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Gibbs Free Energy (Gibb + free_energy)
Terms modified by Gibbs Free Energy Selected AbstractsSimultaneous Determination of Gibbs Free Energies of Formation of Sr2RhO4(s) and Sr4RhO6(s) Using Solid-State Electrochemical Cells.CHEMINFORM, Issue 1 2005Aparna Banerjee No abstract is available for this article. [source] ChemInform Abstract: Solvent Effect on Relative Gibbs Free Energy and Structural Property of Eu3* to Yb3+ Ion Mutation: A Monte Carlo Simulation Study.CHEMINFORM, Issue 10 2002Hag-Sung Kim Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a "Full Text" option. The original article is trackable via the "References" option. [source] Interpretation of biological activity data of bacterial endotoxins by simple molecular models of mechanism of actionFEBS JOURNAL, Issue 3 2000Vladimir Frecer Lipid A moiety has been identified as the bioactive component of bacterial endotoxins (lipopolysaccharides). However, the molecular mechanism of biological activity of lipid A is still not fully understood. This paper contributes to understanding of the molecular mechanism of action of bacterial endotoxins by comparing molecular modelling results for two possible mechanisms with the underlying experimental data. Mechanisms of action involving specific binding of lipid A to a protein receptor as well as nonspecific intercalation into phospholipid membrane of a host cell were modelled and analysed. As the cellular receptor for endotoxin has not been identified, a model of a peptidic pseudoreceptor was proposed, based on molecular structure, symmetry of the lipid A moiety and the observed character of endotoxin-binding sites in proteins. We have studied the monomeric form of lipid A from Escherichia coli and its seven synthetic analogues with varying numbers of phosphate groups and correlated them with known biological activities determined by the Limulus assay. Gibbs free energies associated with the interaction of lipid A with the pseudoreceptor model and intercalation into phospholipid membrane calculated by molecular mechanics and molecular dynamics methods were used to compare the two possible mechanisms of action. The results suggest that specific binding of lipid A analogues to the peptidic pseudoreceptor carrying an amphipathic cationic binding pattern BHPHB (B, basic; H, hydrophobic; P, polar residue, respectively) is energetically more favourable than intercalation into the phospholipid membrane. In addition, binding affinities of lipid A analogues to the best minimum binding sequence KFSFK of the pseudoreceptor correlated with the experimental Limulus activity parameter. This correlation enabled us to rationalize the observed relationship between the number and position of the phosphate groups in the lipid A moiety and its biological activity in terms of specific ligand,receptor interactions. If lipid A,receptor interaction involves formation of phosphate-ammonium ion-pair(s) with cationic amino-acid residues, the specific mechanism of action was fully consistent with the underlying experimental data. As a consequence, recognition of lipid A variants by an amphipathic binding sequence BHPHB of a host-cell protein receptor might represent the initial and/or rate-determining molecular event of the mechanism of action of lipid A (or endotoxin). The insight into the molecular mechanism of action and the structure of the lipid A-binding pattern have potential implications for rational drug design strategies of endotoxin-neutralizing agents or binding factors. [source] Energetics of overall metabolic reactions of thermophilic and hyperthermophilic Archaea and BacteriaFEMS MICROBIOLOGY REVIEWS, Issue 2 2001Jan P. Amend Abstract Thermophilic and hyperthermophilic Archaea and Bacteria have been isolated from marine hydrothermal systems, heated sediments, continental solfataras, hot springs, water heaters, and industrial waste. They catalyze a tremendous array of widely varying metabolic processes. As determined in the laboratory, electron donors in thermophilic and hyperthermophilic microbial redox reactions include H2, Fe2+, H2S, S, S2O32,, S4O62,, sulfide minerals, CH4, various mono-, di-, and hydroxy-carboxylic acids, alcohols, amino acids, and complex organic substrates; electron acceptors include O2, Fe3+, CO2, CO, NO3,, NO2,, NO, N2O, SO42,, SO32,, S2O32,, and S. Although many assimilatory and dissimilatory metabolic reactions have been identified for these groups of microorganisms, little attention has been paid to the energetics of these reactions. In this review, standard molal Gibbs free energies (,Gr°) as a function of temperature to 200°C are tabulated for 370 organic and inorganic redox, disproportionation, dissociation, hydrolysis, and solubility reactions directly or indirectly involved in microbial metabolism. To calculate values of ,Gr° for these and countless other reactions, the apparent standard molal Gibbs free energies of formation (,G°) at temperatures to 200°C are given for 307 solids, liquids, gases, and aqueous solutes. It is shown that values of ,Gr° for many microbially mediated reactions are highly temperature dependent, and that adopting values determined at 25°C for systems at elevated temperatures introduces significant and unnecessary errors. The metabolic processes considered here involve compounds that belong to the following chemical systems: H,O, H,O,N, H,O,S, H,O,N,S, H,O,Cinorganic, H,O,C, H,O,N,C, H,O,S,C, H,O,N,S,Camino acids, H,O,S,C,metals/minerals, and H,O,P. For four metabolic reactions of particular interest in thermophily and hyperthermophily (knallgas reaction, anaerobic sulfur and nitrate reduction, and autotrophic methanogenesis), values of the overall Gibbs free energy (,Gr) as a function of temperature are calculated for a wide range of chemical compositions likely to be present in near-surface and deep hydrothermal and geothermal systems. [source] Determination of isotope fractionation factors and quantification of carbon flow by stable carbon isotope signatures in a methanogenic rice root model systemGEOBIOLOGY, Issue 2 2006H. PENNING ABSTRACT Methanogenic processes can be quantified by stable carbon isotopes, if necessary modeling parameters, especially fractionation factors, are known. Anoxically incubated rice roots are a model system with a dynamic microbial community and thus suitable to investigate principal geochemical processes in anoxic natural systems. Here we applied an inhibitor of acetoclastic methanogenesis (methyl fluoride), calculated the thermodynamics of the involved processes, and analyzed the carbon stable isotope signatures of CO2, CH4, propionate, acetate and the methyl carbon of acetate to characterize the carbon flow during anaerobic degradation of rice roots to the final products CO2 and CH4. Methyl fluoride inhibited acetoclastic methanogenesis and thus allowed to quantify the fractionation factor of CH4 production from H2/CO2. Since our model system was not affected by H2 gradients, the fractionation factor could alternatively be determined from the Gibbs free energies of hydrogenotrophic methanogenesis. The fractionation factor of acetoclastic methanogenesis was also experimentally determined. The data were used for successfully modeling the carbon flow. The model results were in agreement with the measured process data, but were sensitive to even small changes in the fractionation factor of hydrogenotrophic methanogenesis. Our study demonstrates that stable carbon isotope signatures are a proper tool to quantify carbon flow, if fractionation factors are determined precisely. [source] Structures and energies of D -galactose and galabiose conformers as calculated by ab initio and semiempirical methodsJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 7 2003Majda Rahal-Sekkal Abstract Optimized geometries and total energies of some conformers of ,- and ,- D -galactose have been calculated using the RHF/6-31G* ab initio method. Vibrational frequencies were computed at the 6-31G* level for the conformers that favor internal hydrogen bonding, in order to evaluate their enthalpies, entropies, Gibbs free energies, and then their structural stabilities. The semiempirical AM1, PM3, MNDO methods have also been performed on the conformers GG, GT, and TG of ,- and ,- D -galactose. In order to test the reliability of each semiempirical method, the obtained structures and energies from the AM1, PM3, and MNDO methods have been compared to those achieved using the RHF/6-31G* ab initio method. The MNDO method has not been investigated further, because of the large deviation in the structural parameters compared with those obtained by the ab initio method for the galactose. The semiempirical method that has yielded the best results is AM1, and it has been chosen to perform structural and energy calculations on the galabiose molecule (the disaccharides constituted by two galactose units , 1,4 linked). The goal of such calculations is to draw the energy surface maps for this disaccharide. To realize each map, 144 different possible conformations resulting from the rotations of the two torsional angles , and , of the glycosidic linkage are considered. In each calculation, at each increment of , and ,, using a step of 30° from 0 to 330°, the energy optimization is employed. In this article, we report also calculations concerning the galabiose molecule using different ab initio levels such as RHF/6-31G*, RHF/6-31G**, and B3Lyp/6-31G*. © 2003 Wiley Periodicals, Inc. J Comput Chem 24: 806,818, 2003 [source] Synthesis of the hydrophobic,hydrophilic macroporous poly divinylbenzene/poly(sodium acrylate) IPN resin and adsorption performance for berberinePOLYMERS FOR ADVANCED TECHNOLOGIES, Issue 12 2009Guqing Xiao Abstract The macroporous polydivinylbenzene/poly(methyl acrylate) interpenetrating polymer network (PDVB/PMA IPN) was prepared by the sequential suspension polymerization method, and was modified to be hydrophobic,hydrophilic macroporous polydivinylbenzene/poly (sodium acrylate) IPN (PDVB/PNaA IPN) by converting the PMA to PNaA under the condition of base. The effects of different mass ratio of the two networks and different cross-linking degree of the second network on the pore structure and adsorption capacity of PDVB/PNaA IPN resin were studied. The PDVB/PNaA IPN resin whose adsorption quantity is the biggest was chosen to study further. The pore structure, the weak acid exchange capacity, the water retention capacity, and the swelling ability of PDVB/PNaA IPN resin were measured. The study focused on the adsorption isotherms of berberine at different temperatures. Isosteric adsorption enthalpy, adsorption Gibbs free energies can be calculated according to thermodynamic functions. The results show that the saturated adsorption quantity of berberine is up to 109.4,mg,ml,1 (wet resin) by the way of dynamic adsorption and desorption experiment. The resin could be reused by the mixture with 0.5% sodium chloride and 80% ethanol. On the one hand the hydrophobic PDVB in the PDVB/PNaA IPN resin has the ability of adsorption using ,,, interaction, and on the other hand the hydrophilic PNaA in the PDVB/PNaA IPN resin has the ability of adsorption using ion exchange interaction. An important conclusion can be drawn that the PDVB/PNaA IPN resin has a promising application prospect in extracting and separating quaternary ammonium type alkaloids such as berberine. Copyright © 2009 John Wiley & Sons, Ltd. [source] Rapid structural determination of alkaloids in a crude extract of Stemona saxorum by high-performance liquid chromatography/electrospray ionization coupled with tandem mass spectrometryRAPID COMMUNICATIONS IN MASS SPECTROMETRY, Issue 23 2009Shu-Ying Peng The electrospray ionization (ESI) mass spectrometric behavior of five Stemona alkaloids, stemokerrin, oxystemokerrin, oxystemokerrilactone, oxystemokerrin N -oxide and stemokerrin N -oxide, was studied using an ESI tandem mass technique (MSn). These compounds, isolated from Stemonasaxorum endemic in Vietnam, represent a class of alkaloids containing a pyrido[1,2-a]azepine A,B-ring core with a 1-hydroxypropyl side chain attached to C-4. Their fragmentation pathways were elucidated by ESI-MSn results and the elemental composition of the major product ions was confirmed by accurate mass measurement. In order to rationalize some fragmentation pathways, the relative Gibbs free energies of some product ions were estimated using the B3LYP/6-31+G(d) method. Based on the ESI-MSn results of five reference compounds, a reversed-phase high-performance liquid chromatography with tandem mass spectrometry (RP-HPLC/MSn) method was developed for the characterization of Stemona alkaloids with a pyrido[1,2-a]azepine A,B-ring core from the extract of S. saxorum. A total of 41 components were rapidly identified or tentatively characterized, of which 12 compounds were identified as Stemona alkaloids with a pyrido[1,2-a]azepine A,B-ring core, including four new compounds. This method is convenient and sensitive, especially for minor components in complex natural product extracts. Copyright © 2009 John Wiley & Sons, Ltd. [source] Complexes between Lithium Cation and Diphenylalkanes in the Gas Phase: The Pincer EffectCHEMISTRY - A EUROPEAN JOURNAL, Issue 29 2006Jean-François Gal Prof. Abstract The gas-phase lithium cation basicities (LCB values, Gibbs free energies of binding) of ,,,-diphenylalkanes Ph-(CH2)n -Ph (n=2, 3, or 7) and 1,1-diphenylethane Ph-CH(Me)-Ph were investigated by means of Fourier-transform ion cyclotron resonance (FTICR) mass spectrometry. Their structures, and those of the corresponding Li+ complexes were optimized at the B3LYP/6-31G(d) level and their relative stabilities calculated at the B3LYP/6-311+G(3df,2p)//B3LYP/6-31G(d) level. Whereas the most stable conformers of the free diphenylalkanes were found to adopt a completely stretched aliphatic chain connecting the two benzene rings, the most stable Li+ complexes correspond to conformers in which the alkali metal cation interacts simultaneously with both benzene rings through the folding of the aliphatic chain ("pincer effect"). This chelation brings about a significant enhancement of the Li+ binding enthalpies (LBE values), which were calculated to be approximately 75 kJ,mol,1 higher than those evaluated for conventional (singly coordinated) , complexes in which the metal cation interacts with only one of the benzene rings. The increase of the corresponding lithium cation basicities, however, (Gibbs free energies of Li+ binding, LCB values) was calculated to be smaller by approximately 15 kJ,mol,1 as the pincer effect is entropically disfavored. The good agreement between the calculated LCB values, assuming a statistical distribution of the different conformers present in the gas phase, and the experimental LCB values measured by means of FTICR mass spectrometry are considered indirect evidence of the existence of the pincer effect. [source] Thermodynamic Analysis of Receptors Based on Guanidinium/Boronic Acid Groups for the Complexation of Carboxylates, ,-Hydroxycarboxylates, and Diols: Driving Force for Binding and CooperativityCHEMISTRY - A EUROPEAN JOURNAL, Issue 15 2004Sheryl L. Wiskur Dr. Abstract The thermodynamics of guanidinium and boronic acid interactions with carboxylates, ,-hydroxycarboxylates, and diols were studied by determination of the binding constants of a variety of different guests to four different hosts (7,10). Each host contains a different combination of guanidinium groups and boronic acids. The guests included molecules with carboxylate and/or diol moieties, such as citrate, tartrate, and fructose, among others. The Gibbs free energies of binding were determined by UV/Vis absorption spectroscopy, by use of indicator displacement assays. The receptor based on three guanidinium groups (7) was selective for the tricarboxylate guest. The receptors that incorporated boronic acids (8,10) had higher affinities for guests that included ,-hydroxycarboxylate and catechol moieties over guests containing only carboxylates or alkanediols. Isothermal titration calorimetry revealed the enthalpic and entropic contributions to the Gibbs free energies of binding. The binding of citrate and tartrate was investigated with hosts 7,10, for which all the binding events were exothermic, with positive entropy. Because of the selectivity of hosts 8,10, a simple boronic acid (14) was also investigated and determined to be selective for ,-hydroxycarboxylates and catechols over amino acids and alkanediols. Further, the cooperativity of 8 and 9 in binding tartrate was also investigated, revealing little or no cooperativity with 8, but negative cooperativity with 9. A linear entropy/enthalpy compensation relationship for all the hosts 7,10, 14, and the carboxylate-/diol-containing guests was also obtained. This relationship indicates that increasing enthalpy of binding is offset by similar losses in entropy for molecular recognition involving guanidinium and boronic acid groups. [source] Solubility studies on valdecoxib in the presence of carriers, cosolvents, and surfactantsDRUG DEVELOPMENT RESEARCH, Issue 1 2004Kashappa Goud H. Desai Abstract Enhancement of the solubility of valdecoxib was examined using a series of hydrophilic carriers (mannitol, polyethylene glycol (PEG) 4000, PEG 6000, PEG 8000, and urea), surfactants (Tween-20, Tween-80, and sodium lauryl sulfate [SLS]) and cosolvents (ethanol, methanol, and glycerol) at 37°C. The solubility of valdecoxib could be enhanced significantly using PEG 4000 as a carrier, ethanol as cosolvent, and SLS as a surfactant. Because the solubility of valdecoxib increased dramatically in the presence of polyethylene glycols, these are suitable dispersing agents for preparing solid dispersions containing valdecoxib. Gibbs free energy (,G) values were all negative for all hydrophilic carriers tested, indicating the spontaneous nature of valdecoxib solubilisation. Among the cosolvents, ethanol exhibited higher solubilization potential than methanol and glycerol. As the dielectric constant of the cosolvent,water mixtures decreased, the solubility of valdecoxib increased. Finally, SLS exerted maximum solubilization of valdecoxib when compared to Tween-20 or Tween-80. The crystallinity of valdecoxib was explored by X-ray diffraction study and showed numerous crystalline peaks. Examination of surface morphology by scanning electron microscopy depicted a near spherical shape of valdecoxib with irregular surface characteristics. Drug Dev. Res. 62:41,48, 2004. © 2004 Wiley-Liss, Inc. [source] Energetics of overall metabolic reactions of thermophilic and hyperthermophilic Archaea and BacteriaFEMS MICROBIOLOGY REVIEWS, Issue 2 2001Jan P. Amend Abstract Thermophilic and hyperthermophilic Archaea and Bacteria have been isolated from marine hydrothermal systems, heated sediments, continental solfataras, hot springs, water heaters, and industrial waste. They catalyze a tremendous array of widely varying metabolic processes. As determined in the laboratory, electron donors in thermophilic and hyperthermophilic microbial redox reactions include H2, Fe2+, H2S, S, S2O32,, S4O62,, sulfide minerals, CH4, various mono-, di-, and hydroxy-carboxylic acids, alcohols, amino acids, and complex organic substrates; electron acceptors include O2, Fe3+, CO2, CO, NO3,, NO2,, NO, N2O, SO42,, SO32,, S2O32,, and S. Although many assimilatory and dissimilatory metabolic reactions have been identified for these groups of microorganisms, little attention has been paid to the energetics of these reactions. In this review, standard molal Gibbs free energies (,Gr°) as a function of temperature to 200°C are tabulated for 370 organic and inorganic redox, disproportionation, dissociation, hydrolysis, and solubility reactions directly or indirectly involved in microbial metabolism. To calculate values of ,Gr° for these and countless other reactions, the apparent standard molal Gibbs free energies of formation (,G°) at temperatures to 200°C are given for 307 solids, liquids, gases, and aqueous solutes. It is shown that values of ,Gr° for many microbially mediated reactions are highly temperature dependent, and that adopting values determined at 25°C for systems at elevated temperatures introduces significant and unnecessary errors. The metabolic processes considered here involve compounds that belong to the following chemical systems: H,O, H,O,N, H,O,S, H,O,N,S, H,O,Cinorganic, H,O,C, H,O,N,C, H,O,S,C, H,O,N,S,Camino acids, H,O,S,C,metals/minerals, and H,O,P. For four metabolic reactions of particular interest in thermophily and hyperthermophily (knallgas reaction, anaerobic sulfur and nitrate reduction, and autotrophic methanogenesis), values of the overall Gibbs free energy (,Gr) as a function of temperature are calculated for a wide range of chemical compositions likely to be present in near-surface and deep hydrothermal and geothermal systems. [source] Energy Harvesting with Single-Ion-Selective Nanopores: A Concentration-Gradient-Driven Nanofluidic Power SourceADVANCED FUNCTIONAL MATERIALS, Issue 8 2010Wei Guo Abstract Inspired by biological systems that have the inherent skill to generate considerable bioelectricity from the salt content in fluids with highly selective ion channels and pumps on cell membranes, herein, a fully abiotic single-pore nanofluidic energy-harvesting system that efficiently converts Gibbs free energy in the form of a salinity gradient into electricity is demonstrated. The maximum power output with the individual nanopore approaches ,26,pW. By exploiting parallelization, the estimated power density can be enhanced by one to three orders over previous ion-exchange membranes. A theoretical description is proposed to explain the power generation with the salinity-gradient-driven nanofluidic system. Calculation results suggest that the electric-power generation and its efficiency can be further optimized by enhancing the surface-charge density (up to 100,mC m,2) and adopting the appropriate nanopore size (between 10 and 50,nm). This facile and cost-efficient energy-harvesting system has the potential to power biomedical tiny devices or construct future clean-energy recovery plants. [source] Ab initio analysis of monomers and dimers of trialkylphosphine oxides: Structural and thermodynamic stabilityINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 2 2009Thaciana Malaspina Abstract Structural and thermodynamic stabilities of monomers and dimers of trialkylphosphine oxides (TRPO) were studied using quantum chemistry calculations. Density functional theory calculations were carried out and the structures of four TRPO have been determined: TMPO (methyl; R = CH3), TEPO (ethyl; R = CH3CH2), TBPO (n -butyl; R = CH3(CH2)3), and TOPO (n -octyl; R = CH3(CH2)7). TRPO homodimers were investigated considering two isomeric possibilities for each dimer. Relative binding energies and the enthalpic and entropic contributions to the Gibbs free energy were calculated for all dimers. The formation of dimers from the individual monomeric TRPO species as a function of temperature was also analyzed. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009 [source] The Planck,Benzinger thermal work function in the condensation of water vaporINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 15 2006Paul W. Chun Abstract Based on the Planck,Benzinger thermal work function using Chun's method, the innate temperature-invariant enthalpy at 0 K, ,H0(T0), for the condensation of water vapor as well as the dimer, trimer, tetramer, and pentamer form in the vapor phase, was determined to be 0.447 kcal mol,1 for vapor, 1.127 for the dimer, 0.555 for the trimer, 0.236 for the tetramer, and 0.079 kcal mol,1 for the pentamer using ,G(T) data reported by Kell et al. in 1968 and Kell and McLaurin in 1969. These results suggest that the predominant dimeric form is the most stable of these n -mers. Using Nemethy and Scheraga's 1962 data for the Helmholtz free energy of liquid water, the value of ,H0(T0) was determined to be 1.21 kcal mol,1. This is very close to the value for the energy of the hydrogen bond EH of 1.32 kcal mol,1 reported by Nemethy and Scheraga, using statistical thermodynamics. It seems clear that very little energy is required for interconversion between the hypothetical supercooled water vapor and glassy water at 0 K. A hypothetical supercooled water vapor at 0 K is apparently almost as highly associated as glassy water at that temperature, suggesting a dynamic equilibrium between vapor and liquid. This water vapor condensation is highly similar in its thermodynamic behavior to that of sequence-specific pairwise (dipeptide) hydrophobic interaction, except that the negative Gibbs free energy change minimum at ,Ts,, the thermal setpoint for vapor condensation, where T,S = 0, occurs at a considerably lower temperature, 270 K (below 0°C) compared with ,350 K. The temperature of condensation ,Tcond, at which ,G(T) = 0, where water vapor begins to condense, was found to be 383 K. In the case of a sequence-specific pairwise hydrophobic interaction, the melting temperature, ,Tm,, where ,G(Tm) = 0 was found to be 460 K. Only between two temperature limits, ,Th, = 99 K and ,Tcond, = 383 K, where ,G(Tcond) = 0, is the net chemical driving force favorable for polymorphism of glassy water and hypothetical supercooled water vapor. Analysis of the water vapor condensation process based on the Planck,Benzinger thermal work function confirms that a thermodynamic molecular switch occurs at 10 K, wherein a change of sign in [,Cp(T)]cond leads to a true negative minimum in the Gibbs free energy of vapor condensation, and hence a maximum in the related equilibrium constant, Kcond. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006 [source] Optimization and analysis of nickel adsorption on microwave irradiated rice husk using response surface methodology (RSM)JOURNAL OF CHEMICAL TECHNOLOGY & BIOTECHNOLOGY, Issue 2 2009Magesh Ganesa Pillai Abstract BACKGROUND: The removal of heavy metals using adsorption techniques with low cost biosorbents is being extensively investigated. The improved adsorption is essentially due to the pores present in the adsorbent. One way of improving the porosity of the material is by irradiation of the precursor using microwaves. In the present study, the adsorption characteristics of nickel onto microwave-irradiated rice husks were studied and the process variables were optimized through response surface methodology (RSM). RESULT: The adsorption of nickel onto microwave-irradiated rice husk (MIRH) was found to be better than that of the raw rice husk (RRH). The kinetics of the adsorption of Ni(II) from aqueous solution onto MIRH was found to follow a pseudo-second-order model. Thermodynamic parameters such as standard Gibbs free energy (,G°), standard enthalpy (,H°), and standard entropy (,S°) were also evaluated. The thermodynamics of Ni(II) adsorption onto MIRH indicates that it is spontaneous and endothermic in nature. The response surface methodology (RSM) was employed to optimize the design parameters for the present process. CONCLUSION: Microwave-irradiated rice husk was found to be a suitable adsorbent for the removal of nickel(II) ions from aqueous solutions. The adsorption capacity of the rice husk was found to be 1.17 mg g,1. The optimized parameters for the current process were found as follows: adsorbent loading 2.8 g (100 mL),1; Initial adsorbate concentration 6 mg L,1; adsorption time 210 min.; and adsorption temperature 35 °C. Copyright © 2008 Society of Chemical Industry [source] A dual extremum principle in thermodynamics ,AICHE JOURNAL, Issue 8 2007Alexander Mitsos Abstract Phase equilibria of multicomponent mixtures are considered and a reinterpretation of the Gibbs tangent plane stability criterion is proposed via Lagrangian duality. The starting point is the natural primal problem of minimizing the Gibbs free energy subject to material balance. The stable phase split is the solution of the corresponding dual problem, providing a necessary and sufficient dual extremum principle. Only in the absence of duality gap is the physical phase split also the solution of the primal problem. The only requirements are continuity of the Gibbs free energy and the trivial requirement that each species is present in the overall composition. The number of phases is permitted to be infinite, and does not need to be known a priori. No assumption is made on the presence of all species in all phases. Case studies are presented based on the NRTL and UNIQUAC activity coefficient model. © 2007 American Institute of Chemical Engineers AIChE J, 2007 [source] Substituent effects in the Baeyer,Villiger reaction of acetophenones: a theoretical studyJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 7 2009Lino Reyes Abstract This paper reports the first complete theoretical study of substituent effects on the mechanism of the Baeyer, Villiger (BV) reaction in non-polar solvents taking into account the lowest-energy mechanism that has been proposed for this rearrangement which is non-ionic and fully concerted. The BV reaction of p -substituted acetophenones, p -XC6H4COCH3 (X,=,NO2, CN, H, CH3, OCH3), with performic (PFA) and trifluoroperacetic (TFPAA) acids, catalyzed by formic (FA) and trifluoroacetic (TFAA) acids, respectively, using the MPWB1K functional and the 6-311G(d,p) and 6-311++G(d,p) basis sets, are studied. Solvent effects are taken into account by means of the PCM continuum model using dichloromethane as solvent. Electron-donating substituents on the aryl group have a relatively small activation effect on the first step, but a pronounced activation effect on the second to the point of being able to change the rate-determining step (RDS) of the reaction, as observed in the case of p -methoxyacetophenone with TFPAA acids. After analyzing the changes in Gibbs free energy of activation, geometrical parameters, and charge distributions of the transition states (TSs), explanations are provided for the two distinct effects that substituents on the ketone have on the kinetics of the addition and migration steps of the BV oxidation. The effect of the acid/peracid pair used is also discussed. Copyright © 2008 John Wiley & Sons, Ltd. [source] Mechanism of 4-methyl-1,2,4-triazol-3-thione reaction with formaldehydeJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 5 2008Monika Wujec Abstract We have recently described theoretically the mechanism of the reaction between 4-methyl-1,2,4-triazol-3-thiol and formaldehyde that leads to the N- substituted product, N1 -hydroxymethyl-4-methyl-1,2,4-triazol-3-thione. New experimental findings indicate that the thione tautomeric form in reaction with formaldehyde also yields this product. This observation could not be explained on the basis of previous calculations, which predicted that the thione tautomer undergoes nucleophilic substitution at the sulfur atom, leading to the S- substituted product. We present theoretical explanation of the observed reactivity. We show that under experimental conditions this reaction proceeds with the intervention of the anionic form of the triazole with the Gibbs free energy of activation of only 1.8,kcal/mol. Copyright © 2008 John Wiley & Sons, Ltd. [source] Tricritical behavior of Ising metamagnet in both external longitudinal and transverse fieldsPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 9 2007Jing Liu Abstract A two-sublattice Ising metamagnet in both external longitudinal and transverse fields is studied within the mean-field approach based on Bogoliubov's inequality for the Gibbs free energy. The phase diagrams in the longitudinal field-temperature plane and in the transverse field-temperature plane are determined, respectively. The results show that the tricritical points can occur in a certain region of the external longitudinal and transverse fields, the temperature of the tricritical point monotonically increases with decreasing the transverse magnetic field , and increasing the longitudinal magnetic field h. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Kinetics and thermodynamics of isothermal curing reaction of epoxy-4, 4,-diaminoazobenzene reinforced with nanosilica and nanoclay particlesPOLYMER COMPOSITES, Issue 8 2010M. Barghamadi The kinetics of the cure reaction for a system of bisphenol-A epoxy resin (DGEBA), with 4, 4,-diaminoazobenzene (DAAB), reinforced with nanosilica (NS), and nanoclay (NC) by means of isothermal technique of differential scanning calorimetry were studied. The Kamal autocatalytic-like kinetic model was used to estimate the reaction orders (m, n), rate constants (k1, k2), and also active energies (Ea) and pre-exponential factors (A) of the curing reaction. However, the existence of NS and NC with hydroxyl groups in the structure improves the cure reaction and influence the rate of reaction and therefore kinetics parameters. The Ea of cure reaction of DGEBA/DAAB system showed a decrease when nanoparticles were present and therefore the rate of the reaction was increased. Using the rate constants from the kinetic analysis and transition state theory, thermodynamic parameters such as enthalpy (,H#), entropy (,S#), and Gibbs free energy (,G#) changes were also calculated. The thermodynamic functions were shown to be very sensitive parameters for evaluation of the cure reaction. POLYM. COMPOS., 31:1442,1448, 2010. © 2009 Society of Plastics Engineers [source] Kinetic and thermodynamic study of methanolysis of poly(ethylene terephthalate) waste powderPOLYMER INTERNATIONAL, Issue 3 2003S Mishra Abstract Depolymerization of poly(ethylene terephthalate) waste (PETW) was carried out by methanolysis using zinc acetate in the presence of lead acetate as the catalyst at 120,140,°C in a closed batch reactor. The particle size ranging from 50 to 512.5,µm and the reaction time 60 to 150,min required for methanolysis of PETW were optimized. Optimal percentage conversion of PETW into dimethyl terephthalate (DMT) and ethylene glycol (EG) was 97.8% (at 120,°C) and 100% (at 130 and 140,°C) for the optimal reaction time of 120,min. Yields of DMT and EG were almost equal to PET conversion. EG and DMT were analyzed qualitatively and quantitatively. To avoid oxidation/carbonization during the reaction, methanolysis reactions were carried out below 150,°C. A kinetic model is developed and the experimental data show good agreement with the kinetic model. Rate constants, equilibrium constant, Gibbs free energy, enthalpy and entropy of reaction are also evaluated at 120, 130 and 140,°C. The methanolysis rate constant of the reaction at 140,°C (10.3 atm) was 1.4,×,10,3,g PET mol,1 min,1. The activation energy and the frequency factor for methanolysis of PETW were 95.31,kJ,mol,1 and 107.1,g PET mol,1 min,1, respectively. © 2003 Society of Chemical Industry [source] Novel copper(II) heterochelate: synthesis, structural features and fluorescence studiesAPPLIED ORGANOMETALLIC CHEMISTRY, Issue 7 2010G. J. Kharadi Abstract Fluorescence properties of four based derivatives [An] (where n = 1,4) and their Cu(II) heterochelates of the type [Cu(An)(CQ)(OH)],xH2O {where A1 = 3-(2-oxo-2H-chromen-3-yl)-4H-furo[3,2-c]chromen-4-one, A2 = 8-methyl-3-(2-oxo-2H-chromen-3-yl)-4H-furo[3,2-c]chromen-4-one, A3 = 6-methyl-3-(2-oxo-2H-chromen-3-yl)-4H-furo[3,2-c]chromen-4-one, A4 = 8-chloro-3-(2-oxo-2H-chromen-3-yl)-4H-furo[3,2-c]chromen-4-one and x = 3, 2, 4, 1} were studied at room temperature. The fluorescence spectra of heterochelates show red shift, which may be due to the chelation by the ligands to the metal ion. It enhances ligand ability to accept electrons and decreases the electron transition energy. The kinetic parameters such as order of reaction (n), energy of activation (Ea), entropy (,S#), pre-exponential factor (A), enthalpy (,H#) and Gibbs free energy (,G#) have been reported. The antimicrobial activity of Clioquinol and Cu(II) heterochelates have been determined and described. All the heterochelates showed a more effective antimicrobial activity than the free ligand. Copyright © 2010 John Wiley & Sons, Ltd. [source] True and Apparent Temperature Dependence of Protein Adsorption Equilibrium in Reversed-Phase HPLCBIOTECHNOLOGY PROGRESS, Issue 6 2002Szabelski The adsorption behavior of bovine insulin on a C8 -bonded silica stationary phase was investigated at different column pressures and temperatures in isocratic reversed-phase HPLC. Changes in the molar volume of insulin (, Vm) upon adsorption were derived from the pressure dependence of the isothermal retention factor ( k,). The values of , Vm were found to be practically independent of the temperature between 25 and 50 °C at ,96 mL/mol and to increase with increasing temperature, up to ,108 mL/mol reached at 50 °C. This trend was confirmed by two separate series of measurements of the thermal dependence of ln( k,). In the first series the average column pressure was kept constant. The second series involved measurements of ln( k,) under constant mobile-phase flow rate, the average column pressure varying with the temperature. In both cases, a parabolic shape relationship was observed between ln( k,) and the temperature, but the values obtained for ln k, were higher in the first than in the second case. The relative difference in ln( k,), caused by the change in pressure drop induced by the temperature, is equivalent to a systematic error in the estimate of the Gibbs free energy of 12%. Thus, a substantial error is made in the estimates of the enthalpy and entropy of adsorption when neglecting the pressure effects associated with the change in the molar volume of insulin. This work proves that the average column pressure must be kept constant during thermodynamic measurements of protein adsorption constants, especially in RPLC and HIC. Our results show also that there is a critical temperature, Tc , 53 °C, at which ln( k,) is maximum and the insulin adsorption process changes from an exothermic to an endothermic one. This temperature determines also the transition point in the molecular mechanism of insulin adsorption that involves successive unfolding of the protein chain. [source] Chemical Recycling and Kinetics of Aqueous Alkaline Depolymerization of Poly(Butylene Terephthalate) WasteCHEMICAL ENGINEERING & TECHNOLOGY (CET), Issue 7 2004A.S. Goje Abstract Depolymerization reactions of poly(butylene terephthalate) (PBT) waste in aqueous sodium hydroxide solution were carried out in a batch reactor at 80,140,°C at atmospheric pressure by varying PBT particle size in the range of 50,512.5,,m. Reaction time was also varied from 10,110,min to understand the influence of PBT particle size and reaction time on the batch reactor performance. Agitator speed, particle size of PBT and reaction time required were optimized. Disodium terephthalate (salt) and 1,4-butanediol (BD) remain in the liquid phase. BD was recovered by the salting-out method. Disodium terephthalate was separated by acidification to obtain solid terephthalic acid (TPA). The produced monomeric products (TPA and BD) and PBT were analyzed. The yields of TPA and BD were in agreement with PBT conversion. The depolymerization reaction rate was first order to PBT concentration as well as first order to sodium hydroxide concentration. The acid value of TPA changes with the reaction time as well as particle size of PBT. This indicates that PBT molecules get fragmented and hydrolyze simultaneously with aqueous sodium hydroxide to produce BD and disodium terephthalate. Activation energy, Arrhenius constant, equilibrium constant, Gibbs free energy, enthalpy and entropy were determined. The dependence of the hydrolysis rate constant on reaction temperature was correlated by the Arrhenius plot, which shows an activation energy of 25,kJ/mol and an Arrhenius constant of 438,L/min/cm2. [source] Thermal Behavior and Non-isothermal Decomposition Reaction Kinetics of NEPE Propellant with Ammonium DinitramideCHINESE JOURNAL OF CHEMISTRY, Issue 5 2010Weiqiang Pang Abstract Thermal decomposition behavior and non-isothermal decomposition reaction kinetics of nitrate ester plasticized polyether NEPE propellant containing ammonium dinitramide (ADN), which is one of the most important high energetic materials, were investigated by DSC, TG and DTG at 0.1 MPa. The results show that there are four exothermic peaks on DTG curves and four mass loss stages on TG curves at a heating rate of 2.5 K·min,1 under 0.1 MPa, and nitric ester evaporates and decomposes in the first stage, ADN decomposes in the second stage, nitrocellulose and cyclotrimethylenetrinitramine (RDX) decompose in the third stage, and ammonium perchlorate decomposes in the fourth stage. It was also found that the thermal decomposition processes of the NEPE propellant with ADN mainly have two mass loss stages with an increase in the heating rate, that is the result of the decomposition heats of the first two processes overlap each other and the mass content of ammonium perchlorate is very little which is not displayed in the fourth stage at the heating rate of 5, 10, and 20 K·min,1 probably. It was to be found that the exothermal peak temperatures increased with an increase in the heating rate. The reaction mechanism was random nucleation and then growth, and the process can be classified as chemical reaction. The kinetic equations of the main exothermal decomposition reaction can be expressed as: d,/dt=1012.77(3/2)(1,,)[,ln(1,,)]1/3 e,1.723×104/T. The critical temperatures of the thermal explosion (Tbe and Tbp) obtained from the onset temperature (Te) and the peak temperature (Tp) on the condition of ,,0 are 461.41 and 458.02 K, respectively. Activation entropy (,S,), activation enthalpy (,H,), and Gibbs free energy (,G,) of the decomposition reaction are ,7.02 J·mol,1·K,1, 126.19 kJ·mol,1, and 129.31 kJ·mol,1, respectively. [source] Vibrations-determined properties of green fluorescent proteinBIOPOLYMERS, Issue 3 2005Veera Krasnenko Abstract The physicochemical characteristics of the green fluorescent protein (GFP), including the thermodynamic properties (entropy, enthalpy, Gibbs' free energy, heat capacity), normal mode vibrations, and atomic fluctuations, were investigated. The Gaussian 03 computational chemistry program was employed for normal mode analysis using the AMBER force field. The thermodynamic parameters and atomic fluctuations were then calculated from the vibrational eigenvalues (frequencies) and eigenvectors. The regions of highest rigidity were shown to be the ,-sheet barrel with the central ,-helix, which bears the chromophore. The most flexible parts of the GFP molecule were the outlying loops that cover the top and bottom of the ,-barrel. This way, the balance between rigidity and flexibility is maintained, which is the optimal relationship for protein stability in terms of Gibbs' free energy. This dual-schemed structure satisfies the requirements for GFP function. In this sense, the structure of GFP resembles a nanoscale drum: a stiff cylinder with flexible vibrating end(s). © 2005 Wiley Periodicals, Inc. Biopolymers 78: 140,146, 2005 This article was originally published online as an accepted preprint. The "Published Online" date corresponds to the preprint version. You can request a copy of the preprint by emailing the Biopolymers editorial office at biopolymers@wiley.com [source] | |||||||||||||||||||||||||||||||