI III in acid solutions (b) Oxidation by Ce in acid solutions (c) Oxidation by iodine in buffered solutions (d) Oxidation by Fe(CN) 3- 6 in alkaline solutions 6. Complicated analysis in a simple system of TI III -Sb III in acid perchlorate solutions for 1 : 1 stoichiometry.

Modeling and Simulation of Titania Synthesis in Two-dimensional Methane–air Flames

The formation and growth of titanium dioxide nanoparticles in two-dimensional, non-premixed methane–air flames is investigated via direct numerical simulation. The simulations are performed by capturing the spatio-temporal evolution of the fluid, chemical, and particle fields. The fluid is described by the conservation of mass, momentum, and energy equations; species transport is augmented by the effects of methane–air combustion and the oxidation of titanium tetrachloride; and a nodal approximation to the general dynamic equation is used to represent the effects of nucleation, condensation and coagulation. Simulations are performed for two initial reactant concentration levels, 20% and 30% titanium tetrachloride by mass. The evolution of the temperature, chemical and particle fields as a function of space and size are presented. Results indicate that particle formation and growth is mixing limited in this study and the mean particle diameter and geometric standard deviation increase as the concentration level of the initial reactants increases. In general, high geometric standard deviations correspond to a large particle sizes.

Optimization of High‐Yield Biological Synthesis of Single‐Crystalline Gold Nanoplates.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Optimization of High-Yield Biological Synthesis of Single-Crystalline Gold Nanoplates

In this work, single-crystalline gold nanoplates were obtained by reducing aqueous chloroauric acid solution with the extract of Sargassum sp. (brown seaweed) at room temperature. The gold nanoplates so obtained were characterized by UV-vis spectroscopy, X-ray diffraction, atomic force microscopy, and transmission electron microscopy. The formation of gold nanoplates was found to depend on a number of environmental factors, such as the time taken to age the seaweed extract, pH of the reaction medium, reaction temperature, reaction time, and initial reactant concentrations. The size of the gold nanoplates could be controlled to between 200 and 800 nm by manipulating the initial reactant concentrations. The yield of the flat gold nanocrystals relative to the total number of nanoparticles formed was as high as approximately 80-90%.

A kinetic model of quantitative real‐time polymerase chain reaction

Real-time polymerase chain reaction (PCR) is one of the most sensitive and accurate methods for quantifying transcript levels especially for those expressed at low abundance. The selective amplification of target DNA over multiple cycles allows its initial concentration to be determined. The amplification rate is a complex interplay of the operating conditions, initial reactant concentrations, and reaction rate constants. Experimentally, the compounded effect of all factors is quantified in terms of an effective efficiency, which is estimated by curve fitting to the amplification data. We present a comprehensive model of PCR to study the effect of various reactant concentrations on the amplification efficiency. The model is used to calculate the kinetic progression of the target DNA concentration with cycle number under conditions when different species are stoichiometrically or kinetically limiting. The reaction efficiency remains constant for the initial cycles. As the primer concentration becomes limiting, the efficiency is marked by a gradual decrease. This is in contrast to a steep decline under nucleotide limiting conditions. Under some conditions, commonly used experimentally, increasing primer concentration has the adverse effect of reducing the final amplified template concentration. This phenomenon seen at times experimentally is explained by the simulation results under rate limiting enzyme concentrations. Primer dimer formation is shown to significantly affect the reaction rates, effective efficiency, and the estimated initial concentrations. This model, by describing the interplay of the many operating variables, will be a useful tool in designing PCR conditions and evaluating its results.

Modeling and simulation of titania formation and growth in temporal mixing layers

Direct numerical simulation of the formation and growth of titanium dioxide nanoparticles in two-dimensional, temporal mixing layers is performed. Titania is produced by the gas-phase hydrolysis of titanium tetrachloride at a temperature of 300 K. The flow field is obtained by solving the Navier–Stokes equations and evolution of the particle field is obtained via a nodal method. The approach approximates the aerosol general dynamic equation and is advantageous in that there are no a priori assumptions regarding the nature of the particle size distribution. The formation and growth of particles up to and including 128 nm in diameter in iso-thermal flows are studied. Simulations are performed for two initial reactant concentration levels. The evolution of the particle field as a function of space, time and size is presented. Results indicate that particle formation and growth is mixing limited and particles grow faster with increasing initial reactant levels. Additionally, the results indicate rather large geometric standard deviations that vary significantly throughout the computational domain.