Analysis of the Reaction Rate Coefficients for Slow Bimolecular Chemical Reactions

Simple bimolecular reactions $A_1+A_2\rightleftharpoons A_3+A_4$ are analyzed within the framework of the Boltzmann equation in the initial stage of a chemical reaction with the system far from chemical equilibrium. The Chapman-Enskog methodology is applied to determine the coefficients of the expansion of the distribution functions in terms of Sonine polynomials for peculiar molecular velocities. The results are applied to the reaction $H_2+Cl\rightleftharpoons HCl+H$, and the influence of the non-Maxwellian distribution and of the activation-energy dependent reactive cross sections upon the forward and reverse reaction rate coefficients are discussed.Comment: 11 pages, 5 figures, to appear in vol.42 of the Brazilian Journal of Physic

On modified simple reacting spheres kinetic model for chemically reactive gases

Versão dos autores para esta publicação.We consider the modiffed simple reacting spheres (MSRS) kinetic model that, in addition to the conservation of energy and momentum, also preserves the angular momentum in the collisional processes. In contrast to the line-of-center models or chemical reactive models considered in [1], in the MSRS (SRS) kinetic models, the microscopic reversibility (detailed balance) can be easily shown to be satisfied, and thus all mathematical aspects of the model can be fully justi ed. In the MSRS model, the molecules behave as if they were single mass points with two internal states. Collisions may alter the internal states of the molecules, and this occurs when the kinetic energy associated with the reactive motion exceeds the activation energy. Reactive and non-reactive collision events are considered to be hard spheres-like. We consider a four component mixture A, B, A*, B*, in which the chemical reactions are of the type A + B = A* + B*, with A* and B* being distinct species from A and B. We provide fundamental physical and mathematical properties of the MSRS model, concerning the consistency of the model, the entropy inequality for the reactive system, the characterization of the equilibrium solutions, the macroscopic setting of the model and the spatially homogeneous evolution. Moreover, we show that the MSRS kinetic model reduces to the previously considered SRS model (e.g., [2], [3]) if the reduced masses of the reacting pairs are the same before and after collisions, and state in the Appendix the more important properties of the SRS system.Fundação para a Ciência e a Tecnologi

Nonequilibrium thermodynamics and energy efficiency in weight loss diets

Carbohydrate restriction as a strategy for control of obesity is based on two effects: a behavioral effect, spontaneous reduction in caloric intake and a metabolic effect, an apparent reduction in energy efficiency, greater weight loss per calorie consumed. Variable energy efficiency is established in many contexts (hormonal imbalance, weight regain and knock-out experiments in animal models), but in the area of the effect of macronutrient composition on weight loss, controversy remains. Resistance to the idea comes from a perception that variable weight loss on isocaloric diets would somehow violate the laws of thermodynamics, that is, only caloric intake is important ("a calorie is a calorie"). Previous explanations of how the phenomenon occurs, based on equilibrium thermodynamics, emphasized the inefficiencies introduced by substrate cycling and requirements for increased gluconeogenesis. Living systems, however, are maintained far from equilibrium, and metabolism is controlled by the regulation of the rates of enzymatic reactions. The principles of nonequilibrium thermodynamics which emphasize kinetic fluxes as well as thermodynamic forces should therefore also be considered

Interaction between a bisphosphonate, tiludronate, and biomimetic nanocrystalline apatites.

Bisphosphonates (BPs) are well established as successful antiresorptive agents for the prevention and treatment of bone diseases such as osteoporosis and Paget's disease. The aim of this work was to clarify the reaction mechanisms between a BP molecule, tiludronate, and the nanocrystalline apatite surface. The adsorption of tiludronate on well characterized synthetic biomimetic nanocrystalline apatites with homogeneous but different compositions and surface characteristics was investigated to determine the effect of the nanocrystalline apatite substrate on the adsorption behavior. The results show that the adsorption of tiludronate on nanocrystalline biomimetic apatite surfaces varies over a large range. The most immature apatitic samples exhibited the highest affinity and the greatest amount adsorbed at saturation. Maturation of the nanocrystals induces a decrease of these values. The amount of phosphate ion released per adsorbed BP molecule varied, depending on the nanocrystalline substrate considered. The adsorption mechanism, although associated with a release of phosphate ions, cannot be considered as a simple ion exchange process involving one or two phosphate ions on the surface. A two-step process is proposed consisting of a surface binding of BP groups to calcium ions associated with a proton release inducing the protonation of surface orthophosphate ions and their eventual solubilization

Application of Artificial Neural Networks for Analysis of Highly Overlapped and Disturbed Differential Pulse Polarographic Peaks in the Region of Hydrogen Evolution

Multivariate calibration based on a suitable experimental design (ED) and soft modelling with artificial neural networks (ANNs) is proposed for quantitative analysis of highly overlapped and disturbed differential pulse polarographic (DPP) peaks that occur in the region of a hydrogen evolution. It is demonstrated that analysis of mixtures, even if some of the constituents undergo an irreversible reduction and the background current varies significantly with a composition of a sample, can be quantified with reasonable accuracy using a combination of ED and ANNs. Examples of DPP examination of ZnII and CrIII mixtures and/or simultaneous determination of metal ions and a strong acid concentration are presented. The possibility of an on-line monitoring is suggested. It is demonstrated that standard hard model based refinement procedures perform much worse than ANNs combined with ED and, in principle, proved to be unsuitable for the purpose. South African Journal of Chemistry Vol.53(3) 2000: 213-23

Stability Constants of the Inclusion Complexes of ß-Cyclodextrin with Various Adamantane Derivatives. A UV-Vis Study

The stability constants for the inclusion complexes of ß-cyclodextrin (ß-CD) with various adamantane derivatives (ADA), namely the amantadinium (AM), rimantadinium (RIM), and memantinium (MEM) cations have been determined by UV-Vis spectrophotometry. All experiments have been performed at a pH of 1.7 and 25 º C on aqueous solutions adjusted to an ionic strength of 0.05 M (Na+, H+)ClO4. The competitive binding method has been used whereby methyl orange (MO) is first encapsulated by ß-CD and is then substituted by ADA. It has been shown that the derivatives studied form host-guest type complexes. The calculated stability constants, reported as log K1, were estimated to be 3.9 ± 0.1, 5.1 ± 0.2 and 3.3 ± 0.1, for AM, RIM and MEM, respectively. The factors that govern the strength of binding ADA with ß-CD have been discussed and an attempt was made to rationalise the variation in the established stability constants for the ADA-ß-CD complexes. General experimental conditions required for the determination of the stability constants of ADA with ß-CD with the use of MO as an auxiliary agent were evaluated. The optimised experimental conditions are recommended. It has been concluded that MO, even though commonly used in this type of study, does not meet the optimal and recommended conditions. South African Journal of Chemistry Vol.54 2001: 84-10

Modelling of the Blood Plasma Species of Biguanide Derivatives Exhibiting Potential as Diagnostic Radiopharmaceuticals

99mTc-DMSA (DMSA=dimercaptosuccinic acid), the gold standard for static renal imaging, has a long uptake time, which is a limiting factor in diagnostic procedures and also leads to a relatively high radiation dose to patients. The ligands dimethyl biguanide (DMBG), biuret (BIU), 2-imino-4-thiobiuret (ITB) and carboxy-biguanide (CBIG) have nitrogen donor atoms, which are able to complex transition elements. The formation constants needed in order to establish a blood plasma model for these ligands were determined by potentiometry and the results are reported herein. Based on blood plasma modelling, it was shown that the ligands had selectivity for 99mTc over blood plasma metal ions at physiological pH and it was therefore hypothesized that the Tc-ligand complex would survive in blood plasma. Furthermore, no or few side effects related to the mobilization of blood plasma metal ions by these ligands are expected, once the radiopharmaceutical has been administered. It is also expected that these ligands should clear rapidly from the blood plasma.Keywords: Biguanides, kidney agents, stability formation constants, blood plasma speciation, CBIG, DMBG, BIU, ITBPDF and Supplementry file attache