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Equilibration
Kinds of Equilibration Terms modified by Equilibration Selected AbstractsBioconcentration of persistent organic pollutants in four species of marine phytoplanktonENVIRONMENTAL TOXICOLOGY & CHEMISTRY, Issue 11 2005Antje Gerofke Abstract The uptake of polychlorinated biphenyls (PCBs) was studied in four species of marine algae. A novel experimental system to establish and maintain constant dissolved concentrations of PCBs was employed. Headspace sampling was used to verify that the freely dissolved concentrations remained constant with time. The headspace analysis also allowed sorption to dissolved organic carbon (DOC) to be quantified for all but the most lipophilic PCB congeners. Equilibration with the dissolved phase was rapid for three of the four algae species (<1 d for the majority of congeners). Organic carbon,normalized algae/water partition coefficients (KAlgW) were similar for three of the four species, but were lower by a factor of 10 to 20 for Phaeodactylum tricornutum. The KAlgW values of the first three species were similar to the octanol/water partition coefficient (KOW) for those PCB congeners for which DOC sorption could be quantified. These KAlgW values also agreed well with organic carbon,normalized bioconcentration factors for PCBs in suspended particulate matter (BCFSPM) sampled in Baltic Sea surface water during the summer. [source] Dichlorodiphenyltrichloroethane in the aquatic ecosystem of the Okavango Delta, Botswana, South AfricaENVIRONMENTAL TOXICOLOGY & CHEMISTRY, Issue 1 2003Bontle Mbongwe Abstract Concentrations of DDT and its metabolites were measured in water, plants, invertebrates, and fish from lagoons in the Okavango Delta, Botswana (Africa), where DDT has been used for approximately 50 years. The sampling area was sectioned to distinguish spraying for malaria and for African sleeping sickness. Average concentrations of total DDT (sum of DDT and its metabolites) in the Okavango ranged from 0.009 ng/L in water to 18.76 ng/g wet weight in fish. These levels are approximately 1% of those found in piscivorous fish from temperate North America. The dichlorodiphenyl ethylene (DDE) metabolite was the most abundant fraction of total DDT. Although total DDT concentrations were higher in areas treated for malaria than areas treated for sleeping sickness, these concentrations were likely driven by factors other than the historic application of the pesticide. Equilibration with air concentrations is the most likely explanation for these levels. Since the mean annual temperature exceeds the temperature of vaporization of DDT, this research points to the need for reliable transport models. Our results showed that total DDT concentration in fish was best explained by lipid content of the fish and trophic position inferred by ,15N, regardless of DDT application history in those areas. The reservoir above Gaborone Dam, an area downstream of the Okavango but where DDT had not been used, was sampled to compare total DDT levels to the treated areas. The two species (a herbivorous threespot talapia and the omnivorous sharptooth catfish) from Gaborone had levels higher than those found in the Okavango Delta, but these differences can again be explained using trophic position inferred by ,15N rather than by fish size or location. [source] Dynamic Charge Equilibration-Morse stretch force field: Application to energetics of pure silica zeolitesJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 16 2002Jan Sefcik Abstract We present the Dynamic Charge Equilibration (DQEq) method for a self-consistent treatment of charge transfer in force field modeling, where atomic charges are designed to reproduce electrostatic potentials calculated quantum mechanically. Force fields coupled with DQEq allow charges to readjust as geometry changes in classical simulations, using appropriate algorithms for periodic boundary conditions. The full electrostatic energy functional is used to derive the corresponding forces and the second derivatives (hessian) for vibrational calculations. Using DQEq electrostatics, we develop a simple nonbond force field for simulation of silica molecular sieves, where nonelectrostatic interactions are described by two-body Morse stretch terms. Energy minimization calculations with the new force field yield accurate unit cell geometries for siliceous zeolites. Relative enthalpies with respect to quartz and third-law entropies calculated from harmonic vibrational analysis agree very well with available calorimetric data: calculated SiO2 enthalpies relative to ,-quartz are within 2 kJ/mol and entropies at 298 K are within 3 J/mol K of the respective experimental values. Contributions from the zero point energy and vibrational degrees of freedom were found to be only about 1 kJ/mol for the free energy of mutual transformations between microporous silica polymorphs. The approach presented here can be applied to interfaces and other oxides as well and it is suitable for development of force fields for accurate modeling of geometry and energetics of microporous and mesoporous materials, while providing a realistic description of electrostatic fields near surfaces and inside pores of adsorbents and catalysts. © 2002 Wiley Periodicals, Inc. J Comput Chem 23: 1507,1514, 2002 [source] Water Sorption and Crispness of Fat-Free Apple ChipsJOURNAL OF FOOD SCIENCE, Issue 1 2002D. Konopacka ABSTRACT: Equilibration of fat-free apple chips to predetermined moisture level was achieved by keeping samples over saturated salt solutions. The sorption isotherm obtained indicated absence of a monolayer and was typical for type III according to the Brunauer classification. At water activity below 0.12 apple chips demonstrated excellent crispness and were highly acceptable as a snack food item. They were extremely hygroscopic and lost crispness easily. The critical water activity was found to be ac= 0.18 which corresponds to a water content of 3.5 g H2O/100 g solids. These values were much lower than those found for other crispy snack foods. [source] Li-Deficient, Off-Congruent MgO:LiNbO3 Crystals Prepared by Postgrown Li-Poor Vapor Transport Equilibration for Integrated OpticsJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 7 2010De-Long Zhang Li-deficient, off-congruent Z -cut MgO:LiNbO3 (MgO:LN) crystals for integrated optics were prepared by carrying out postgrown Li-poor vapor transport equilibration (VTE) treatments on congruently grown MgO (5 mol% in melt):LiNbO3 plates at 1100°C for durations ranging from 40 to 395 h. Secondary ion mass spectrometry analysis, surface ordinary refractive index measurement, and neutron activation analysis were carried out on the VTE crystals to verify that the Mg and Nb ions did not diffuse out of the crystal during the VTE procedure and their distributions over the whole plate retain its homogeneity. The VTE duration dependence of the Li2O content reduction was determined using gravimetric method, and the crystalline phase was by powder X-ray diffraction. The results show that the Li2O content decreases with a prolonged VTE and the Li2O content reduction in the saturation regime is about 2.9 mol%. All of the VTE crystals still retain the LN phase, and Li-vacancy and NbLi are the major defects in the VTE crystal. OH absorption study reveals that the doped MgO concentration is below the photorefractive threshold for all VTE crystals. The optical absorption edge (OAE) of the VTE-treated MgO:LN was also measured as a function of the VTE duration. On the basis of the known Li2O content and measured OAE, the photon-energy fit reported previously, valid for the evaluation of Li2O content in a pure LN, is corrected for the Li-poor VTE-treated MgO:LNs. Finally, the applicability of the Li-deficient off-congruent MgO:LN crystals prepared by the Li-poor VTE method is demonstrated by characterizing the optical damage and Er diffusion properties of a single-mode Ti:MgO:Er:LiNbO3 strip waveguide fabricated on an Li-poor VTE-prepared MgO:LN crystal. [source] Enantioselective Equilibration , Access to Chiral Aldol Adducts of Mandelic Acid Esters.CHEMINFORM, Issue 10 2007Stefan Scholtis Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF. [source] Thermodynamic Equilibration of Dihydropyridone Enolates: Application to the Total Synthesis of (+/-)-Epiuleine.CHEMINFORM, Issue 13 2004Edward S. Tasber Abstract For Abstract see ChemInform Abstract in Full Text. [source] Endurance training adaptations modulate the redox,force relationship of rat isolated slow-twitch skeletal musclesCLINICAL AND EXPERIMENTAL PHARMACOLOGY AND PHYSIOLOGY, Issue 1-2 2003David R Plant Summary 1.,Studies have shown that, in isolated skeletal muscles, maximum isometric force production (Po) is dependent on muscle redox state. Endurance training increases the anti-oxidant capacity of skeletal muscles, a factor that could impact on the force-producing capacity following exogenous exposure to an oxidant. We tested the hypothesis that 12 weeks treadmill training would increase anti-oxidant capacity in rat skeletal muscles and alter their response to exogenous oxidant exposure. 2.,At the conclusion of the 12 week endurance-training programme, soleus (slow-twitch) muscles from trained rats had greater citrate synthase (CS) and catalase (CAT) activity compared with soleus muscles from untrained rats (P < 0.05). In contrast, CAT activity of extensor digitorum longus (EDL; fast-twitch) muscles from trained rats was not different to EDL muscles of untrained rats. The CS activity was lower in EDL muscles from trained compared with untrained rats (P < 0.05). 3.,Equilibration with exogenous hydrogen peroxide (H2O2, 5 mmol/L) increased the Po of soleus muscles from untrained rats for the duration of treatment (30 min), whereas the Po of EDL muscles was affected biphasically, with a small increase initially (after 5 min), followed by a more marked decrease in Po (after 30 min). The H2O2 -induced increase in Po of soleus muscles from trained rats was less than that in untrained rats (P < 0.05), but no differences were observed in the Po of EDL muscles following training. 4.,The results indicate that 12 weeks endurance running training conferred adaptations in soleus but not EDL muscles. These adaptations were associated with an attenuation of the oxidant-induced increase in Po of soleus muscles from trained compared with untrained rats. We conclude that endurance training-adapted soleus muscles have a slightly altered redox,force relationship. [source] Growth, etching morphology and spectra of LiAlO2 crystalCRYSTAL RESEARCH AND TECHNOLOGY, Issue 8 2008Taohua Huang Abstract ,-LiAlO2 single crystal was successfully grown by Czochralski method. The crystal quality was characterized by X-ray rocking curve and chemical etching. The effects of air-annealing and vapor transport equilibration (VTE) on the crystal quality, etch pits and absorption spectra of LiAlO2 were also investigated in detail. The results show that the as-grown crystal has very high quality with the full width at half maximum (FWHM) of 17.7-22.6 arcsec. Dislocation density in the middle part of the crystal is as low as about 3.0×103 cm,2. The VTE-treated slice has larger FWHM value, etch pits density and absorption coefficient as compared with those of untreated and air-annealed slices, which indicates that the crystal quality became inferior after VTE treatment. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Mechanisms of intercellular hypertonicity and isotonic fluid absorption in proximal tubules of mammalian kidneysACTA PHYSIOLOGICA, Issue 1 2002F. KIILArticle first published online: 30 APR 200 ABSTRACT The main purpose of this theoretical analysis (second of two articles) is to examine whether transjunctional diffusion of NaCl causes intercellular hypertonicity, which permits transcellular water transport across solute-impermeable lateral cell membranes until osmotic equilibration. In the S2 segment with tubular NaCl concentration 140 mM, the calculated apical intercellular NaCl concentration is c0 , 132 mM, which exceeds peritubular NaCl concentration by 12 mM or 22 mOsm kg,1. Variations in volume flow, junctional reflection coefficient (,NaCl=0.25,0.50), gap distance (g=6,8 Ĺ), junctional depth (d=18,100 Ĺ), intercellular diffusion coefficient (DLIS=500,1500 ,m2 s,1) and hypothetical active NaCl transport alter c0 only by a fraction of 1 mM. However, dilution and back-leakage of NaHCO3 lower apical intercellular hyperosmolality to ,18 mOsm kg,1. Water transport through solute-impermeable lateral cell membranes continues until intercellular and cellular osmolalities are equal. Transcellular and transjunctional volume flow are of similar magnitude (2 nL min,1 mm,1 tubule length) in the S2 segment. Thus, diffusion ensures isotonic absorption of NaCl. Two-thirds of NaHCO3 and other actively transported sodium salts are extruded into the last third of the exponentially widening intercellular space where the exposure time is only 0.9 s. Osmotic equilibration is dependent on aquaporins in the cell membranes. If permeability to water is low, transcellular water transport stops; tubular fluid becomes hypotonic; NaCl diffusion diminishes, but transjunctional water transport remains unaltered as long as transcellular transport of NaHCO3 and other solutes provides the osmotic force. [source] Toxicity of the molybdate anion in soil is partially explained by effects of the accompanying cation or by soil pHENVIRONMENTAL TOXICOLOGY & CHEMISTRY, Issue 6 2010Jurgen Buekers Abstract Previous studies have shown that toxicity of cationic trace metals in soil is partially confounded by effects of the accompanying anions. A similar assessment is reported here for toxicity of an oxyanion, i.e., molybdate (MoO), the soil toxicity of which is relatively unexplored. Solubility and toxicity were compared between the soluble sodium molybdate (Na2MoO4) and the sparingly soluble molybdenum trioxide (MoO3). Confounding effects of salinity were excluded by referencing the Na2MoO4 effect to that of sodium chloride (NaCl). The pH decrease from the acid MoO3 amendment was equally referenced to a hydrochloric (HCl) treatment or a lime-controlled MoO3 treatment. The concentrations of molybdenum (Mo) in soil solution or calcium chloride (CaCl2) 0.01,M extracts were only marginally affected by either MoO3 or Na2MoO4 as an Mo source after 10 to 13 days of equilibration. Effects of Mo on soil nitrification were fully confounded by associated changes in salinity or pH. Effects of Mo on growth of wheat seedlings (Triticum aestivum L) were more pronounced than those on nitrification, and toxicity thresholds were unaffected by the form of added Mo. The Mo thresholds for wheat growth were not confounded by pH or salinity at incipient toxicity. It is concluded that oxyanion toxicity might be confounded in relatively insensitive tests for which reference treatments should be included. Environ. Toxicol. Chem. 2010;29:1274,1278. © 2010 SETAC [source] Sorption irreversibility of 1,4-dichlorobenzene in two natural organic matter,rich geosorbentsENVIRONMENTAL TOXICOLOGY & CHEMISTRY, Issue 3 2009Michael Sander Abstract Hysteresis, a frequently observed phenomenon in sorption studies, is inconsistent with the key assumption of sorption reversibility in most fate and bioavailability models. Therefore, a study of the underlying causes of hysteresis is essential. Carbon-radiolabeled 1,4-dichlorobenzene (DCB) isotope tracer exchange was carried out at select points along the isotherms of DCB in a brown coal and a peat soil, holding total DCB concentration constant. Tracer exchange was performed both in the forward (sorption) and reverse (desorption) directions at the bulk sorption points and in the desorption direction at the corresponding bulk desorption points. Bulk DCB isotherms showed concentration-dependent hysteresis. However, tracer reequilibration in all cases was consistent with free exchange between sorbed and aqueous-phase molecules. These results rule out common experimental artifacts and demonstrate that sorption of bulk DCB is truly hysteretic (i.e., irreversible). The differences in rates between bulk and tracer sorption and desorption are consistent with the coupling of bulk DCB diffusion to other processes that retard equilibration, which we assign to matrix swelling or shrinking. Hysteresis is attributed to matrix deformation,specifically, to inelastic expansion and creation of voids accommodating sorbate molecules in the matrix, which leads to enhanced affinity in the desorption step. Comparing the results to previous results for naphthalene in the coal, we find that irreversible effects are similar for DCB and naphthalene in the coal but differ for DCB between the two sorbents. An explanation based on the different physical properties of these sorbents is provided. Solid-phase extraction of equilibrated DCB with Tenax® revealed a highly desorption-resistant fraction. While too small to account for the observed hysteresis, this fraction may represent molecules that become trapped as the matrix collapses and simultaneously stiffens during abrupt desorption. [source] Partitioning of copper at concentrations below the marine water quality criteria,ENVIRONMENTAL TOXICOLOGY & CHEMISTRY, Issue 5 2001Anthony J. Paulson Abstract Partitioning of Cu between the aqueous and particulate phases and among their components was examined in six ambient Puget Sound, Washington State, USA, samples (6,10 nM Cu). Most of the particulate Cu (4,12% of the total Cu) was associated with particulate organic matter, and resulted in distribution coefficients (Kd) ranging between 104,55 and 105,1. For the dissolved phase, the portion of Cu extracted by C18 -packed cartridges averaged 44% (+ 11%). Radioactive 64Cu was added to these samples to total stable Cu concentrations (17,33 nM). After 24 h of equilibration, the portion of 64Cu associated with the particulate matter in five of the six samples (Kd between 1047 and 1053) was an average of 70% higher than that of natural Cu in the ambient samples. In contrast, only 19 ± 7% of the 64Cu was extracted by C18 -packed cartridges. The partitioning of natural Cu and 64Cu onto particles was not significantly different when the equilibria were based on dissolved Cu passing through the C18 cartridges. Further research is warranted on utilizing the hydrophilic component of the dissolved phase as a parameter on which water quality criteria are based. [source] Reliability and repeatability of thermographic examination and the normal thermographic image of the thoracolumbar region in the horseEQUINE VETERINARY JOURNAL, Issue 4 2004B. V. TUNLEY Summary Reasons for performing study: Thermographic imaging is an increasingly used diagnostic tool. When performing thermography, guidelines suggest that horses should be left for 10,20 mins to ,acclimatise' to the thermographic imaging environment, with no experimental data to substantiate this recommendation. In addition, little objective work has been published on the repeatability and reliability of the data obtained. Thermography has been widely used to identify areas of abnormal body surface temperature in horses with back pathology; however, no normal data is available on the thermographic ,map' of the thoracolumbar region with which to compare horses with suspected pathology. Objectives: To i) investigate whether equilibration of the thermographic subject was required and, if so, how long it should take, ii) investigate what factors affect time to equilibration, iii) investigate the repeatability and reliability of the technique and iv) generate a topographic thermographic ,map' of the thoracolumbar region. Methods: A total of 52 horses were used. The following investigations were undertaken: thermal imaging validation, i.e. detection of movement around the baseline of an object of constant temperature; factors affecting equilibration; pattern reproducibility during equilibration and over time (n = 25); and imaging of the thoracolumbar region (n = 27). Results: A 1°C change was detected in an object of stable temperature using this detection system, i.e the ,noise' in the system. The average time taken to equilibrate, i.e. reach a plateau temperature, was 39 mins (40.2 in the gluteal region, 36.2 in lateral thoracic region and 40.4 in metacarpophalangeal region). Only 19% of horses reached plateau within 10,20 mins. Of the factors analysed hair length and difference between the external environment and the internal environment where the measurements were being taken both significantly affected time to plateau (P<0.05). However, during equilibration, the thermographic patterns obtained did not change, nor when assessed over a 7 day period. A ,normal' map of the surface temperature of the thoracolumbar region has been produced, demonstrating that the midline is the hottest, with a fall off of 3°C either side of the midline. Conclusions: This study demonstrates that horses may not need time to equilibrate prior to taking thermographic images and that thermographic patterns are reproducible over periods up to 7 days. A topographical thermographic ,map' of the thoracolumbar region has been obtained. Potential relevance: Clinicians can obtain relevant thermographic images without the need for prior equilibration and can compare cases with thoracolumbar pathology to a normal topographic thermographic map. [source] Synthesis and Dynamic Features of (Chloro)zirconocene Cations Stabilised by Pendant (Diarylphosphanyl)alkyl and (Dimethylamino)alkyl Substituents at Their Cyclopentadienyl Ring SystemsEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 8 2003Steve Döring Abstract Treatment of the substituted (diarylphosphanyl)methyl group-4 metallocene complexes [(C5H4,CR1R2,PAr2)2ZrCl2] (2: R1/R2 = CH3/CH3, H/CH3, H/aryl) with Li[B(C6F5)4] in dichloromethane solution results in chloride ligand abstraction (with LiCl precipitation) to yield the complexes [(C5H4,CR1R2,PAr2)2Zr,Cl+] (5), with both phosphanyl groups internally coordinated to the metal centre. Three possible diastereoisomers are observed in the case of 5c (R1 = H; R2 = CH3), while bulkier R2 substituents give higher selectivities. The thermally induced (reversible) cleavage of the Zr,phosphane linkage results in dynamic NMR behaviour. Gibbs activation energies of ,G,(298 K) = 14.8 ± 0.5 and 14.5 ± 0.5 kcal/mol were obtained for these intramolecular equilibration processes in the complexes trans - 5d (R1 = H; R2 = Ph) and trans - 5e (R1 = H; R2 = ferrocenyl), respectively. Treatment of the substituted (dimethylamino)methyl metallocene complexes [(C5H4,CR1R2,NMe2)2ZrCl2] (6a, 6b) with Li[B(C6F5)4] proceeds analogously to yield the cation systems [{C5H4,C(CH3)2,NMe2}2ZrCl+] (12a) and [{C5H4,CH(CH3),NMe2}2ZrCl+] (12b, three possible diastereoisomers). Both complexes have their pairs of amino groups coordinated to the metal centre. The complexes exhibit dynamic NMR spectra. Selective equilibration of the diastereotopic N(CH3)A(CH3)B resonances of complex 12a is observed [,G,(233 K) = 11.5 ± 0.2 kcal/mol], whereas the adjacent C(CH3)A(CH3)B methyl groups remain diastereotopic. The dynamic equilibration of the latter was observed at a markedly higher temperature [,G,(333 K) = 17.3 ± 0.2 kcal/mol]. Treatment of [{C5H4,C(CH3)2,NMe2}CpZrCl2] (10) with Li[B(C6F5)4] resulted in the formation of complex [{C5H4,C(CH3)2,NMe2}CpZr,Cl+] (11), which shows the internal ,N(CH3)A(CH)B equilibration proceeding with a markedly higher activation barrier [,G,(333 K) = 17.6 ± 0.2 kcal/mol] than in 12a, and a stereochemical memory effect indicative of solvent coordination to the metal centre of the resulting highly electrophilic chlorozirconocene cation intermediate. Complex 11 was characterised by an X-ray crystal structure analysis, which shows the internal Zr,amine coordination. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003) [source] Liquid,vapour partition of ethanol in bakery productsFLAVOUR AND FRAGRANCE JOURNAL, Issue 1 2006Paola Pittia Abstract Ethanol is a common ingredient in formulated foods, naturally present or added in liquid form in order to achieve the desired sensorial properties. In many complex foods this volatile compound could also serve interesting technological functions, as well as extending their shelf-lives, owing to its capacity to inhibit or reduce the rate of microbial growth. At the European level there are no known restrictions regarding the use of ethanol in foods as a preservative, while in Italy, current regulations allow its addition as an anti-moulding agent in pre-packed bread, at a maximum concentration of 2% on a dry weight basis. This research studied the effect of water activity (aw) and water content on the ethanol vapour pressure of sliced white bread, previously equilibrated at various aw values and with 2% ethanol added. Different aw values were obtained by both rehydration from previously freeze-dried bread, and dehydration from the fresh product. The results showed that both aw and moisture affected the vapour pressure of ethanol as a consequence of water,solute and ethanol,solute interactions in the matrix. These interactions varied according to the modality of equilibration (desorption or absorption) at a given aw. The results are discussed in terms of ethanol activity (ae), computed as the ratio between the ethanol vapour pressure in bread and the vapour pressure of pure ethanol at the same temperature. This index, analogous to aw, proved to be useful in evaluating the ,freedom' of the ethanol present in a food matrix to be released in the vapour phase. Copyright © 2006 John Wiley & Sons, Ltd. [source] Evolution of Permanent Deformations (or Memory) in Nafion 117 Membranes with Changes in Temperature, Relative Humidity and Time, and Its Importance in the Development of Medium Temperature PEMFCs,FUEL CELLS, Issue 4 2009G. Alberti Abstract An important problem for medium temperature polymer electrolyte fuel cells (MT PEMFCs) operating in the temperature range 90,140,°C is the short time-life of proton conducting membranes. To shed some light on the empirical annealing treatments used for increasing the membrane durability, a systematic research on the effects of thermal treatments of Nafion 117 membranes was undertaken with the hope that the information obtained could be useful for a better understanding of the real limits for MT PEMFCs. Kinetic experiments showed that, for each couple of T,RH values, the water taken up from the membrane reaches a constant value only after long times of equilibration (,200,h). Taking into account that the enlargements provoked by the water-uptake remain as permanent deformations when the samples are cooled, it was found that the evolution of the deformations provoked by changes in temperature and RH can be conveniently estimated at 20,°C by determining the water taken up after equilibration in liquid water. By relating the counter-elastic index of the matrix (nc(m)) to the extent of these deformations, a set of equations were obtained which allowed us to predict their evolution with changes of temperature and relative humidity. A good agreement with experimental values was found. The importance of this discovery for the development of MT PEMFCs is discussed. [source] The mechanism of fluid infiltration in peridotites at Almklovdalen, western NorwayGEOFLUIDS (ELECTRONIC), Issue 3 2002O. Kostenko Abstract A major Alpine-type peridotite located at Almklovdalen in the Western Gneiss Region of Norway was infiltrated by aqueous fluids at several stages during late Caledonian uplift and retrogressive metamorphism. Following peak metamorphic conditions in the garnet,peridotite stability field, the peridotite experienced pervasive fluid infiltration and retrogression in the chlorite,peridotite stability field. Subsequently, the peridotite was infiltrated locally by nonreactive fluids along fracture networks forming pipe-like structures, typically on the order of 10 m wide. Fluid migration away from the fractures into the initially impermeable peridotite matrix was facilitated by pervasive dilation of grain boundaries and the formation of intragranular hydrofractures. Microstructural observations of serpentine occupying the originally fluid-filled inclusion space indicate that the pervasively infiltrating fluid was characterized by a high dihedral angle (, > 60°) and ,curled up' into discontinuous channels and fluid inclusion arrays following the infiltration event. Re-equilibration of the fluid phase topology took place by growth and dissolution processes driven by the excess surface energy represented by the ,forcefully' introduced external fluid. Pervasive fluid introduction into the peridotite reduced local effective stresses, increased the effective grain boundary diffusion rates and caused extensive recrystallization and some grain coarsening of the infiltrated volumes. Grain boundary migration associated with this recrystallization swept off abundant intragranular fluid inclusions in the original chlorite peridotite, leading to a significant colour change of the rock. This colour change defines a relatively sharp front typically located 1,20 cm away from the fractures where the nonreactive fluids originally entered the peridotite. Our observations demonstrate how crustal rocks may be pervasively infiltrated by fluids with high dihedral angles (, > 60°) and emphasize the coupling between hydrofracturing and textural equilibration of the grain boundary networks and the fluid phase topology. [source] Invasion of a karst aquifer by hydrothermal fluids: evidence from stable isotopic compositions of cave mineralizationGEOFLUIDS (ELECTRONIC), Issue 2 2001S. H. Bottrell Abstract Mineral deposits in the Cupp-Coutunn/Promeszutochnaya cave system (Turkmenia, central Asia) record a phase of hydrothermal activity within a pre-existing karstic groundwater conduit system. Hydrothermal fluids entered the caves through fault zones and deposited sulphate, sulphide and carbonate minerals under phreatic conditions. Locally, intense alteration of limestone wall rocks also occurred at this stage. Elsewhere in the region, similar faults contain economic quantities of galena and elemental sulphur mineralization. Comparisons between the Pb and S isotope compositions of minerals found in cave and ore deposits confirm the link between economic mineralization and hydrothermal activity at Cupp-Coutunn. The predominance of sulphate mineralization in Cupp-Coutunn implies that the fluids were more oxidized in the higher permeability zone associated with the karst aquifer. A slight increase in the ,34S of sulphate minerals and a corresponding ,34S decrease in sulphides suggest that partial isotopic equilibration occurred during oxidation. Carbonate minerals indicate that the hydrothermal fluid was enriched in 18O (,18OSMOW , + 10,) relative to meteoric groundwater and seawater. Estimated values for ,13CDIC (,13CPDB , , 13,) are consistent with compositions expected for dissolved inorganic carbon (DIC) derived from the products of thermal decomposition of organic matter and dissolution of marine carbonate. Values derived for ,13CDIC and ,18Owater indicate that the hydrothermal fluid was of basinal brine origin, generated by extensive water,rock interaction. Following the hydrothermal phase, speleothemic minerals were precipitated under vadose conditions. Speleothemic sulphates show a bimodal sulphur isotope distribution. One group has compositions similar to the hydrothermal sulphates, whilst the second group is characterized by higher ,34S values. This latter group may either record the effects of microbial sulphate reduction, or reflect the introduction of sulphate-rich groundwater generated by the dissolution of overlying evaporites. Oxygen isotope compositions show that calcite speleothems were precipitated from nonthermal groundwater of meteoric origin. Carbonate speleothems are relatively enriched in 13C compared to most cave deposits, but can be explained by normal speleothem-forming processes under thin, arid-zone soils dominated by C4 vegetation. However, the presence of sulphate speleothems, with isotopic compositions indicative of the oxidation of hydrothermal sulphide, implies that CO2 derived by reaction of limestone with sulphuric acid (,condensation corrosion') contributed to the formation of 13C-enriched speleothem deposits. [source] Exhumation paths of high-pressure metapelites obtained from local equilibria for chlorite,phengite assemblagesGEOLOGICAL JOURNAL, Issue 3-4 2000O. Vidal Abstract The compositional variation of phengite and chlorite pairs in rocks sampled across the metamorphic zonation of three different blueschist belts (Cycladic, Sambagawa and Schistes Lustrés) is characterized from the regional to the thin-section scale. The different sample suites show different compositional trends, but similar trends are observed at the regional and thin-section scale in the same unit. At the thin-section scale, several local chlorite,phengite equilibria involving minerals of different compositions can be identified. These observations suggest that at temperature T <,,550°C, equilibration of chlorite and phengite compositions with varying pressure and temperature is controlled by crystallization/recrystallization processes rather than by changing the composition of older grains by lattice diffusion. In some instances, the relative time of growth of the different phyllosilicate generations can be determined using microstructural criteria. The observed compositional variations are interpreted and quantified in terms of pressure (P) and temperature (T) variations using new thermodynamic solution models accounting for the Tschermak, di/trioctahedral, and pyrophyllitic substitutions. Chlorite,phengite local equilibria constrain the shape of the exhumation P,T paths of the rocks under consideration. The assemblage chlorite,phengite,paragonite,albite,quartz,H2O can be used to constrain different P,T paths for Tinos and Sambagawa blueschists. Copyright © 2000 John Wiley & Sons, Ltd. [source] Electrically Conductive Thin Films Prepared from Layer-by-Layer Assembly of Graphite PlateletsADVANCED FUNCTIONAL MATERIALS, Issue 7 2009Mubarak Alazemi Abstract Layer-by-layer (LBL) assembly of carbon nanoparticles for low electrical contact resistance thin film applications is demonstrated. The nanoparticles consist of irregularly shaped graphite platelets, with acrylamide/,, -methacryl-oxyethyl-trimethyl-ammonium copolymer as the cationic binder. Nanoparticle zeta (,,) potential and thereby electrostatic interactions are varied by altering the pH of graphite suspension as well as that of the binder suspension. Film thickness as a function of zeta potential, immersion time, and the number of layers deposited is obtained using Monte Carlo simulation of the energy dispersive spectroscopy measurements. Multilayer film surface morphology is visualized via field-emission scanning electron microscopy and atomic-force microscopy. Thin film electrical properties are characterized using electrical contact resistance measurements. Graphite nanoparticles are found to self-assemble onto gold substrates through two distinct yet overlapping mechanisms. The first mechanism is characterized by logarithmic carbon uptake with respect to the number of deposition cycles and slow clustering of nanoparticles on the gold surface. The second mechanism results from more rapid LBL nanoparticle assembly and is characterized by linear weight uptake with respect to the number of deposition cycles and a constant bilayer thickness of 15 to 21,nm. Thin-film electrical contact resistance is found to be proportional to the thickness after equilibration of the bilayer structure. Measured values range from 1.6,m,,cm,2 at 173,nm to 3.5,m,,cm,2 at 276,nm. Coating volume resistivity is reduced when electrostatic interactions are enhanced during LBL assembly. [source] Dermally adhered soil: 2.INTEGRATED ENVIRONMENTAL ASSESSMENT AND MANAGEMENT, Issue 4 2006Reconstruction of dry-sieve particle-size distributions from wet-sieve data Abstract In the evaluation of soil particle-size effects on environmental processes, particle-size distributions are measured by either wet or dry sieving. Commonly, size distributions determined by wet and dry sieving differ because some particles disaggregate in water. Whereas the dry-sieve distributions are most relevant to the study of soil adherence to skin, soil can be recovered from skin only by washing with the potential for disaggregation whether or not it is subsequently wet or dry sieved. Thus, the possibility exists that wet-sieving measurements of the particle sizes that adhered to the skin could be skewed toward the smaller fractions. This paper provides a method by which dry-sieve particle-size distributions can be reconstructed from wet-sieve particle-size distributions for the same soil. The approach combines mass balances with a series of experiments in which wet sieving was applied to dry-sieve fractions from the original soil. Unless the soil moisture content is high (i.e., greater than or equal to the water content after equilibration with water-saturated air), only the soil particles of diameters less than about 63 ,m adhere to the skin. Because of this, the adhering particle-size distribution calculated using the reconstruction method was not significantly different from the wet-sieving determinations. [source] Exchange Rate Volatility and Democratization in Emerging Market CountriesINTERNATIONAL STUDIES QUARTERLY, Issue 2 2003Jude C. Hays We examine some of the consequences of financial globalization for democratization in emerging market economies by focusing on the currency markets of four Asian countries at different stages of democratic development. Using political data of various kinds,including a new events data series,and the Markov regime switching model from empirical macroeconomics, we show that in young and incipient democracies politics continuously causes changes in the probability of experiencing two different currency market equilibria: a high volatility "contagion" regime and a low volatility "fundamentals" regime. The kind of political events that affect currency market equilibration varies cross-nationally depending on the degree to which the polity of a country is democratic and its policymaking transparent. The results help us better gauge how and the extent to which democratization is compatible with financial globalization. [source] Kinetic and heat transfer modeling of rubber blends' sulfur vulcanization with N - t -butylbenzothiazole-sulfenamide and N,N -di- t -butylbenzothiazole-sulfenamideJOURNAL OF APPLIED POLYMER SCIENCE, Issue 1 2007Likozar Abstract Vulcanization kinetics and heat transfer for various blends of natural (NR) and polybutadiene (BR) rubber were studied simultaneously using a mechanistic approach when developing vulcanization model kinetics. Rubber process analyzer (RPA), dynamic scanning calorimetry (DSC), and Fourier transform infrared spectroscopy (FTIR) methods were used for the study. The model reaction scheme was based on one of the best possible proposed individual reaction mechanisms. Molecular modeling was applied to distinguish between the reactivity of chemically similar species. The kinetics of N - t -butylbenzothiazole-sulfenamide (TBBS) and N,N -di- t -butylbenzothiazole-sulfenamide (TBSI) were treated separately using FTIR experiment data, and then incorporated in a model suitable for two-accelerator vulcanization. The proposed model quite well describes the thermal equilibration during the induction period despite a few simplifications. During cure and over-cure periods the course of vulcanization was described using a rigorous kinetic model. Physical and chemical model parameters were calculated from experimental data. Average heat transfer coefficient minimum during induction period was found to be at a weight ratio of BR and NR 1 : 1. The activation energy of significant reactions between rubber and other species was found to vary linearly with vulcanization compound composition. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 293,307, 2007 [source] Enhancement of activity and selectivity in lipase-catalyzed transesterification in ionic liquids by the use of additivesJOURNAL OF CHEMICAL TECHNOLOGY & BIOTECHNOLOGY, Issue 10 2007Francisco J Hernández-Fernández Abstract BACKGROUND: Seven ionic liquids (ILs) based on 1-alkyl-3-methylimidazolium cation in combination with hexafluorophosphate and bis{(trifluoromethyl)sulfonyl}imide anions were tested as reaction media for lipase-catalyzed transesterification in low water conditions. With the aim of improving the activity and/or selectivity of the lipase, various treatments were applied to ionic liquid media such as equilibration with aqueous solutions of salts, NaHCO3 or Na2CO3, or the addition of a catalytic amount of a non-reactive organic base to the reaction mixture, triethylamine. RESULTS: The treated ionic liquids were shown to be excellent media for lipase-catalyzed ester synthesis by transesterification compared with conventional organic solvents, such as n -hexane. All treatments were found to enhance the synthetic activity of the enzyme, the best results being achieved with the addition of triethylamine. The addition of a catalytic amount of this base to the ILs resulted in a significant increase in both the synthetic activity and selectivity values. For instance, the synthetic activity in [emim+][TfN2,] was enhanced more than 12 times and the selectivity increased from 86% to 95% when triethylamine was used. CONCLUSION: These treatments could be easy-to-use approaches to improve the efficiency of enzymatic reactions in ionic liquids when the reaction does not proceed smoothly. Copyright © 2007 Society of Chemical Industry [source] Using the local elevation method to construct optimized umbrella sampling potentials: Calculation of the relative free energies and interconversion barriers of glucopyranose ring conformers in waterJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 1 2010Halvor S. Hansen Abstract A method is proposed to combine the local elevation (LE) conformational searching and the umbrella sampling (US) conformational sampling approaches into a single local elevation umbrella sampling (LEUS) scheme for (explicit-solvent) molecular dynamics (MD) simulations. In this approach, an initial (relatively short) LE build-up (searching) phase is used to construct an optimized biasing potential within a subspace of conformationally relevant degrees of freedom, that is then used in a (comparatively longer) US sampling phase. This scheme dramatically enhances (in comparison with plain MD) the sampling power of MD simulations, taking advantage of the fact that the preoptimized biasing potential represents a reasonable approximation to the negative of the free energy surface in the considered conformational subspace. The method is applied to the calculation of the relative free energies of ,- D -glucopyranose ring conformers in water (within the GROMOS 45A4 force field). Different schemes to assign sampled conformational regions to distinct states are also compared. This approach, which bears some analogies with adaptive umbrella sampling and metadynamics (but within a very distinct implementation), is shown to be: (i) efficient (nearly all the computational effort is invested in the actual sampling phase rather than in searching and equilibration); (ii) robust (the method is only weakly sensitive to the details of the build-up protocol, even for relatively short build-up times); (iii) versatile (a LEUS biasing potential database could easily be preoptimized for small molecules and assembled on a fragment basis for larger ones). © 2009 Wiley Periodicals, Inc. J Comput Chem 2010 [source] Low barrier kinetics: Dependence on observables and free energy surfaceJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 2 2006Hairong Ma Abstract Dynamics on free energy surfaces with high activation barriers are usually treated by few-state kinetics models, yielding characteristic rate coefficients and amplitudes depending on the connectivity of the states. When the barriers are low (<3 kT), the assumption of instantaneous equilibration of the transition state, and hence, the few-state kinetics treatment, break down. Langevin dynamics is used here to explore the characteristic trends that occur in such cases, as a function of barrier height, number of barriers, dimensionality of the free energy surface, and switching functions that describe how spectroscopic probes vary from reactant to product. The result is a systematic phenomenological description of low barrier kinetics and dynamics. © 2005 Wiley Periodicals, Inc. J Comput Chem 27: 125,134, 2006 [source] Stochastic generator of chemical structure.JOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 6 2001Abstract A novel computational technique to generate close-to-equilibrium crosslinked polymeric systems is proposed. Compared to the current state-of-the-art equilibration methods, the new technique appears to be faster by several orders of magnitude. The main advantage of the technique is that one can circumvent the bottlenecks in configuration space that inhibit relaxation in molecular dynamics or Monte Carlo simulations. The problem of polymer equilibration described by continuous equations in molecular dynamics is reduced to a discrete representation where solutions are approximated by simple algorithms. In the current study, a series of coarse-grained, united-atom, and fully atomistic crosslinked networks has been generated. Network statistics and topology, X-ray scattering intensities, and elastic properties are tested vs. experimental results and similar models generated using molecular dynamics and Monte Carlo simulations. The results demonstrate the efficiency of this new method for generating large realistic polymeric systems up to 1.4 M atoms. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 580,590, 2001 [source] Evolution of the distribution of dispersal distance under distance-dependent cost of dispersalJOURNAL OF EVOLUTIONARY BIOLOGY, Issue 4 2002F. Rousset Abstract We analyse the evolution of the distribution of dispersal distances in a stable and homogeneous environment in one- and two-dimensional habitats. In this model, dispersal evolves to avoid the competition between relatives although some cost might be associated with this behaviour. The evolutionarily stable dispersal distribution is characterized by an equilibration of the fitness gains among all the different dispersal distances. This cost-benefit argument has heuristic value and facilitates the comprehension of results obtained numerically. In particular, it explains why some minimal or maximal probability of dispersal may evolve at intermediate distances when the cost of dispersal function is an increasing function of distance. We also show that kin selection may favour long range dispersal even if the survival cost of dispersal is very high, provided the survival probability does not vanish at long distances. [source] Effect of pH on Microstructure and Characteristics of Cream CheeseJOURNAL OF FOOD SCIENCE, Issue 2 2009R.R. Monteiro ABSTRACT:, This study evaluated the effect of pH on the microstructure of cream cheese and compared pH-induced changes in its microstructure with concomitant changes in cheese firmness and meltability. On 4 different days, experimental batches of cultured hot pack cream cheese were manufactured and analyzed for initial chemical composition. The cheeses were then sectioned into samples that were randomly assigned to 7 different treatment groups. Three groups were exposed to ammonia vapor for 1, 3, and 5 min to increase the pH; 3 groups were exposed to acetic acid vapor for 30, 60, and 90 min to decrease the pH; and 1 unexposed group served as the control. After equilibration at 4 °C, samples were analyzed for pH, firmness, meltability, and microstructure by scanning electron microscopy. The effects of experimental treatments on cheese pH, firmness, and meltability were analyzed by randomized complete block analysis of variance (ANOVA). Relationships between cheese pH and firmness and meltability were evaluated by regression. Experimental treatments significantly affected cheese pH, firmness, and meltability. Cheese firmness decreased and meltability increased with increasing pH from about pH 4.2 to 6.8. Cheese microstructure also changed dramatically over the same approximate pH range. Specifically, the volume of the protein network surrounding the fat droplets increased markedly with increasing pH, presumably due to casein swelling. These data support the hypothesis that protein-to-water interactions increased as the cheese pH increased, which gave rise to progressive swelling of the casein network, softer texture, and increased meltability. [source] | ||||||||||||||||||||||||||||||||||||||||||||||