Home About us Contact
|
Home About us Contact | ||
|
|
||
Solvent Composition (solvent + composition)
Selected AbstractsComparison of different algorithms to calculate electrophoretic mobility of analytes as a function of binary solvent compositionELECTROPHORESIS, Issue 10 2003Abolghasem Jouyban Abstract Ten different mathematical models representing the electrophoretic mobility of analytes in capillary electrophoresis in mixed solvents of different composition have been compared using 32 experimental data sets. The solvents are binary mixtures of water-methanol, water-ethanol and methanol-ethanol, respectively. Mean percentage deviation (MPD), overall MPD (OMPD) and individual percentage deviation (IPD) have been considered as comparison criteria. The results showed that a reorganized solution model, namely the combined nearly ideal binary solvent/Redlich-Kister equation, is the most accurate model among other similar models concerning both correlation ability and prediction capability. [source] Solvolysis of Some Arenediazonium Salts in Binary EtOH/H2O Mixtures under Acidic ConditionsEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 17 2003Román Pazo-Llorente Abstract We have determined the product distribution, the rate constants for dediazoniation product formation, and the solvolytic rate constants for 2-, 3-, and 4-methylbenzenediazonium ions (2-, 3-, and 4-MBD, respectively) loss in acidic ethanol/water mixtures over the whole composition range by a combination of spectrophotometric (UV/Vis) and high performance liquid chromatography (HPLC) measurements. The observed rate constants (kobs) for substrate loss are equal to those for product formation, and they remain essentially constant (2-MBD) with changing solvent composition but increase by a factor of ,2 (4MBD) on going from water to 100% EtOH. Up to four dediazoniation products , cresols (ArOH), chlorotoluene (ArCl), methylphenetole (ArOEt), and toluene (ArH) , were detected, depending on the solvent composition; the major dediazoniation products were the ArOH and ArOEt derivatives. The product selectivity (S) of the reaction towards nucleophiles is low and essentially constant with changing solvent composition, and good linear correlations between log kobs and YCl (solvent ionizing power) were observed for the three ArN2+ ions. All data are consistent with the rate-determining formation of an aryl cation, which reacts immediately with available nucleophiles. The data suggest that the distribution of neutral and anionic nucleophiles in the neighborhood of the ground state arenediazonium ion remains essentially unchanged upon dediazoniation, the observed product distribution reflecting the concentrations of nucleophiles in their immediate environment (i.e., in the first solvation shells of the arenediazonium ions). (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003) [source] Azeotropic Binary Solvent Mixtures for Preparation of Organic Single CrystalsADVANCED FUNCTIONAL MATERIALS, Issue 22 2009Xiaoran Li Abstract Here, a new approach is introduced to prepare large single crystals of ,-conjugated organic molecules from solution. Utilizing the concept of azeotropism, single crystals of tri-isopropylsilylethynyl pentacene (TIPS-PEN) with dimensions up to millimeters are facilely self-assembled from homogeneous solutions comprising two solvents with opposing polarities and a positive azeotropic point. At solvent compositions close to the azeotropic point, an abrupt transition of morphology from polycrystalline thin-films to large single crystals is found. How to adjust the initial ratio of the binary solvents so that the change in solvent composition during evaporation favors the specific H-aggregation and promotes an efficient self-assembly of TIPS-PEN is explained. The charge-carrier (hole) mobilities are substantially enhanced by a factor of 4 from the morphology of thin-films to large single crystals used as active layer in field-effect transistors. Additionally, this approach is extended to other ,,, stacked organic molecules to elucidate its broad applicability. [source] Metal-Free and Copper-Promoted Single-Pot Hydrocarboxylation of Cycloalkanes to Carboxylic Acids in Aqueous MediumADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 17 2009Marina Abstract A simple and effective method for the transformation, under mild conditions and in aqueous medium, of various cycloalkanes (cyclopentane, cyclohexane, methylcyclohexane, cis - and trans -1,2-dimethylcyclohexane, cycloheptane, cyclooctane and adamantane) into the corresponding cycloalkanecarboxylic acids bearing one more carbon atom, is achieved. This method is characterized by a single-pot, low-temperature hydrocarboxylation reaction of the cycloalkane with carbon monoxide, water and potassium peroxodisulfate in water/acetonitrile medium, proceeding either in the absence or in the presence of a metal promoter. The influence of various reaction parameters, such as type and amount of metal promoter, solvent composition, temperature, time, carbon monoxide pressure, oxidant and cycloalkane, is investigated, leading to an optimization of the cyclohexane and cyclopentane carboxylations. The highest efficiency is observed in the systems promoted by a tetracopper(II) triethanolaminate-derived complex, which also shows different bond and stereoselectivity parameters (compared to the metal-free systems) in the carboxylations of methylcyclohexane and stereoisomeric 1,2-dimethylcyclohexanes. A free radical mechanism is proposed for the carboxylation of cyclohexane as a model substrate, involving the formation of an acyl radical, its oxidation and consequent hydroxylation by water. Relevant features of the present hydrocarboxylation method, besides the operation in aqueous medium, include the exceptional metal-free and acid-solvent-free reaction conditions, a rare hydroxylating role of water, substrate versatility, low temperatures (ca. 50,°C) and a rather high efficiency (up to 72% carboxylic acid yields based on cycloalkane). [source] Hydrolysis of 2-(p -nitrophenoxy)tetrahydropyran: solvent and ,-deuterium secondary kinetic isotope effects and relationships with the solvolysis of simple secondary alkyl arenesulfonates and the enzyme-catalyzed hydrolysis of glycosides,JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 6-7 2004Imran A. Ahmad Abstract The effect of solvent composition in aqueous ethanol, trifluoroethanol and hexafluoropropan-2-ol on the rate constant and activation parameters for the uncatalysed hydrolysis of 2-(p -nitrophenoxy)tetrahydropyran (1) was investigated, and the m(YOTs) value is 0.60. This appreciable but less than maximal value is in accordance with an SN1 mechanism with rate-limiting ionization. The ,-deuterium secondary kinetic isotope effect (,-kie) for the uncatalysed hydrolysis of 1 is 1.17 in water (46°C), 1.15 in aqueous trifluoroethanol (50% mole fraction, 70.6°C) and 1.13 in aqueous ethanol (50% mole fraction, 70.6°C). These values correspond to about 1.19 at 25°C, which is characteristic of rate-limiting ionization in an SN1 reaction and appreciably higher than values for enzyme-catalysed glycolysis. The ,-kie is smaller under aqueous acidic conditions (1.07, 0.1,mol,dm,3 hydrochloric acid, 20.2°C) when 1 hydrolyses with acid catalysis. The previously reported ,-kie for the hydrolysis of 1 in buffered aqueous dioxan (1.063, 25°C) is now seen to correspond to acid-catalysed hydrolysis. These new results for 1 indicate that transition structures in enzyme-catalysed glycolyses with ,-kie values of less than about 1.15 at 25°C involve a lower degree of carbenium ion character than has hitherto been assumed. Copyright © 2004 John Wiley & Sons, Ltd. [source] The influence of solution-state conditions and stirring rate on the assembly of poly(acrylic acid)-containing amphiphilic triblock copolymers with multi-aminesJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 20 2010Jennifer L. Sorrells Abstract In the effort towards making nanoscale objects and assemblies feasible for use as functional materials, it is imperative to obtain control over the fundamental architectures and essential to understand what experimental conditions cause the manifestation of specific morphologies. A number of factors are known to influence the shape during the self-assembly of amphiphilic block copolymers in solution, including solvent composition, polymer length, hydrophobicity versus hydrophilicity, as well as the addition of additives that can interact with segments of the block copolymers. This research, focused on developing an understanding of the micellar architectures accessed by the amphiphilic triblock copolymer of acrylic acid, methyl acrylate, and styrene, PAA85 - b -PMA40 - b -PS35, as a function of the stirring rate, together with other factors, when undergoing coassembly with ethylenediamine or diethylenetriamine in water/tetrahydrofuran solutions. The work demonstrates that the rate at which the polymer solution was stirred impacts the shape of the solution-state assemblies formed by the triblock copolymer. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010 [source] Effect of temperature on the chromatographic retention of ionizable compounds.JOURNAL OF SEPARATION SCIENCE, JSS, Issue 6-7 2008Abstract We propose a general simple equation for accurately predicting the retention factors of ionizable compounds upon simultaneous changes in mobile phase pH and column temperature at a given hydroorganic solvent composition. Only four independent experiments provide the input data: retention factors measured in two pH buffered mobile phases at extreme acidic and basic pH values (e. g., at least ± 2 pH units far from the analyte pKa) and at two column temperatures. The equations, derived from the basic thermodynamics of the acid,base equilibria, additionally require the knowledge of the solute pKa and enthalpies of acid,base dissociation of both the solute and the buffer components in the hydroorganic solvent mixture. The performance of the predictive model is corroborated with the comparison between theoretical and experimental retention factors of several weak acids and bases of important pharmacological activity, in mobile phases containing different buffer solutions prepared in 25% w/w ACN in water and at several temperatures. [source] Synthesis and Mechanism of Ferroelectric Potassium Tantalate Niobate Nanoparticles by the Solvothermal and Hydrothermal ProcessesJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 5 2007Nian Wei High-purity KTa0.3Nb0.7O3 nanoparticles have been successfully synthesized by hydrothermal and solvothermal methods. The KOH concentration and the solvent composition have significant effects on the final products. The synthesis mechanism was discussed. X-ray diffraction, scanning electron microscopy, and transmission electron microscopy investigations show that the typical samples solvothermally synthesized are nanosized, well crystallized, and single crystalline. The KTa0.3Nb0.7O3 shows a pseudo-cubic to tetragonal transition with increasing crystallite size. It is believed that supercritical isopropanol plays an important role in synthesizing KTa0.3Nb0.7O3 nanoparticles under milder conditions than the hydrothermal route. The present solvothermal method provides a new potential route for synthesizing ferroelectric potassium tantalate niobate material. [source] Optimization of extraction process for phenolic acids from black cohosh (Cimicifuga racemosa) by pressurized liquid extraction,JOURNAL OF THE SCIENCE OF FOOD AND AGRICULTURE, Issue 1 2006Sudarsan Mukhopadhyay Abstract An investigation to optimize the extraction of phenolic acids from black cohosh using a pressurized liquid extractor system was studied with the aim of developing a generalized approach for sample preparation of phenolic compounds from plant matrices. Operating parameters such as solvent composition, solid-to-solvent ratio, temperature, particle size distribution, and number of extraction cycles were identified as main variables that influence extraction efficiency. A mixture of methanol and water (60:40 v/v) was found to be the best solvent for total phenolics (TP) and individual phenolic acids. The four phenolic acids extracted from black cohosh were identified by HPLC and LC-MS as caffeic acid, ferulic acid, sinapic acid and isoferulic acid. Over 96% of the measured phenolics were extracted in first two cycles. The extraction efficiency for black cohosh with MeOH:H2O (60:40 v/v) was found to be maximum at a solid-to-solvent ratio of 80 mg ml,1. TP content of the extract was found to increase with temperature up to 90 °C. Particle size was found to have a large impact on extraction efficiency of TP. Samples with particle size between 0.25 mm and 0.425 mm provided optimum extraction of phenolics from black cohosh. Published in 2005 for SCI by John Wiley & Sons, Ltd. [source] Microwave-assisted extraction of total bioactive saponin fraction from Gymnema sylvestre with reference to gymnemagenin: a potential biomarkerPHYTOCHEMICAL ANALYSIS, Issue 6 2009Vivekananda Mandal Abstract Objective , To develop a fast and ecofriendly microwave assisted extraction (MAE) technique for the effective and exhaustive extraction of gymnemagenin as an indicative biomarker for the quality control of Gymnema sylvestre. Methodology , Several extraction parameters such as microwave power, extraction time, solvent composition, pre-leaching time, loading ratio and extraction cycle were studied for the determination of the optimum extraction condition. Scanning electron micrographs were obtained to elucidate the mechanism of extraction Results , The final optimum extraction conditions as obtained from the study were: 40% microwave power, 6,min irradiation time, 85% v/v methanol as the extraction solvent, 15,min pre-leaching time and 25,:,1 (mL/g) as the solvent-to-material loading ratio. The proposed extraction technique produced a maximum yield of 4.3% w/w gymnemagenin in 6,min which was 1.3, 2.5 and 1.95 times more efficient than 6,h of heat reflux, 24,h of maceration and stirring extraction, respectively. A synergistic heat and mass transfer theory was also proposed to support the extraction mechanism Conclusion , Comparison with conventional extraction methods revealed that MAE could save considerable amounts of time and energy, whilst the reduction of volume of organic solvent consumed provides an ecofriendly feature. Copyright © 2009 John Wiley & Sons, Ltd. [source] Analysis of estrogenic contaminants in river water using liquid chromatography coupled to ion trap based mass spectrometryRAPID COMMUNICATIONS IN MASS SPECTROMETRY, Issue 14 2002Tom Benijts A precise and reliable method, using liquid chromatography combined with ion trap based mass spectrometry, for the determination of three endogenous estrogens, namely, estrone, estradiol, and estriol, and two synthetic estrogens, ethinyl estradiol and diethylstilbestrol, in environmental water samples was developed. Optimization of the parameter settings of the ion source and mass analyzer as well as evaluation of solvent composition were carried out by continuous introduction of standards through a syringe pump. In negative ion mode the electrospray ionization source gave acceptable results. The optimum solvent used consisted of water/acetonitrile, with no volatile bases or buffers added. A simple, off-line, manual solid-phase extraction method was developed for sample preparation of environmental water samples. Recoveries were over 86% for all compounds. The method was validated and found to be linear, selective, and robust. For analysis of a 50-mL sample, the limit of detection (LOD) ranged from 3.2 to 10.6,ng/L for all compounds, and the limit of quantitation (LOQ) from 10.6 to 35.0,ng/L. Within-day (n,=,5) and total (n,=,5) reproducibility were investigated at three different concentration levels and ranged from 6.2 to 9.5% and 9.4 to 12.1%, respectively. Finally, the method was applied to real-world samples. Copyright © 2002 John Wiley & Sons, Ltd. [source] Tetraethylenepentamine-Directed Controllable Synthesis of Wurtzite ZnSe Nanostructures with Tunable MorphologyCHEMISTRY - A EUROPEAN JOURNAL, Issue 31 2008Baojuan Xi Dr. Abstract A novel tetraethylenepentamine (TEPA)-directed method has been successfully developed for the controlled synthesis of ZnSe particles with distinctive morphologies, including nanobelts, nanowires, and hierarchically solid/hollow spheres. These structures, self-assembled from nanobelts and nanorods, have been synthesized by adjusting the reaction parameters, such as the solvent composition, reaction temperature, and the aging time. Results reveal that the volume ratio of H2O and TEPA plays a crucial role in the final morphology of ZnSe products. The mechanisms of phase formation and morphology control of ZnSe particles are proposed and discussed in detail. The products have been characterized by means of X-ray diffraction, field-emission scanning electron microscopy, transmission electron microscopy (TEM), selected area electron diffraction, high-resolution TEM, Raman spectra and luminescence spectroscopy. The as-prepared ZnSe nanoparticles display shape- and size-dependent photoluminescent optical properties. This is the first time to report preparation of complex hollow structures of ZnSe crystals with hierarchy through a simple solution-based route. This synthetic route is designed to exploit a new H2O/TEPA/N2H4,H2O system possibly for the preparation of other semiconductor nanomaterials. [source] Spontaneous Organization of Uniform CeO2 Nanoflowers by 3D Oriented Attachment in Hot Surfactant Solutions Monitored with an In Situ Electrical Conductance TechniqueCHEMISTRY - A EUROPEAN JOURNAL, Issue 11 2008Huan-Ping Zhou Abstract Uniform CeO2 nanoflowers were synthesized by rapid thermolysis of (NH4)2Ce(NO3)6 in oleic acid (OA)/oleylamine (OM), by a unique 3D oriented-attachment mechanism. CeO2 nanoflowers with controlled shape (cubic, four-petaled, and starlike) and tunable size (10,40,nm) were obtained by adjusting the reaction conditions including solvent composition, precursor concentration, reaction temperature, and reaction time. The nanoflower growth mechanism was investigated by in situ electrical conductance measurements, transmission electron microscopy, and UV/Vis spectroscopy. The CeO2 nanoflowers are likely formed in two major steps, that is, initial formation of ceria cluster particles capped with various ligands (e.g., OA, OM, and NO3,) via hydrolysis of (NH4)2Ce(NO3)6 at temperatures in the range 140,220,°C, and subsequent spontaneous organization of the primary particles into nanoflowers by 3D oriented attachment, due to a rapid decrease in surface ligand coverage caused by sudden decomposition of the precursor at temperatures above 220,°C in a strong redox reaction. After calcination at 400,°C for 4,h the 33.8,nm CeO2 nanoflowers have a specific surface area as large as 156,m2,g,1 with high porosity, and they are highly active for conversion of CO to CO2 in the low temperature range of 200,400,°C. The present approach has also been extended to the preparation of other transition metal oxide (CoO, NiO, and CuOx) nanoflowers. [source] Temperature-Dependent and Friction-Controlled Electrochemically Induced Shuttling Along Molecular Strings Associated with ElectrodesCHEMPHYSCHEM, Issue 10 2005Eugenii Katz Dr. Abstract The temperature and solvent composition dependence of the electrochemically stimulated rate of shuttling of the redox-active cyclophane, cyclobis(paraquat- p -phenylene), on a molecular string has been studied. The molecular string includes a ,-donor diiminebenzene-site that is associated on one side with an electrode, and stoppered on the other side with an adamantane unit. The cyclophane rests on the ,-donor site, owing to stabilizing ,-donor,acceptor interactions. Electrochemical reduction of the cyclophane units, to the bis-radical cation cyclophane, results in the shuttling of the reduced cyclophane towards the electrode, a process that is driven by the removal of the stabilizing donor,acceptor interactions, and the electrostatic attraction of the reduced product by the electrode. The latter process is energetically downhill, and is temperature-independent. Upon oxidation of the reduced cyclophane that is associated with the electrode, the energetically uphill shuttling of the oxidized cyclophane to the ,-donor site proceeds. The rate of this translocation process has been found to be temperature-dependent, and controlled by the solvent composition. The experimental results have been theoretically analyzed in terms of Kramers' molecular friction model. The theoretical fitting of the experimental results, using solutions of variable composition, reveals that the rate-constants for the uphill reaction in a pure aqueous solution follow the temperature-dependence of the viscosity of water. The results demonstrate the significance of friction phenomena in shuttling processes within molecular machines. [source] Electronic gel protein transfer and identification using matrix-assisted laser desorption/ionization-mass spectrometryELECTROPHORESIS, Issue 9 2004Jonathan W. Cooper Abstract An electronic protein transfer technique is described for achieving the rapid and efficient recovery of sodium dodecyl sulfate (SDS)-protein complexes from polyacrylamide gels. This process involves the use of small-dimension capillaries in physical contact with a resolved protein band within the polyacrylamide gel, providing a large potential drop and high electric field strength at the capillary/gel interface. Several factors controlling the electronic protein transfer, including the applied electric field strength, the electrophoresis buffer concentration, and the capillary dimension, are studied to further enhance the use of field-amplification for sample stacking of extracted SDS-protein complexes. As a result of sample stacking, the extracted proteins from a 50 ng gel loading are present in a narrow (,80 nL) and highly concentrated (0.46 mg/mL or 3.3×10,5 M for cytochrome c) solution plug. Three model proteins with molecular mass ranging from 14 kDa (cytochrome c) to 116 kDa (,-galactosidase) are stained by Coomassie blue and electrophoretically extracted from gels with protein loadings as low as 50 ng. The capillary format of the electronic protein transfer technique allows direct deposition of extracted proteins onto a matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) target. Various matrices and solvent compositions are evaluated for the analysis of extracted and concentrated SDS-protein complexes using MALDI-MS. The electronic protein transfer technique, when operated under optimized conditions, is demonstrated for the effective (>70% recovery), speedy (less than 5 min), and sensitive MS identification of gel resolved proteins (as low as 50 ng). [source] Azeotropic Binary Solvent Mixtures for Preparation of Organic Single CrystalsADVANCED FUNCTIONAL MATERIALS, Issue 22 2009Xiaoran Li Abstract Here, a new approach is introduced to prepare large single crystals of ,-conjugated organic molecules from solution. Utilizing the concept of azeotropism, single crystals of tri-isopropylsilylethynyl pentacene (TIPS-PEN) with dimensions up to millimeters are facilely self-assembled from homogeneous solutions comprising two solvents with opposing polarities and a positive azeotropic point. At solvent compositions close to the azeotropic point, an abrupt transition of morphology from polycrystalline thin-films to large single crystals is found. How to adjust the initial ratio of the binary solvents so that the change in solvent composition during evaporation favors the specific H-aggregation and promotes an efficient self-assembly of TIPS-PEN is explained. The charge-carrier (hole) mobilities are substantially enhanced by a factor of 4 from the morphology of thin-films to large single crystals used as active layer in field-effect transistors. Additionally, this approach is extended to other ,,, stacked organic molecules to elucidate its broad applicability. [source] Thermal decomposition of cyclic organic peroxides in pure solvents and binary solvent mixturesINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 6 2010Mariángeles Iglesias The thermal decomposition reaction of acetone cyclic triperoxide, acetone cyclic diperoxide, 4-heptanone cyclic diperoxide, and pinacolone cyclic diperoxide ca. 0.02 M was studied in pure solvents (acetone and 1-propanol) and in binary mixtures of acetone/1-propanol at 150°C. The kinetics of each system was explored by gas chromatography (GC) at different solvent compositions. The reactions showed a behavior accordingly with a pseudo-first-order kinetic law up to at least 90% peroxide decomposition. The main organic products derived from these thermolysis reactions were detected by GC analysis. Among them, the corresponding ketones, methane, ethane, and propane were the main identified products. The rates of decomposition of pinacolone diperoxide in the pure solvents were practically independent of the solvent characteristics, so it was of no interest to analyze its kinetic behavior in binary solvent mixtures. In acetone/1-propanol mixtures, the solvation effect on the cyclic peroxides derived from 4-heptanone and acetone molecules was slightly dominated by specific interactions between 1-propanol and a diradical-activated complex initially formed. This species was preferentially solvated by 1-propanol instead of acetone. Specific interactions between the O atoms from the peroxidic bond and the H from the OH in 1-propanol can be taken into account. © 2010 Wiley Periodicals, Inc. Int J Chem Kinet 42: 347,353, 2010 [source] Contrast analysis of the composition of ribosomes extracted with different purification proceduresJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 4 2000Giuseppe Briganti The composition and hydration of E. coli ribosomes isolated with different purification protocols has been analysed by combining two experimental techniques: measurements of small-angle neutron scattering (SANS), for two different isotopic solvent compositions, and refractive index (RI) increments. From the contrast between the solvent and solute scattering densities and the molar polarizability, determined experimentally with SANS and RI measurements, three independent equations are obtained and three unknown quantities are determined: (i) the volume of the solute hydrated skeleton Vs, (ii) the material contained in it, namely the biological components, intrinsic (rRNA and proteins) and extrinsic, such as aminoacylsynthetase and elongation factors, (iii) the number of water molecules structurally bound to the ribosome and non-exchangeable with the solvent. From the form factor at infinite contrast, a second definition of the solute volume is obtained, , which represents the volume within the contour surface of the ribosome. This value is generally larger than Vs and can include a certain amount of water molecules, i.e. those inside the volume (,Vs). Considering the molar volume of this water to be equal to that of the bulk water, it is possible to evaluate its amount. The particle density calculated from the ribosome components in , including proteins, RNA, bound and unbound water molecules, corresponds to the buoyant density measured for E. coli 70S particles. The two ribosomal preparations display different performances in protein synthesis; hence the results indicate that the optimal condition corresponds to a wider skeleton and contour volume but containing a smaller amount of segregated water molecules. It is believed that the method provides a reliable technique to determine the composition of ribosomes under various experimental conditions. [source] Enantioseparation via EIC-OSN: Process design and improvement of enantiomers resolvability and separation performanceAICHE JOURNAL, Issue 4 2010Issara Sereewatthanawut Abstract This article presents a mathematical model to assess and optimize the separation performance of an enantioselective inclusion complexation-organic solvent nanofiltration process. Enantiomer solubilities, feed concentrations, solvent compositions, permeate solvent volumes, and numbers of nanofiltrations were identified as key factors for process efficiency. The model was first tested by comparing calculated and experimental results for a nonoptimized process, and then, calculations were carried out to select the best operating conditions. An important finding was that the optimal configuration varied with the objective function selected, e.g., resolvability versus yield, with a boundary on product optical purity. The model also suggested that the process efficiency could benefit from diafiltration of the distomer and from the use of higher feed concentrations. However, the latter strategy would result in higher losses of eutomer. To address this drawback, a multistage process was evaluated using the verified process model. © 2009 American Institute of Chemical Engineers AIChE J, 2010 [source] Rheology of polyurethane solutions with different solventsPOLYMER INTERNATIONAL, Issue 1 2002L de Vasconcelos Abstract Hard segment hydrogen bonding interactions in a particular polyurethane segmented copolymer have been analysed through a viscometric approach to determine the behaviour of polyurethane solutions with different solvent compositions. Analysis of log,,spversus log c[,] master curves (solely comprising dilute and semidilute regimes) showed that these systems could be differentiated by the slope of the first part of the curve (dilute regime), the characteristic reduced concentration c*[,], and the slope of the second part of the curve (semidilute regime), differences which were related to a possible occurrence of polymer aggregates. Experiments with solutions in the concentrated regime, at different temperatures, were used to relate non-Newtonian behaviour to hydrogen bonding. © 2001 Society of Chemical Industry [source] Model-based characterization of an amino acid racemase from Pseudomonas putida DSM 3263 for application in medium-constrained continuous processesBIOTECHNOLOGY & BIOENGINEERING, Issue 4 2007M. Bechtold Abstract The amino acid racemase with broad substrate specificity from Pseudomonas putida DSM 3263 was overproduced and characterized with respect to application in an integrated multi-step process (e.g., dynamic kinetic resolution) that,theoretically,would allow for 100% chemical yield and 100% enantiomeric excess. Overexpression of the racemase gene in Escherichia coli delivered cell free extract with easily sufficient activity (20,50 U,mg,1 total protein) for application in an enzyme membrane reactor (EMR) setting. Model-based experimental analysis of a set of enzyme assays clearly indicated that racemization of the model substrates D - or L -methionine could be accurately described by reversible Michaelis,Menten kinetics. The corresponding kinetic parameters were determined from progress curves for the entire suitable set of aqueous-organic mixtures (up to 60% methanol and 40% acetonitrile) that are eligible for an integrated process scheme. The resulting kinetic expression could be successfully applied to describe enzyme membrane reactor performance under a large variety of settings. Model-based calculations suggested that a methanol content of 10% and an acetonitrile content of 20% provide maximum productivity in EMR operations. However product concentrations were decreased in comparison to purely aqueous operation due to decreasing solubility of methionine with increasing organic solvent content. Finally, biocatalyst stability was investigated in different solvent compositions following a model-based approach. Buffer without organic content provided excellent stability at moderate temperatures (20,35°C) while addition of 20% acetonitrile or methanol drastically reduced the half-life of the racemase. Biotechnol. Bioeng. 2007; 98: 812,824. © 2007 Wiley Periodicals, Inc. [source] | ||||||||||||||||