kJ Mol (kj + mol)

Distribution by Scientific Domains
Distribution within Chemistry

Selected Abstracts

Experimental Investigation of Eclogite Rheology and Its Fabrics at High Temperature and Pressure

Abstract Eclogite plays an important role in mantle convection and geodynamics in subduction zones. An improved understanding of processes in the deeper levels of subduction zones and collision belts requires information on eclogite rheology. However, the deformation processes and associated fabrics in eclogite are not well understood. Incompatible views of deformation mechanism have been proposed for both garnet and omphacite. We present here deformation behaviour of eclogite at temperatures of 1027,1427 °C, confining pressures of 2.5,3.5 GPa, and strain rates of 1 × 10,5 s,1 to 5 × 10,4 s,1. We obtained a power-law creep for the high temperature and pressure deformation of a ,dry' eclogite (50 vol.% garnet, 40% omphacite and 10% quartz) with A = 103.3 ± 1.0, n = 3.5 ± 0.4, ,E =403 ± 30 KJ mol,1 and ,V = 27.2 cm3 mol,1. The two principal minerals of eclogite have greatly different strengths. Progressive increase of garnet results in a smooth increase in strength. Analysis by electron back-scattered diffraction shows that: (1) garnet displays pole figures with near random distributions of misorientation angle under both dry and wet conditions; (2) omphacite shows pronounced lattice preferred orientations (LPOs), suggesting a dominant dislocation creep mechanism. Further investigation into the water effects on eclogite show: (3) water content does not influence the style of omphacite fabric but increases slightly the fabric strength; (4) grain boundary processes dominate the deformation of garnet under high water fugacity or high shear-strain conditions, yielding a random LPO similar to that of non-deforming garnet, despite the strong shape preferred orientation (SPO) observed. {110} [001] slip may dominate the deformation of rutile. Quartz displays complicated and inconsistent LPOs in eclogite. These results are remarkably similar to observations from deformed eclogites in nature. [source]

Evidence from ESR studies for [Co(,-C2H4)3] produced at 77 K in a rotating cryostat,

Lynda J. Hayton
Abstract Co atoms were reacted with ethene at 77 K and the paramagnetic products studied by electron spin resonance (ESR) at X- and K-bands. The ESR spectra of the major product at both frequencies showed eight cobalt multiplets (ICo = 7/2) indicating a mono-cobalt complex. The spectra have orthorhombic g and cobalt hyperfine tensors and were simulated by the parameters; g1 = 2.284, g2 = 2.0027, g3 = 2.1527; A1 < , 25 MHz, A2 = , 109 MHz, A3 = , 198 MHz. Proton and 13C (1% natural abundance) hyperfine couplings were lower than the line widths (<2 MHz) indicating less than 0.5 spin transfer to the ethene ligands. We assigned the spectrum to a Jahn,Teller-distorted planar trigonal mono-cobalt tris-ethene [Co(,-C2H4)3] complex in C2v symmetry. The SOMO is either a 3dx2,y2 (2a1) orbital in a T-geometry or a 3dxy (b1) orbital in a Y-geometry but there is only a spin density, a2, of 0.30 in these d orbitals. The spin deficiency of 0.70 is attributed to two factors; spin transfer from the Co to ethene ,/,* orbitals and a 4p orbital contribution, b2, to the SOMO. Calculations of a2 and b2 have been made at three levels of spin transfer, ,. At , = 0.00a2 is 0.23 and b2 is 0.78, at , = 0.25a2 is 0.25 and b2 is 0.52 and at , = 0.50a2 is 0.28 and b2 is 0.23. The other possible assignment to a mono-cobalt bis-ethene complex [Co(,-C2H4)2] cannot be discounted from the ESR data alone but is considered unlikely on other grounds. The complex is stable up to ,220 K indicating a barrier to decomposition of ,50 kJ Mol,1 Copyright © 2006 John Wiley & Sons, Ltd. [source]

A Contribution to the Study of the Adsorption of s -Triazine Herbicides on Glassy Carbon Electrodes by Differential-Capacity Measurements

S. Pintado
Abstract Measurements of differential capacity vs. potential have been made for a series of s -triazine herbicides at different concentrations. In all cases the decrease in capacity was independent of the applied potential, so the adsorption is also independent of the potential. From the measurements it can be established that the adsorption follows Langmuir type isotherms. Adsorption constants were obtained for the different herbicides at 25,°C (simetryn, simazine, terburyn and prometon) as well as those to simetryn at different temperatures, from which the adsorption enthalpy of this herbicide was calculated being its value of 17.5,kJ mol,1. [source]

Kinetics of the reduction of chromium(VI) by vitamin C

Xiang-Rong Xu
Abstract The kinetics of the reduction of Cr(VI) to Cr(III) by vitamin C was studied using potassium dichromate solution as the model pollutant. Effects of the concentration of vitamin C, pH, temperature, and irradiation on the reduction of Cr(VI) were examined. The kinetics of Cr(VI) reduction by vitamin C can be described as - d[Cr(VI)]/dt = 0.0156 (,M s,1).[Cr(VI)][vitamin C] (pH 5), where dt is the differential function (d) of time (t). The activation entropy (,S,) and enthalpy (,H,) are 42.4 kJ mol,1 and -71.0 J mol,1 K,1, respectively, and the activation energy at 298 K is 63.5 kJ mol,1. The advantages of vitamin C as a reductant are as follows: It is an important biological reductant in humans and animals, and it is not toxic. Toxic Cr(VI) can be reduced by vitamin C not only in acidic conditions but also in alkaline solutions (pH 9); furthermore, the reduction was shown to occur both under the irradiation and in the dark. The present results suggest that vitamin C could be used effectively in the remediation of Cr(VI)-contaminated soil and groundwater in a wide range of pH, with or without sunlight. [source]

Using measured octanol-air partition coefficients to explain environmental partitioning of organochlorine pesticides

Mahiba Shoeib
Abstract Octanol-air partition coefficients (Koa) were measured directly for 19 organochlorine (OC) pesticides over the temperature range of 5 to 35°C. Values of log Koa at 25°C ranged over three orders of magnitude, from 7.4 for hexachlorobenzene to 10.1 for 1,1-dichloro-2,2-bis (p-chlorophenyl) ethane. Measured values were compared to values calculated as KowRT/H (where R is the ideal gas constant [8.314 J mol,1 K,1], T is absolute temperature, and H is Henry's law constant) were, in general, larger. Discrepancies of up to three orders of magnitude were observed, highlighting the need for direct measurements of Koa. Plots of Koa versus inverse absolute temperature exhibited a log-linear correlation. Enthalpies of phase transition between octanol and air (,Hoa) were determined from the temperature slopes and were in the range of 56 to 105 kJ mol,1 K,1. Activity coefficients in octanol (,o) were determined from Koa and reported supercooled liquid vapor pressures (p), and these were in the range of 0.3 to 12, indicating near-ideal solution behavior. Differences in Koa values for structural isomers of hexachlorocyclohexane were also explored. A Koa -based model was described for predicting the partitioning of OC pesticides to aerosols and used to calculate particulate fractions at 25 and ,10°C. The model also agreed well with experimental results for several OC pesticides that were equilibrated with urban aerosols in the laboratory. A log-log regression of the particle-gas partition coefficient versus Koa had a slope near unity, indicating that octanol is a good surrogate for the aerosol organic matter. [source]

Microbiology and geochemistry of Little Hot Creek, a hot spring environment in the Long Valley Caldera

GEOBIOLOGY, Issue 2 2010
A culture-independent community census was combined with chemical and thermodynamic analyses of three springs located within the Long Valley Caldera, Little Hot Creek (LHC) 1, 3, and 4. All three springs were approximately 80 °C, circumneutral, apparently anaerobic and had similar water chemistries. 16S rRNA gene libraries constructed from DNA isolated from spring sediment revealed moderately diverse but highly novel microbial communities. Over half of the phylotypes could not be grouped into known taxonomic classes. Bacterial libraries from LHC1 and LHC3 were predominantly species within the phyla Aquificae and Thermodesulfobacteria, while those from LHC4 were dominated by candidate phyla, including OP1 and OP9. Archaeal libraries from LHC3 contained large numbers of Archaeoglobales and Desulfurococcales, while LHC1 and LHC4 were dominated by Crenarchaeota unaffiliated with known orders. The heterogeneity in microbial populations could not easily be attributed to measurable differences in water chemistry, but may be determined by availability of trace amounts of oxygen to the spring sediments. Thermodynamic modeling predicted the most favorable reactions to be sulfur and nitrate respirations, yielding 40,70 kJ mol,1 e, transferred; however, levels of oxygen at or below our detection limit could result in aerobic respirations yielding up to 100 kJ mol,1 e, transferred. Important electron donors are predicted to be H2, H2S, S0, Fe2+ and CH4, all of which yield similar energies when coupled to a given electron acceptor. The results indicate that springs associated with the Long Valley Caldera contain microbial populations that show some similarities both to springs in Yellowstone and springs in the Great Basin. [source]

A thermodynamic analysis of the anaerobic oxidation of methane in marine sediments

GEOBIOLOGY, Issue 5 2008
ABSTRACT Anaerobic oxidation of methane (AOM) in anoxic marine sediments is a significant process in the global methane cycle, yet little is known about the role of bulk composition, temperature and pressure on the overall energetics of this process. To better understand the biogeochemistry of AOM, we have calculated and compared the energetics of a number of candidate reactions that microorganisms catalyse during the anaerobic oxidation of methane in (i) a coastal lagoon (Cape Lookout Bight, USA), (ii) the deep Black Sea, and (iii) a deep-sea hydrothermal system (Guaymas basin, Gulf of California). Depending on the metabolic pathway and the environment considered, the amount of energy available to the microorganisms varies from 0 to 184 kJ mol,1. At each site, the reactions in which methane is either oxidized to , acetate or formate are generally only favoured under a narrow range of pressure, temperature and solution composition , particularly under low (10,10 m) hydrogen concentrations. In contrast, the reactions involving sulfate reduction with H2, formate and acetate as electron donors are nearly always thermodynamically favoured. Furthermore, the energetics of ATP synthesis was quantified per mole of methane oxidized. Depending on depth, between 0.4 and 0.6 mol of ATP (mol CH4),1 was produced in the Black Sea sediments. The largest potential productivity of 0.7 mol of ATP (mol CH4),1 was calculated for Guaymas Basin, while the lowest values were predicted at Cape Lookout Bight. The approach used in this study leads to a better understanding of the environmental controls on the energetics of AOM. [source]

Temperature dependence of Fe(III) and sulfate reduction rates and its effect on growth and composition of bacterial enrichments from an acidic pit lake neutralization experiment

GEOBIOLOGY, Issue 4 2005
ABSTRACT Microbial Fe(III) and sulfate reduction are important electron transport processes in acidic pit lakes and stimulation by the addition of organic substrates is a strategy to remove acidity, iron and sulfate. This principle was applied in a pilot-scale enclosure in pit lake 111 (Brandenburg, Germany). Because seasonal and spatial variation of temperature may affect the performance of in situ experiments considerably, the influence of temperature on Fe(III) and sulfate reduction was investigated in surface sediments from the enclosure in the range of 4,28 °C. Potential Fe(III) reduction and sulfate reduction rates increased exponentially with temperature, and the effect was quantified in terms of the apparent activation energy Ea measuring 42,46 kJ mol,1 and 52 kJ mol,1, respectively. Relatively high respiration rates at 4 °C and relatively low Q10 values (,2) indicated that microbial communities were well adapted to low temperatures. In order to evaluate the effect of temperature on growth and enrichment of iron and sulfate-reducing bacterial populations, MPN (Most Probable Number) dilution series were performed in media selecting for the different bacterial groups. While the temperature response of specific growth rates of acidophilic iron reducers showed mesophilic characteristics, the relatively high specific growth rates of sulfate reducers at the lowest incubation temperature indicated the presence of moderate psychrophilic bacteria. In contrast, the low cell numbers and low specific growth rates of neutrophilic iron reducers obtained in dilution cultures suggest that these populations play a less significant role in Fe and S cycling in these sediments. SSCP (Single-Strand Conformation Polymorphism) or DGGE (Denaturing Gradient Gel Electrophoresis) fingerprinting based on 16S rRNA genes of Bacteria indicated different bacterial populations in the MPN dilution series exhibiting different temperature ranges for growth. [source]

The Influence of Alkyl-Chain Length on Beta-Phase Formation in Polyfluorenes

Daniel W. Bright
Abstract Di- n -alkyl substituted polyfluorenes with alkyl chain lengths of 6, 7, 8, 9, and 10 carbon atoms (PF6, PF7, PF8, PF9, and PF10) are studied in dilute solution in MCH using optical spectroscopy. Beta-phase is formed upon cooling in solutions (, 7,µg mL,1) of PF7, PF8, and PF9 only, which is observed as an equilibrium absorption peak at , 437,nm and strong changes in the emission spectra. Beta-phase is formed upon thermal cycling to low temperature in solutions (,7,µg mL,1) of PF7, PF8, and PF9, which is observed as an equilibrium absorption peak at , 437,nm and strong changes in the emission spectra. Beta phase is found to occur more favorably in PF8 than in PF7 or PF9, which is attributed to a balance between two factors. The first is the dimer/aggregate formation efficiency, which is poorer for longer (more disordered) alkyl chain lengths, and the second is the Van der Waals bond energy available to overcome the steric repulsion and planarize the conjugated backbone, which is insufficient in the PF6 with a shorter alkyl chain. Beta phase formation is shown to be a result of aggregation, not a precursor to it. A tentative value of the energy required to planarize the fluorene backbone of (15.6,±,2.5) kJ mol,1 monomer is suggested. Excitation spectra of PF6, PF7, PF8, and PF9 in extremely dilute (, 10,ng mL,1) solution show that beta phase can form reversibly in dilute solutions of PF7, PF8 and PF9, which is believed to be a result of chain collapse or well dispersed aggregates being present in solution from dilution of more concentrated solutions. PF7, PF8, and PF9 also form beta phase in thermally cycled solid films spin-cast from MCH. However, in the films the PF7 formed a larger fraction of beta phase than the PF9, in contrast to the case in solutions, because it is less likely that the close-packed chains in the solid state will allow the formation of planarized chains with the longer PF9 side chains. [source]

Theoretical study on the mechanism of reaction between 3-hydroxy-3-methyl-2-butanone and malononitrile catalyzed by magnesium ethoxide

Lin-Sen Heng
The mechanism of reaction between 3-hydroxy-3-methyl-2-butanone and malononitrile for the synthesis of 2-dicyanomethylene-4,5,5-trimethyl-2,5-dihydrofuran-3-carbonitrile catalyzed by magnesium ethoxide was investigated by density functional theory (DFT). The geometries and the frequencies of reactants, intermediates, transition states, and products were calculated at the B3LYP/6,31G(d) level. The vibration analysis and the IRC analysis demonstrated the authenticity of transition states, and the reaction processes were confirmed by the changes of charge density at bond-forming critical point. The results indicated that magnesium ethoxide is an effective catalyst in the synthesis of 2-dicyanomethylene-4,5,5-trimethyl-2,5-dihydrofuran-3-carbonitrile from malononitrile and 3-hydroxy-3-methyl-2-butanone. The activation energy of reaction with magnesium ethoxide decreased by 102.37 kJ mol,1 compared with that of the reaction without it. The mechanism of reaction with catalyst magnesium ethoxide differs from that of reaction without it. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 41: 227,235, 2009 [source]

Kinetics and mechanism of myristic acid and isopropyl alcohol esterification reaction with homogeneous and heterogeneous catalysts

Tuncer Yalçinyuva
The reaction of myristic acid (MA) and isopropyl alcohol (IPA) was carried out by using both homogeneous and heterogeneous catalysts. For a homogeneously catalyzed system, the experimental data have been interpreted with a second order, using the power-law kinetic model, and a good agreement between the experimental data and the model has been obtained. In this approach, it was assumed that a protonated carboxylic acid is a possible reaction intermediate. After a mathematical model was proposed, reaction rate constants were computed by the Polymath* program. For a heterogeneously catalyzed system, interestingly, no pore diffusion limitation was detected. The influences of initial molar ratios, catalyst loading and type, temperature, and water amount in the feed have been examined, as well as the effects of catalyst size for heterogeneous catalyst systems. Among used catalysts, p -toluene sulfonic acid (p -TSA) gave highest reaction rates. Kinetic parameters such as activation energy and frequency factor were determined from model fitting. Experimental K values were found to be 0.54 and 1.49 at 60°C and 80°C, respectively. Furthermore, activation energy and frequency factor at forward were calculated as 54.2 kJ mol,1 and 1828 L mol,1 s,1, respectively. © 2008 Wiley Periodicals, Inc. 40: 136,144, 2008 [source]

Kinetics of the sulfate radical-mediated photo-oxidation of humic substances

Pedro M. David Gara
The kinetics of the aqueous phase reaction of sulfate radicals with commercial humic acids and with organic matter extracted from vermicompost (VC) was studied by flash-photolysis. The results can be interpreted by a mechanism that in a first step considers the reversible binding of the sulfate radicals by the humic substances. Both the bound and free sulfate radicals decay to oxidized products. From experiments performed with Aldrich humic acids in the temperature range from 283 to 303 K, the enthalpy change associated with the binding process was estimated to be ,(36 ± 11) kJ mol,1. © 2007 Wiley Periodicals, Inc. Int J Chem Kinet 40: 19,24, 2008 [source]

The silver(I)-catalyzed exchange of coordinated cyanide in hexacyanoferrate(II) by phenylhydrazine in aqueous medium

R. M. Naik
The [Ag]+ -catalyzed exchange of coordinated cyanide in [Fe(CN)6]4, by phenylhydrazine (PhNHNH2) has been studied spectrophotometrically at 488 nm by monitoring increase in the absorbance for the formation of cherry red colored complex [Fe(CN)5PhNHNH2]3,. The other reaction conditions were pH 2.80±,0.02, temperature = 30.0 ± 0.1°C, and ionic strength (I) = 0.02 M (KNO3). The reaction was followed as a function of pH, ionic strength, temperature, [Fe(CN)4,6], [PhNHNH2], [Ag+] by varying one variable at a time. The initial rates were evaluated for each variation using the plane mirror method. The initial rates evaluated as a function of [Fe(CN)4,6] clearly indicate that the initial rate increases with the increase in [Fe(CN)4,6] and finally reaches to a limiting value when [Fe(CN)4,6]/[AgNO3] , 1000. It indicates the formation of a strong adduct between [Fe(CN)6]4, and AgNO3 prior to the abstraction of CN,. The variation in initial rates with [PhNHNH2] also showed limiting values at [Fe(CN)4,6]/[PhNHNH2] , 8.30. The complex behavior due to pH and [Ag+] variations on the rate has been explained in detail. The composition of the final reaction product [Fe(CN)5PhNHNH2] formed during the course of reaction has been found to be 1:1 using the mole ratio method. The evaluated values of activation parameters for the catalyzed reaction are Ea = 53.85 kJ mol,1, , H,, = 51.33 kJ mol,1, and , S, = ,134.63 J K,1 mol,1, which suggest an interchange dissociative mechanism. A most plausible mechanistic scheme has been proposed based on the experimental observations. © 2007 Wiley Periodicals, Inc. Int J Chem Kinet 39: 447,456, 2007 [source]

The kinetics of complex formation between Ti(IV) and hydrogen peroxide

Daniel W. O'Sullivan
The kinetics of the formation of the titanium-peroxide [TiO2+2] complex from the reaction of Ti(IV)OSO4 with hydrogen peroxide and the hydrolysis of hydroxymethyl hydroperoxide (HMHP) were examined to determine whether Ti(IV)OSO4 could be used to distinguish between hydrogen peroxide and HMHP in mixed solutions. Stopped-flow analysis coupled to UV-vis spectroscopy was used to examine the reaction kinetics at various temperatures. The molar absorptivity (,) of the [TiO2+2] complex was found to be 679.5 ± 20.8 L mol,1 cm,1 at 405 nm. The reaction between hydrogen peroxide and Ti(IV)OSO4 was first order with respect to both Ti(IV)OSO4 and H2O2 with a rate constant of 5.70 ± 0.18 × 104 M,1 s,1 at 25°C, and an activation energy, Ea = 40.5 ± 1.9 kJ mol,1. The rate constant for the hydrolysis of HMHP was 4.3 × 10,3 s,1 at pH 8.5. Since the rate of complex formation between Ti(IV)OSO4 and hydrogen peroxide is much faster than the rate of hydrolysis of HMHP, the Ti(IV)OSO4 reaction coupled to time-dependent UV-vis spectroscopic measurements can be used to distinguish between hydrogen peroxide and HMHP in solution. © 2007 Wiley Periodicals, Inc. Int J Chem Kinet 39: 457,461, 2007 [source]

Thermochemistry for enthalpies and reaction paths of nitrous acid isomers

Rubik Asatryan
Recent studies show that nitrous acid, HONO, a significant precursor of the hydroxyl radical in the atmosphere, is formed during the photolysis of nitrogen dioxide in soils. The term nitrous acid is largely used interchangeably in the atmospheric literature, and the analytical methods employed do not often distinguish between the HONO structure (nitrous acid) and HNO2 (nitryl hydride or isonitrous acid). The objective of this study is to determine the thermochemistry of the HNO2 isomer, which has not been determined experimentally, and to evaluate its thermal and atmospheric stability relative to HONO. The thermochemistry of these isomers is also needed for reference and internal consistency in the calculation of larger nitrite and nitryl systems. We review, evaluate, and compare the thermochemical properties of several small nitric oxide and hydrogen nitrogen oxide molecules. The enthalpies of HONO and HNO2 are calculated using computational chemistry with the following methods of analysis for the atomization, isomerization, and work reactions using closed- and open-shell reference molecules. Three high-level composite methods G3, CBS-QB3, and CBS-APNO are used for the computation of enthalpy. The enthalpy of formation, ,Hof(298 K), for HONO is determined as ,18.90 ± 0.05 kcal mol,1 (,79.08 ± 0.2 kJ mol,1) and as ,10.90 ± 0.05 kcal mol,1 (,45.61 ± 0.2 kJ mol,1) for nitryl hydride (HNO2), which is significantly higher than values used in recent NOx combustion mechanisms. H-NO2 is the weakest bond in isonitrous acid; but HNO2 will isomerize to HONO with a similar barrier to the HONO bond energy; thus, it also serves as a source of OH in atmospheric chemistry. Kinetics of the isomerization is determined; a potential energy diagram of H/N/O2 system is presented, and an analysis of the triplet surface is initiated. © 2007 Wiley Periodicals, Inc. Int J Chem Kinet 39: 378,398, 2007 [source]

The elimination kinetics and mechanisms of ethyl piperidine-3-carboxylate, ethyl 1-methylpiperidine-3-carboxylate, and ethyl 3-(piperidin-1-yl)propionate in the gas phase

Angiebelk Monsalve
The gas-phase elimination kinetics of the above-mentioned compounds were determined in a static reaction system over the temperature range of 369,450.3°C and pressure range of 29,103.5 Torr. The reactions are homogeneous, unimolecular, and obey a first-order rate law. The rate coefficients are given by the following Arrhenius expressions: ethyl 3-(piperidin-1-yl) propionate, log k1(s,1) = (12.79 ± 0.16) , (199.7 ± 2.0) kJ mol,1 (2.303 RT),1; ethyl 1-methylpiperidine-3-carboxylate, log k1(s,1) = (13.07 ± 0.12),(212.8 ± 1.6) kJ mol,1 (2.303 RT),1; ethyl piperidine-3-carboxylate, log k1(s,1) = (13.12 ± 0.13) , (210.4 ± 1.7) kJ mol,1 (2.303 RT),1; and 3-piperidine carboxylic acid, log k1(s,1) = (14.24 ± 0.17) , (234.4 ± 2.2) kJ mol,1 (2.303 RT),1. The first step of decomposition of these esters is the formation of the corresponding carboxylic acids and ethylene through a concerted six-membered cyclic transition state type of mechanism. The intermediate ,-amino acids decarboxylate as the ,-amino acids but in terms of a semipolar six-membered cyclic transition state mechanism. © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 38: 106,114, 2006 [source]

Validation of a thermal decomposition mechanism of formaldehyde by detection of CH2O and HCO behind shock waves

Gernot Friedrichs
The thermal decomposition of formaldehyde was investigated behind shock waves at temperatures between 1675 and 2080 K. Quantitative concentration time profiles of formaldehyde and formyl radicals were measured by means of sensitive 174 nm VUV absorption (CH2O) and 614 nm FM spectroscopy (HCO), respectively. The rate constant of the radical forming channel (1a), CH2O + M , HCO + H + M, of the unimolecular decomposition of formaldehyde in argon was measured at temperatures from 1675 to 2080 K at an average total pressure of 1.2 bar, k1a = 5.0 × 1015 exp(-308 kJ mol,1/RT) cm3 mol,1 s,1. The pressure dependence, the rate of the competing molecular channel (1b), CH2O + M , H2 + CO + M, and the branching fraction , = k1a/(kA1a + k1b) was characterized by a two-channel RRKM/master equation analysis. With channel (1b) being the main channel at low pressures, the branching fraction was found to switch from channel (1b) to channel (1a) at moderate pressures of 1,50 bar. Taking advantage of the results of two preceding publications, a decomposition mechanism with six reactions is recommended, which was validated by measured formyl radical profiles and numerous literature experimental observations. The mechanism is capable of a reliable prediction of almost all formaldehyde pyrolysis literature data, including CH2O, CO, and H atom measurements at temperatures of 1200,3200 K, with mixtures of 7 ppm to 5% formaldehyde, and pressures up to 15 bar. Some evidence was found for a self-reaction of two CH2O molecules. At high initial CH2O mole fractions the reverse of reaction (6), CH2OH + HCO , CH2O + CH2O becomes noticeable. The rate of the forward reaction was roughly measured to be k6 = 1.5 × 1013 cm3 mol,1 s,1. © 2004 Wiley Periodicals, Inc. Int J Chem Kinet 36: 157,169 2004 [source]

Kinetics and mechanism of decomposition of intermediate complex during oxidation of pectate polysaccharide by potassium permanganate in alkaline solutions

Khalid S. Khairou
The kinetics of decomposition of an [Pect·MnVIO42,] intermediate complex have been investigated spectrophotometrically at various temperatures of 15,30°C and a constant ionic strength of 0.1 mol dm,3. The decomposition reaction was found to be first-order in the intermediate concentration. The results showed that the rate of reaction was base-catalyzed. The kinetic parameters have been evaluated and found to be ,S, = , 190.06 ± 9.84 J mol,1 K,1, ,H, = 19.75 ± 0.57 kJ mol,1, and ,G, = 76.39 ± 3.50 kJ mol,1, respectively. A reaction mechanism consistent with the results is discussed. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 35: 67,72, 2003 [source]

The mechanism of neutral amino acid decomposition in the gas phase.

N -dimethylglycine, N -dimethylglycine ethyl ester, The elimination kinetics of N, ethyl 1-piperidineacetate
The gas-phase elimination kinetics of the ethyl ester of two ,-amino acid type of molecules have been determined over the temperature range of 360,430°C and pressure range of 26,86 Torr. The reactions, in a static reaction system, are homogeneous and unimolecular and obey a first-order rate law. The rate coefficients are given by the following equations. For N,N-dimethylglycine ethyl ester: log k1(s,1) = (13.01 ± 3.70) , (202.3 ± 0.3)kJ mol,1 (2.303 RT),1 For ethyl 1-piperidineacetate: log k1(s,1) = (12.91 ± 0.31) , (204.4 ± 0.1)kJ mol,1 (2.303 RT),1 The decompositon of these esters leads to the formation of the corresponding ,-amino acid type of compound and ethylene. However, the amino acid intermediate, under the condition of the experiments, undergoes an extremely rapid decarboxylation process. Attempts to pyrolyze pure N,N-dimethylglycine, which is the intermediate of dimethylglycine ethyl ester pyrolysis, was possible at only two temperatures, 300 and 310°C. The products are trimethylamine and CO2. Assuming log A = 13.0 for a five-centered cyclic transition-state type of mechanism in gas-phase reactions, it gives the following expression: log k1(s,1) = (13.0) , (176.6)kJ mol,1 (2.303 RT),1. The mechanism of these ,-amino acids differs from the decarbonylation elimination of 2-substituted halo, hydroxy, alkoxy, phenoxy, and acetoxy carboxylic acids in the gas phase. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33:465,471, 2001 [source]

Diffusion and distribution of element-labelled surfactants in human hair

F.-J. Wortmann
Synopsis To directly follow the diffusion process of cosmetically relevant agents into human hair, a specific methodological approach is presented and elucidated for selected surfactants. For this, practically relevant anionic and cationic surfactants were synthesized with a chlorine atom at the end of their alkyl chain. The property changes of the surfactants through the modification are corresponding to an extension of the alkyl chain by about two methylene groups, thus representing a moderate increase of hydrophobicity. After the application of a modified surfactant to hair, it can be localized and quantified through its chlorine atom in cross-sections by scanning electron microscopy combined with micro X-ray fluorescence analysis. The determination of the diffusion coefficient D is realized through the application of the Matano-equation to element intensity profiles. Values for D vary within the chosen range of pH and temperature between 10,14 and 10,16 m2 s,1. The diffusion coefficients for the anionic surfactants increase with decreasing pH and increasing temperature, The temperature dependence follows in all cases the Arrhenius relationship with activation energies EA of 50,100 kJ mol,1, which decrease with pH. The pH-related effects, with comparable values for D and EA, are opposite for the cationic surfactant. These observations are consistently interpreted on the basis of ionic and hydrophobic interactions in hair. Résumé Afin de suivre directement le processus de diffusion d'agents cosmétiques pertinents dans le cheveu humain, une approche méthodologique spécifique est présentée et approfondie pour des tensio actifs sélectionnés. En pratique, des tensio actifs pertinents, anioniques et cationiques ont été synthétisés en introduisant un atome de Chlore à l'extrémité de leur chaîne alkyle. Les changements de propriétés de ces tensio actifs, via cette modification, correspondent à un allongement de la chaîne alkyle d'environ deux groupes méthylène, représentant ainsi une augmentation modérée de leur caractère hydrophobe. Suite à l'application sur le cheveu d'un tensio actif modifié, il peut être localisé et quantifié par l'atome de Chlore dans des coupes transverses par microscopie électronique à balayage couplée à l'analyse par micro Fluorescence X. La détermination du coefficient de diffusion D est effectuée par l'application de l'équation de Matano aux profils de l'intensité de l'élément. Les valeurs de D varient, selon l'échelle de pH et de température, entre 10,14 et 10,16 m2 s,1. Les coefficients de diffusion pour les tensio actifs anioniques augmentent avec des pH décroissants et des températures croissantes. La dépendance vis à vis de la température suit, dans tous les cas, la relation d'Arrhenius avec des énergies d'activation EA de 50 à 100 kJ mol,1, qui décroît avec le pH. Les effets liés au pH, avec des valeurs comparables de D et EA, sont opposés pour le tensio actif cationique. Ces observations sont constamment interprétées par les interactions de types ioniques et hydrophobes dans le cheveu. [source]

Shock tube pyrolysis of thiophene

Hafeez Ur Rahman Memon
Abstract The kinetics of the thermal decomposition of thiophene diluted in argon have been studied behind reflected shock waves in a single pulse shock tube over the temperature range 1598,2022 K and pressures between 2.5 and 3.44 bar. Product yields and composition were determined using capillary column gas chromatography with flame ionization detection and flame photometric sulphur selective detection. The principal hydrocarbon product at all temperatures was ethyne. Ethanethiol was found to be the major sulphur product together with H2S formed in significant concentrations at lower temperatures. Carbon disulphide was also formed at higher temperatures. Additional reaction products were CH4, C2H4, C3H4, C4H3, C4H6, C4H4, C6H6 and C4H2 with some traces were found of C5 and C6H5 species. It was concluded that pyrolysis of thiophene is initiated by C,S bond fission to form the C4H4S radical which reacts to give C4H3 + SH together with the reaction giving C3H4 + CS. The rate expression obtained for the pyrolysis reaction was k (C4H4S)=2.2×1011 exp (270 kJ mol,1) s,1. Copyright © 2002 John Wiley & Sons, Ltd. [source]

Equilibrium moisture content and heat of desorption of saffron

Hamid Reza Gazor
Summary The equilibrium moisture contents of saffron (Crocus sativus L.) stigmas were determined experimentally using the standard gravimetric method at temperatures 30, 45 and 60 °C and water activity ranging from 11% to 83%. The sorption isotherm curves of saffron were sigmoidal in shape and decreased with increased temperature at constant relative humidity. Five selected isotherm models GAB, modified Henderson, modified Chung-Pfost, modified Halsaey and modified Oswin were tested to fit the experimental isotherm data. Modified Oswin and modified Henderson models were found acceptable for predicting desorption moisture isotherms and fitting to the experimental data, respectively. The isosteric heats of desorption, determined from equilibrium data using the Clausius-Clapeyron equation, were found to be a function of moisture content. The net isosteric heat of desorption of saffron varied between 1.38 and 5.38 kJ mol,1 at moisture content varying between 2% and 20% (d.b). [source]

Modelling of air drying of fresh and blanched sweet potato slices

Kolawole O. Falade
Summary Effects of blanching, drying temperatures (50,80 °C) and thickness (5, 10 and 15 mm) on drying characteristics of sweet potato slices were investigated. Lewis, Henderson and Pabis, Modified Page and Page models were tested with the drying patterns. Page and Modified Page models best described the drying curves. Moisture ratio vs. drying time profiles of the models showed high correlation coefficient (R2 = 0.9864,0.9967), and low root mean squared error (RMSE = 0.0018,0.0130) and chi-squared (,2 = 3.446 × 10,6,1.03 × 10,2). Drying of sweet potato was predominantly in the falling rate period. The temperature dependence of the diffusion coefficient (Deff) was described by Arrhenius relationship. Deff increased with increasing thickness and air temperature. Deff of fresh and blanched sweet potato slices varied between 6.36 × 10,11,1.78 × 10,9 and 1.25 × 10,10,9.75 × 10,9 m2 s,1, respectively. Activation energy for moisture diffusion of the slices ranged between 11.1 and 30.4 kJ mol,1. [source]

Mathematical modelling of moisture sorption isotherms and determination of isosteric heat of blueberry variety O,Neil

Antonio Vega-Gálvez
Summary The sorption isotherms of blueberry variety O'Neil were determined at 20, 40 and 60 °C, for a range of water activity of 0.10,0.95. The isotherms showed that the equilibrium moisture content increased when temperature decreased at constant water activity. The BET, GAB, Halsey, Henderson, Caurie, Smith, Oswin and Iglesias-Chirife equations were tested for modelling the sorption isotherms. The results showed that GAB, BET and Halsey models gave the best fit quality for the experimental desorption data, and BET, Oswin and Henderson for adsorption data as suggested by the statistical tests employed. The net sorption heat was calculated using the Clausius,Clapeyron equation giving 38.62 kJ mol,1 (desorption) and 30.88 kJ mol,1 (adsorption) at a moisture content of 0.01 g water (g d.m.,1). Tsami equation was applied to estimate the net isosteric heat of sorption as function of equilibrium moisture content with satisfactory results. [source]

Modelling the respiration rate of fresh-cut Annurca apples to develop modified atmosphere packaging

Elena Torrieri
Summary In this work, the effect of temperature, oxygen, red coloration process and post-harvest storage time on the respiration rate of fresh-cut Annurca apples was studied to properly develop modified atmosphere packaging. Our results showed that the red coloration process and the post-harvest storage time did not affect the respiration rate or the respiratory quotient of fresh-cut Annurca apples in the range of temperature studied (5,20 °C). A Michaelis,Menten-type equation, with the model constants described by means of an Arrhenius-type relationship, was used for predicting respiration rate on varying the temperature and O2 concentration in the head space. The maximal respiration rate (mL kg h,1) (RRmax) and the O2% corresponding to values estimated at the reference temperature (12.5 °C), i.e. the average of the experimental temperature ranges, were, respectively, 6.77 ± 0.1 mL kg,1 h,1 and 0.68 ± 0.07% v/v, and the activation energy of the aerobic respiration rate of fresh-cut Annurca apples was estimated at 51 ± 1 kJ mol,1. The model works well to develop a modified atmosphere for fresh-cut Annurca apples. [source]

Moisture sorption isotherms and thermodynamic properties of apple Fuji and garlic

Mariana A. Moraes
Summary The moisture equilibrium isotherms of garlic and apple were determined at 50, 60 and 70 °C using the gravimetric static method. The experimental data were analysed using GAB, BET, Henderson,Thompson and Oswin equations. The isosteric heat and the differential entropy of desorption were determined by applying Clausius,Clapeyron and Gibbs,Helmholtz equations, respectively. The GAB equation showed the best fitting to the experimental data (R2 > 99% and E% < 10%). The monolayer moisture content values for apple were higher than those for garlic at the studied temperatures; the values varied from 0.050 to 0.056 and from 0.107 to 0.168 for garlic and apple, respectively. The isosteric heat and the differential entropy of desorption were estimated in function of the moisture content. The values of these thermodynamic properties were higher for apple (in range 48,100 kJ mol,1 and 14,150 J mol,1 K,1) than for garlic (in range 43,68 kJ mol,1 and 0,66 J mol,1 K,1). The water surface area values decreased with increasing temperature. The Kelvin and the Halsey equations were used to calculate the pore size distribution. [source]

Convective hot air drying of blanched yam slices

Olajide Philip Sobukola
Summary In this study, a laboratory convective hot air dryer was used for the thin layer drying of blanched yam slices and experimental moisture ratio was compared with Newton, Logarithmic, Henderson and Pabis, modified Henderson and Pabis, approximation of diffusion, modified page 1, two-term exponential, Verma et al. and Wang and Singh models. Among all the models, the approximation of diffusion model was found to satisfactorily describe the kinetics of air-drying of blanched yam slices. The increase in air temperature significantly reduced the drying time with no constant rate period but drying occurs in falling rate period. The effective diffusivity values varied between 7.62 × 10,8 to 9.06 × 10,8 m2 s,1 and increased with increase in temperature. An Arrhenius relation with an activation energy value of 8.831 kJ mol,1 showed the effect of temperature on moisture diffusivity. [source]

Steady and dynamic shear rheology of glutinous rice flour dispersions

Byoungseung Yoo
Summary The steady and dynamic shear rheological properties of Korean glutinous rice flour dispersions were evaluated at different concentrations (4, 5, 6, 7 and 8%). Glutinous rice flour dispersions at 25 °C showed a shear-thinning behaviour (n = 0.487,0.522) with low magnitudes of Casson yield stresses (,oc = 0.056,0.339 Pa). The magnitudes of ,oc, consistency index (K) and apparent viscosity (,a,100) increased with the increase in concentration. The power law model was found to be more suitable than the exponential model in expressing the relationship between concentration and apparent viscosity. The apparent viscosity over the temperature range of 25,70 °C obeyed the Arrhenius temperature relationship, with high determination coefficients (R2 = 0.982,0.998), indicating that the magnitudes of activation energies (Ea) were in the range of 9.05,11.89 kJ mol,1. A single equation, combining the effects of temperature and concentration on ,a,100, was used to describe the flow behaviour of glutinous rice flour dispersions. Magnitudes of storage (G,) and loss (G,,) moduli increased with the increase in concentration and frequency. Magnitudes of G, were higher than those of G,, over most of the frequency range. [source]

Rheological behaviour and colour changes of ginger paste during storage

Jasim Ahmed
Summary Ginger paste was prepared from fresh ginger by addition of 8% common salt and citric acid. The paste was thermally processed and packed in glass, polyethyleneterephthalate or high-density-polyethylene containers and stored at 5 ± 1 and 25 ± 1 °C for 120 days. The rheological characteristics of the paste were studied by using a computer controlled rotational viscometer over the temperature range of 20,80 °C. Samples were subjected to a programmed shear rate, increasing linearly from 0 to 200 s,1 in 3 min, followed by a steady shear at 200 s,1 for 3 min and finally decreasing linearly from 200 to 0 s,1 in 3 min. Ginger paste exhibited pseudoplasticity with yield stress and flow adequately described by the Herschel,Bulkley model. The yield stress decreased exponentially with process temperature and ranged between 3.86 and 27.82 Pa. The flow behaviour index (n) varied between 0.66 and 0.82 over the temperature range. Both consistency index and apparent viscosity decreased with increase in temperature and the process activation energies were found to be in the range of 16.7 to 21.9 kJ mol,1. The effect of temperature was significant (P < 0.05) on the Hunter colour combination value of the paste during storage; however it was not affected by type of packaging material (P > 0.05). It is recommended that ginger paste is stored at 5 ± 1 °C in polyethyleneterephthalate or glass containers. [source]

Effect of thermal processing on the texture of canned apricots

Constantin G. Mallidis
The effect of thermal processing on the texture of canned apricots was studied by using the main cultivar canned in Greece, Bebecou. The test temperature ranged from 82 to 95 °C. The loss of hardness was tested immediately after processing and after 3 months storage. The z -value was 16.7 °C and the energy of activation 116.5 kJ mol,1. Some restoration in the hardness was found after 3 months storage, which might be attributable to the absorption of sucrose by the fruit. [source]