Reactant Concentration Ratio Research Articles

Synthesis and size control of CaHPO 4 particles in a two-liquid phase micro-mixing process

CaHPO 4 particles were synthesized in oil-in-water (o/w) system and water-in-oil (w/o) system with membrane micro-dispersion mixing technique. The particles of CaHPO 4 were prepared using two kinds of micro-filtration plate membrane modules with different active areas as a dispersion medium. The working system of calcium acetate aqueous solution/n-butanol with phosphoric acid as the transferred solute was applied. The influence of the reactant concentration and the operation conditions on the performance of this process was investigated. Particle morphology was observed by scanning electron microscope, and the particle-size distributions were measured by the photo-imaging method. The results showed that the CaHPO 4 particles with narrow size distribution could be prepared using membrane dispersion method in the o/w or w/o system. The average sizes were affected by the module structure, residence time, reactant concentration and phase ratio of the aqueous phase to the oil phase. The particles with the size ranging from 0.2 to 0.6 μm were prepared in the module with 12.5 mm 2 of active membrane area. When the active area was 400 mm 2, the particles were in range of 10–20 μm.

Preliminary Study on the Characterization and Preparation of Quaternary Ammonium Membrane

Four kinds of anion exchange membranes were prepared by polymerizing the chloromethylstyrene cross-linking reagent of divinylbenzene with a coating on polypropylene nonwoven fabric using the paste method, and then immobilized with trimethyl (triethyl, tripropyl, or tributyl) amine in the quaternarization process. The properties of these membranes were characterized in terms of thickness, ion-exchange capacity, imbibed water content, and degree of swelling. The kinetics of allylation of phenol was investigated in the organic and alkali aqueous solutions in the vertical and horizontal membrane reactors. The operating conditions, such as agitation rate, reaction temperature, immobilization temperature, seven kinds of catalysts (four lab-produced and two commercial membranes, one resin), and concentration ratio of reactants were conducted to achieve the optimum condition. The methylamine membrane as a phase-transfer catalyst was the best choice catalyst for the higher yield of allyl phenyl ether among six ammonium membranes. However, the butylamine membrane showed the best reactivity among the six membranes.

Kinetics and Mechanism of the Reaction of Cysteine and Hydrogen Peroxide in Aqueous Solution

The oxidation of thiol-containing small molecules, peptides, and proteins in the presence of peroxides is of increasing biological and pharmaceutical interest. Although such reactions have been widely studied there does not appear to be a consensus in the literature as to the reaction products formed under various conditions, the reaction stoichiometry, and the reaction mechanisms that may be involved. This study examines the reaction kinetics of cysteine (CSH) with hydrogen peroxide (H2O2) in aqueous buffers (in the absence of metal ions) over a wide range of pH (pH 4–13) and at varying ratios of initial reactant concentrations to explore the range of conditions in which a two-step nucleophilic model describes the kinetics. The disappearance of CSH and H2O2 and appearance of cystine (CSSC) versus time were monitored by reverse-phase high-performance liquid chromatography (HPLC). The effects of oxygen, metal ions (Cu2+), pH (4–13), ionic strength, buffer concentration, and temperature were evaluated. Data obtained at [H2O2]0/[CSH]0 ratios from 0.01–2.3 demonstrate that the reaction of CSH with H2O2 in the absence of metal ions is quantitatively consistent with a two-step nucleophilic reaction mechanism involving rate-determining nucleophilic attack of thiolate anion on the unionized H2O2 to generate cysteine sulfenic acid (CSOH) as an intermediate. Second-order rate constants for both reaction steps were generated through model fitting. At [H2O2]0/[CSH]0 > 10, the % CSSC formed as a product of the reaction declines due to the increased importance of alternative competing pathways for consumption of CSOH. A thorough understanding of the mechanism in aqueous solution will provide valuable background information for current studies aimed at elucidating the influence of such factors on thiol oxidation in solid-state formulations.

Sonochemical and microwave-assisted synthesis of linked single-crystalline ZnO rods

Both sonochemical and microwave-assisted solution-phase routes to the fabrication of linked ZnO rods have been developed. The two methods do not need templates, seeds or surfactants and are suitable for large-scale production. Linked ZnO rods with various morphologies such as bipods, tripods, tetrapods have been prepared. ZnO rods grow along [0 0 0 1] direction and the formation of linked ZnO rods may be controlled by the oriented attachment mechanism. The influence of the irradiation time, reactant concentration and reactant molar ratio on the formation of linked ZnO rods were investigated. X-ray powder diffraction (XRD), transmission electron microscopy (TEM), selected-area electron diffraction (SEAD) and energy dispersive spectroscopy (EDS) were used for the characterization of the as-prepared products. Photoluminescence (PL) spectrum of linked ZnO rods was also investigated at room temperature.

Voltammetric investigation of formation of complexes in low ionic strength environments: Theoretical description and modeling

Digital simulation of steady-state voltammetry of diffusion–migration systems with the reactant involved in the complexation reaction has been carried out using a FIFD-related algorithm. It has been found, that in the case of neutral ligands, the shift of the halfwave potential caused by the formation of the complex has the same magnitude in solutions with and without supporting electrolyte. The measurement data can thus be interpreted using the standard methods for voltammetric analysis, like, e.g. De Ford–Hume scheme. For charged ligands it is possible to find a value for the ligand:reactant concentration ratio above which the migrational effects can be neglected. This allows one to use classic methods also to the systems with charged ligands. The simulation results show that the limiting ligand:reactant concentration ratio mentioned above differs for different charge numbers of species and for different complex formation constants, but for the majority of the considered systems this ratio should be not less than 5 in the total absence of supporting electrolyte. The proposed approach has been applied to the experimental results obtained for Tl + with 18-crown-6 ligand in water without addition of supporting electrolyte.

Optimal methods for estimating kinetic isotope effects from different forms of the Rayleigh distillation equation

The interpretation of stable isotope values hinges on precise, accurate estimates of kinetic isotope effects (α), which are equal to 1k/ 2k, where 1k and 2k are the reaction rate constants for the two isotopes. Kinetic isotope effects are commonly determined by monitoring the reactant concentration ( C) and isotope ratio ( R) relative to their initial values ( C 1 and R 1, respectively). Values of α are estimated from the C and R values by using the Rayleigh distillation equation (RDE). (A) (R/R 1)=(C/C 1) 1/α−1 We conducted simulation experiments to evaluate the precision of the many different published approaches of estimating α from linearized versions of the RDE and reached the following conclusions: (a) kinetic isotope effects estimated from the slope of the line ln( R) vs. ln( C) were accurate and precise; (b) regressing ln( R i/ R j) on ln( C i/ C j), where all i datapoints were compared to all preceding datapoints j resulted in inflated 95% confidence intervals; (c) forcing the regression of ln( R/ R 1) on ln( C/ C 1) through the origin resulted in 95% confidence intervals for α that covered the true value less than 90% of the time; and (d) regression methods that compensate for errors in both x and y values need to be used with caution. When combining multiple datasets, values of α were sensitive to the form of the equation and the level of error. If all datasets had the same level of error, the optimal estimate of α was achieved by a linear regression with dummy variables. However, when the three datasets had different levels of error, the optimal estimate of α and much narrower 95% confidence intervals were obtained by using the Pitman estimator. Our study demonstrates that some of the other methods jeopardize the accuracy and precision of empirically determined kinetic isotope effects, thus confounding the interpretation of stable isotope values in the environment.

Formation of zinc sulfide nanorods and nanoparticles in ternary W/O microemulsions

A simple, easy approach to the synthesis of semiconductor ZnS nanorods and nanoparticles exhibiting versatile morphology-formation ability is reported. Water-insoluble zinc sulfide nanocrystals were synthesized in ternary water-in-oil (w/o) microemulsion systems stabilized by either nonionic or, in contrast, cationic surfactant. Products were visualized by transmission electron microscopy (TEM) and identified by energy-dispersive X-ray spectroscopy (EDAX); electron diffraction (ED) was also performed for individual nanorods. With varying molar ratios of water to surfactant ( ω 0) in solution, hence changing droplet sizes of water pool of microemulsions consequently, several morphologies with different size spans were encountered in the formation of ZnS, such as nanorods and spherical or ellipsoidal particles. Meanwhile, product morphology was also found to be sensitive to the absolute reactant concentration and concentration ratio of [Zn 2+] to [S 2−], the incubation time, and the ambient temperature. A schematic mechanism for the formation of ZnS nanocrystals and their morphological diversity is described. It is feasible to extend this method to the synthesis of one-dimensional nanocrystals of other semiconductors, given suitable formulae of microemulsions and other appropriate reaction conditions.

A kinetic investigation of CVD mullite coatings on Si-based ceramics

Chemically vapor deposited (CVD) mullite coatings were deposited on SiC substrates using the AlCl 3,SiCl 4,CO 2,H 2 system in a hot-walled vertical type reactor. An empirical kinetic model was developed indicating that an intermediate gaseous reaction was kinetically significant to the growth rate of mullite on SiC. The kinetically limiting equation was defined between a temperature range of 673–1327 K and 30–400 mbar where temperature was defined as a function of reactor position. The growth rate and crystal structure of mullite were dependent upon temperature, pressure, reactant concentration, and reactant ratios.

Chlorination of N-acetyltyrosine with HOCl, chloramines, and myeloperoxidase-hydrogen peroxide-chloride system.

N-acetyl-L-tyrosine (N-acTyr), with the alpha amine residue blocked by acetylation, can mimic the reactivity of exposed tyrosyl residues incorporated into polypeptides. In this study chlorination of N-acTyr residue at positions 3 and 5 in reactions with NaOCl, chloramines and the myeloperoxidase (MPO)-H2O2-Cl- chlorinating system were invesigated. The reaction of N-acTyr with HOCl/OCl- depends on the reactant concentration ratio employed. At the OCl-/N-acTyr (molar) ratio 1:4 and pH 5.0 the chlorination reaction yield is about 96% and 3-chlorotyrosine is the predominant reaction product. At the OCl-/N-acTyr molar ratio 1:1.1 both 3-chlorotyrosine and 3,5-dichlorotyrosine are formed. The yield of tyrosine chlorination depends also on pH, amounting to 100% at pH 5.5, 91% at pH 4.5 and 66% at pH 3.0. Replacing HOCl/OCl- by leucine/chloramine or alanine/chloramine in the reaction system, at pH 4.5 and 7.4, produces trace amount of 3-chlorotyrosine with the reaction yield of about 2% only. Employing the MPO-H2O2-Cl- chlorinating system at pH 5.4, production of a small amount of N-acTyr 3-chloroderivative was observed, but the reaction yield was low due to the rapid inactivation of MPO in the reaction system. The study results indicate that direct chlorination of tyrosyl residues which are not incorporated into the polypeptide structure occurs with excess HOCl/OCl- in acidic media. Due to the inability of the myeloperoxidase-H2O2-Cl- system to produce high enough HOCl concentrations, the MPO-mediated tyrosyl residue chlorination is not effective. Semistable amino-acid chloramines also appeared not effective as chlorine donors in direct tyrosyl chlorination.

Polymerization of ?-caprolactone with maleic anhydride: Synthesis and characterization

High-molecular-weight polymers of ϵ-caprolactone (CL) and maleic anhydride (MA) with anhydride group content of about 1% wt have been synthesized and studied. The polymerization reaction was carried out in bulk under nitrogen atmosphere. Stannous octoate (Sn(oct)2), and 2,2'-azobisisobutyronitrile (AIBN) were used as a catalyst and an initiator, respectively. A two-level design of experiments was used to study the effect of various conditions on the characteristics of the copolymer. Reaction time, temperature, and concentration ratio of various reactants (two monomers, monomer to catalyst, and monomer to initiator) were the independent variables used, and the dependent variables included the molecular weight and the anhydride content in the polymer. Nuclear magnetic resonance (NMR) studies indicate that the succinic anhydride units were incorporated individually either to the polymer chain end or backbone. Anhydride content in the polymer and gel permeation chromatograph (GPC) studies indicate that the maleic anhydride acts as the true initiating species rather than as a comonomer in the system. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 3189–3194, 2000

Micromixing in a single‐feed semi‐batch precipitation process

AbstractA mixing‐precipitation model (based on engulfment, the E‐model by Baldyga and Rohani, 1987) is applied to a single‐feed semi‐batch precipitation process to describe the mixing effects on the final particle‐size distribution (PSD) and particle morphology. The model describes mixing on two scales: the microscale (at which the flow directly affects nucleation and growth phenomena in the mixed‐precipitation (MP) zone) and a macroscale (which controls the environment for the MP‐zone). Application of the model is limited to 1≪Sc<4,000 and slow feed addition rates. When the model was applied to experimental results of BaSO4 precipitation, satisfactory agreement between predicted and observed results was obtained. Experiments were carried out in a single‐feed 18‐L Rushton reactor, whereby the effects of various operating conditions such as feed rate, intensity of mixing, mean initial reactant concentration and reactant volume ratio on barium sulfate precipitation process were investigated.

Experiments on a reacting plume—1. Conventional concentration statistics

Abstract Experiments have been carried out under laboratory conditions to study turbulent mixing with chemical reaction. Such flows are common in the environment but there are few high-resolution measurements of them under controlled conditions. The reaction NO + O 3 → NO 2 + O 2 + 200 kJ (mol) −1 (in the absence of UV radiation) has been used by introducing a non-buoyant reactive plume of NO into a turbulent grid flow doped with O 3. The experiments have been conducted over a wide range of conditions by varying the Damko¨hler number (the ratio of the flow timescale to that of the chemical reaction) by a factor of 6 and varying the ratio of initial reactant concentrations by a factor of 30. The data are presented in two park. Part 1 (present paper) presents conventional concentration statistics. Part 2 presents concentration statistics conditionally averaged by a mixing parameter and discusses their usefulness for investigating; the reactive behaviour of the plume. Specially developed chemiluminescent analysers are used to measure NO and O 3 concentrations with high-frequency response and a spatial resolution of four Kolmogorov scales (in this flow). The mixing field of the plume is studied using conserved scalar theory and is compared. to that obtained in other non-reactive flows. Results for the means, variances and joint statistics of the reactive species are presented and compared with frozen and equilibrium limits obtained from the conserved scalar theory for reacting flows. Statistics for NO show no significant trend with Damko¨hler number while those for O 3 and NO 2 have increased reaction at higher Damko¨hler number. The reactive scalar means are bounded by the frozen and equilibrium limits as they should be according to conserved scalar theory. Other reactive scalar statistics are not necessarily bounded by the limits. A simple relation between moments of O 3 and N0 2 is found in the experimental results and is confirmed by conserved scalar theory. Contribution to the mean reaction rate from the reactant covariance and product of means is compared and it is found that both are significant under the present conditions. Intensity of segregation varies over a wide range and is sensitive to initial reactant concentrations.

Experiments on a reacting plume—2. Conditional concentration statistics

Abstract A detailed study is made of reactive scalar statistics conditional on the value of a conserved scalar in a non-buoyant reactive plume of NO in a turbulent grid flow doped with O 3 using the reaction NO + O 3 → NO 2 + O 2 + 200 kJ mol −1 (in the absence of UV radiation). Use of such conditional statistics makes it possible to separate the effects of turbulent mixing and chemical reaction. Simultaneous measurements of two reactive scalars and two orthogonal turbulent velocities are made using high-resolution instruments. Radial profiles of scalar statistics are taken through the plume. It is found that the reactive scalar means conditional on a conserved scalar lack significant dependence on radial position and that conditional variance around the conditional mean is relatively small. This is in agreement with similar findings in completely different types of turbulent reacting flows. Detailed experiments have been conducted at a single location where the ratio of initial reactant concentrations has been varied by a factor of 30 and the Damko¨hler number (the ratio of the flow timescale to that of the chemical reaction) by a factor of 6, but without measurable effect on the flow field. The conditional reactive scalar results are presented and their usefulness for investigating the reactive behaviour of the plume is demonstrated. Use is made of the stoichiometiic distance, the location where the unconditional mean conserved scalar is equal to the initial concentration of ambient reactant. Preliminary results of the conditional moment closure (CMC) model of Klimenko (1990) and Bilger (1993) are compared with conditional experimental data. The CMC model leads to significant simplification of the equations governing variation of reactive scalars in the flow. The equations are solved numerically with low computational requirements. Weighting of the conditional values with the conserved scalar probability density function yields the conventional reactive scalar means. CMC modelling results are shown to be in good agreement with the experimental data. Limiting cases (equilibrium and reaction dominated) for the reactive scalar statistics are derived from the conserved scalar statistics and form bounds on both the experimental and CMC predictions as expected from theoretical considerations. Application of the CMC model to atmospheric flows is discussed. A good estimate of the covariance is also made from the conditional reactive scalar means.

Dendritic growth of lead molybdate in aqueous solutions

Crystals of PbMoO4, point group 4/m, grown by chemical reactions in nitric acid solutions under ambient conditions, easily lose morphological stability, the dendritic forms exhibiting strong dependence on the reactant concentration ratio and pH of solutions. Lead in excess of molybdate ions in solutions produces either 1D branches along [001], or 3D tree-like forms branching along [001], [100] and [010]. Excess molybdate results in 2D “snowflakes” growing in the (001) plane. At pH higher than about 2 the branches extend along [100] and [010], whereas below 2 they follow the [310] (respectively [130]) directions. Transitions from stationary to dendritic growth forms and vice versa have also been observed permitting one to trace back the dendrites to their original stationary forms. The dendritic growth of this low symmetry crystal provides fine examples of the effect of anisotropy of interface kinetics on morphology.

Low Temperature Hydrothermal Synthesis of Nanophase BaTiO3 and BaFe12O19 Powders

ABSTRACTNanocrystalline powders with an average particle size of 50 nm has been synthesized in two materials systems under hydrothermal conditions below 100°C. Processing variables, such as temperature, concentration and molar ratio of reactants and reaction time were optimized to obtain particles of reduced size and stoichiometric compositions. Hydrothermal reaction takes place between Ba(OH)2 solution and titanium/iron precursors in sealed polyethylene bottles in the BaTiO3 and BaFe12O19 systems, respectively. While crystalline BaTiO3 forms relatively fast within a few hours, formation of fully crystalline and stoichiometric BaFei20i9 require considerably longer reaction times up to several weeks and strongly dependent on the Ba:Fe ratio of the precursors. The structural and compositional evaluation of the nanophase powders were studied by XRD and TEM techniques.

Anodic oxidation of hydroquinone in sulfite solution

Abstract The anodic oxidation of hydroquinone in the presence of sulfite was investigated by chronoamperometry at a rotating gold electrode and chronopotentiometry at a stationary gold electrode. The experiments show that sulfite is strongly adsorbed on the electrode inhibiting the hydroquinone oxidation, and at the same time is consumed by sulfonation of the anodic reaction product, p -benzoquinone. The resulting effects can be explained by the respective rates of both processes, depending on the experimental conditions (rotational speed, pH, concentration ratio of reactants).

Low-Knudsen-number transport and deposition

We present our model for transport and deposition in features for situations in which intermolecular collisions dominate the species transport. Species transport in this ‘‘low’’-Knudsen-number regime is modeled using continuum diffusion. Our model allows both homogeneous reactions and heterogeneous (surface) reactions. We use the Galerkin finite element method to estimate reactant species concentration profiles for infinite trenches with arbitrary cross section, for which two-dimensional profile evolution is appropriate. We simulate deposition processes using a solution domain which includes the trench as well as a specified region above the surface of the wafer. As an example application, we present results for the deposition of tungsten using the hydrogen reduction of tungsten hexafluoride in trenches of rectangular cross section with initial aspect ratio 4. The model predicts that step coverage increases with decreasing temperature and increasing tungsten hexafluoride partial pressure, while keeping hydrogen partial pressure and the height of the solution domain constant. These trends are in agreement with experimental observations for tungsten deposition processes. Trends in film conformality with deposition conditions are explained in terms of the ‘‘step coverage modulus’’ and reactant concentration ratios for given initial trench dimensions. The step coverage modulus is the ratio of a characteristic deposition rate to a characteristic transport rate.

Chemiluminescence studies of the reaction between oxygen O(3P) atoms and methyl iodide

The reaction between O( 3 P) atoms and CH 3 I was studied in a fast discharge-flow system. A threshold behavior with respect to the ratio of initial reactant concentrations in the chemiluminescence due to I 2 (B 2 Π 0u+ →X 1 Σ g + ) was observed. The reactions in Chart 1 were considered significant in our interpretation. Fitting of temporal profiles produced an order of magnitude for k 1 of 10 10 cm 3 mol -1 s -1 at room temperature

Preparation and surface‐active properties of the sodium soaps, mono‐ and diethanolamides and diol and triol sulfates of cycloaliphatic C21 Di‐ and C22 Tricarboxylic acids

Cycloaliphatic C21 di‐ and C22 tricarboxylic acids were prepared by the Diels‐Alder reaction of dehydrated castor oil (DCO) fatty acids (containing 48% conjugated and 42% nonconjugated dienes) with acrylic and fumaric acids, respectively. The reaction temperature, time, catalyst concentration, and mole ratio of reactants were varied to get maximum yields of the di‐ and tricarboxylic acids. The unreacted DCO fatty acids were removed from the products by partition between aqueous methanol and n‐hexane. The products were converted to methyl esters and characterized as the substituted cyclohexene derivatives before and after dehydrogenation with Pd/C in xylene, hydrogenation with Pd/C in decalin and oxidation with periodate‐permanganate reagent and by mass and proton nuclear magnetic resonance spectrometry. The acids were converted to sodium soaps, mono‐ and diethanolamides and the corresponding diol and triol sulfates, and the resulting products were evaluated for their surface‐active properties. The sodium soaps showed better calcium tolerance and poorer foaming power than sodium oleate. The diethanolamides were inferior to lauroyl diethanolamide in wetting and emulsifying power. The monoethanolamides were better than the respective diethanolamides and lauroyl diethanolamide in wetting property. The diol and triol sulfates were poorer in wetting and emulsifying ability and better in calcium tolerance than sodium lauryl sulfate.

Aluminum speciation studies in biological fluids. Part 2. Quantitative investigation of aluminum-citrate complexes and appraisal of their potential significance in vivo

Because of the recent implications of aluminium in the pathogenesis of various disease states, its in vivo chemistry has been receiving growing attention from bioinorganic chemists over the last few years. In this context, the elucidation of the main factors that govern aluminum bioavailability constitutes an urgent objective. Clearly, prevention measures require that mechanisms of aluminum absorption be definitely characterized, whereas specific sequestering agents are needed to detoxify patients with high-aluminum-body burdens. In particular, speciation studies are necessary to discriminate among the chemical forms under which aluminum predominates in vivo. Low molecular weight (LMW) species, which are the most active in terms of bioavailability, cannot be assessed by analytical techniques, and so computer simulations must be used. In recent clinical studies as well as in preliminary simulations dealing with aluminum distribution in blood plasma, citrate has been recognized as the most important LMW ligand of aluminum. The present paper thus reports a quantitative investigation of aluminum-citrate equilibria, carried out at 37° C in NaCl 0.15 mol dm −3 in accordance with the experimental protocol defined in our previous study on aluminum hydrolysis. The ML, MLH, ML 2, M 3L 3H −4, M 2L 2H −2, ML 2H −1 species have been characterized over the whole physiological pH range using as large reactant concentration ratios as possible. Corresponding formation constants have then been used to investigate the role of citrate towards aluminum bioavailability. Blood plasma simulations reveal that citrate can promote aluminum urinary excretion, which substantiates recent clinical observations made on mice. However, the higher plasma aluminum concentrations are, the less effective citrate is to be expected. Gastrointestinal simulations confirm that the electrically neutral ML complex does represent an important risk of aluminum absorption in the upper region of the gastrointestinal tract at usual therapeutic doses. At moderate- and low-aluminum concentrations, citrate is also capable of dissolving the aluminum trihydroxide precipitate, which may combine with the capacity of other ligands to complex Al 3+ into absorbable complexes at less acidic pH.