Simulação Monte Carlo no ensino de luminescência e cinética de decaimento de estados excitados

A software based in the Monte Carlo method have been developed aiming the teaching of important cases of mechanisms found in luminescence and in excited states decay kinetics, including: multiple decays, consecutive decays and coupled systems decays. The Monte Carlo Method allows the student to easily simulate and visualize the luminescence mechanisms, focusing on the probabilities of the related steps. The software CINESTEX was written for FreeBASIC compiler; it assumes first-order kinetics and any number of excited states, where the pathways are allowed with probabilities assigned by the user.

Ordens não inteiras em cinética química

Starting from zero-, first-, and second-order integrated laws for chemical kinetics, some cases are shown which produce fractional orders. Taking the Michaelis-Menten mechanism as a first example, it is shown that substrate order can go from 1 to zero, depending on relative concentration of enzyme and substrate. Using other examples which show fractional orders higher than one and even negative (inhibition), it is shown that the presence of an equilibrium before or parallel to the rate determining step can be the reason for fractional orders, which is an indication of a more complex mechanism.

Aplicação do delineamento simplex-centroide no estudo da cinética da oxidação de biodiesel B100 em mistura com antioxidantes sintéticos

Antioxidants are an alternative to prevent or slow the degradation of the biofuel. In this study, it was evaluated the oxidative stability of B100 biodiesel from soybean oil in the presence of three commercial synthetic antioxidants, butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT) and tert-butylhydroquinone (TBHQ), pure or blended, from the experimental design of simplex-centroid mixture. The reaction order and rate constant were also calculated for all tests. The treatment containing pure TBHQ proved to be the most effective, proven by design, the optimum mix obtained and the rate constant. Binary and ternary mixtures containing TBHQ also showed appreciable antioxidant effect.

Uso de modelagem molecular no estudo dos conceitos de nucleofilicidade e basicidade

Molecular modeling enables the students to visualize the abstract relationships underlying theoretical concepts that explain experimental data on the molecular and atomic levels. With this aim we used the free software "Arguslab 4.0.1" (semi-empirical method) to study the reaction of 1-chloropropane with ethoxide in solution, known to lead to methyl propyl ether, through the S N2 mechanism, and propene, through the E2 mechanism. This tool allows users to calculate some properties (i. e. heat formation or electric charges) and to produce 3D images (molecular geometry, electrostatic potential surface, etc.) that render the comprehension of the factors underlying the reaction's progress, which are related to the structure of the reagents, and the process kinetic clearer and easier to understand by the students

Modelagem termodinâmica por extração por solvente de metais divalentes em meio sulfato usando D2EHPA

The extraction of divalent metals (Mn2+, Ni2+, Co2+ and Cu2+) in the system MSO4 - H2SO4 - H2O - D2EHPA in isoparaffin (17/21) was studied by a thermodynamic model based on chemical equilibria with mass and charge balance equations. The activity coefficients of all solutes in the aqueous phase were calculated by Davies equation. By applying this model, the equilibrium concentrations of solutes were calculated from de concentration of divalent metals and pH. The predicted distribution coefficients for the divalents metals were in good agreement with experimental results.

Efeito da ativação mecânica de uma formulação de talco/caulim/alumina sobre o mecanismo e a cinética de formação de cordierita

In this study, the influence of mechanical activation by intensive ball milling of a stoichiometric mixture of talc, kaolin, and alumina on the mechanism and kinetics of cordierite (2MgO·2Al2O3·5SiO2) formation was evaluated. The raw materials were characterized by chemical analysis, X-ray diffraction (XRD), laser diffraction, and helium pycnometry. The kinetics and mechanism of cordierite formation were studied by XRD, differential thermal analysis, and dilatometry in order to describe the phase formation as a function of temperature (1000-1400 ºC), time of thermochemical treatment (0-4 h), and grinding time of the mixture (0-45 min). Finally, the optimal conditions of the thermochemical treatment that ensured the formation of cordierite were determined: milling time of 45 min and thermal treatment at 1280 ºC for 1 h.

Estudo da cinética de oxidação de biodiesel b100 obtido de óleo de soja e gordura de porco: determinação da energia de ativação

The quality of biodiesel is extremely important for its commercialization and use; oxidation of biodiesel is a critical factor because it decreases the fuel storage time. A commercial biodiesel was mixed with synthetic antioxidants, according to a simplex-centroid experimental mixture design, and its stability was evaluated through induction period and activation energy. In all trials, addition of antioxidants increased activation energy in the mixtures containing tertiary butylhydroquinone (TBHQ). When a mixture containing 50% TBHQ and 50% butylated hydroxyanisole was used, synergistic effect was observed, and the major activation energy obtained was 104.43 kJ mol-1.

Cinética e caracterização de ramnolipídeos produzidos por Pseudomonas aeruginosa MSIC02 utilizando glicerol como fonte de carbono

Glycerol, a co-product of biodiesel production, was used as a carbon source for the kinetics studies and production of biosurfactants by P. aeruginosa MSIC02. The highest fermentative parameters (Y PX = 3.04 g g-1; Y PS = 0.189 g g-1, P B = 31.94 mg L-1 h-1 and P X = 10.5 mg L-1 h-1) were obtained at concentrations of 0.4% (w/v) NaNO3 and 2% (w/v) glycerol. The rhamnolipid exhibited 80% of emulsification on kerosene, surface tension of 32.5 mN m-1, CMC = 28.2 mg L-1, C20 (concentration of surfactant in the bulk phase that produces a reduction of 20 dyn/cm in the surface tension of the solvent) = 0.99 mg L-1, Γm (surface concentration excess) = 2.4 x 10-26 mol Å-2 and S (surface area) = 70.4 Ų molecule-1 with solutions containing 10% NaCl. A mathematical model based on logistic equation was considered to representing the process. Model parameters were estimated by non-linear regression method. This approach was able to give a good description of the process.

Delineamentos experimentais eficientes para estudos de cinética química

In this paper we show how to obtain efficient designs of experiments for fitting Michaelis-Menten and Hill equations useful in chemical studies. The search of exact D-optimal designs by using local and pseudo-Bayesian approaches is considered. Optimal designs were compared to those commonly used in practice using an efficiency measure and theoretical standard errors of the kinetic parameter estimates. In conclusion, the D-optimal designs based on the Hill equation proved efficient for estimating the parameters of both models. Furthermore, these are promising with respect to practical issues, allowing efficient estimation as well as goodness-of-fit tests and comparisons between some kinetic models.

Solo contaminado com cádmio: extratibilidade do metal e cinética química de degradação da matéria orgânica de torta de filtro

The chemical kinetics of sugarcane filter cake (FC) organic matter degradation at rates (0, 40, 80, and 120 t ha-1) in non-contaminated and different degrees of cadmium-contaminated Oxisol (0.19, 28, 56, 112 and 200 mg Cd kg-1) and DTPA-extractable Cd was studied. FC degradation was determined by quantifying CO2 emitted from soil samples during 72 days of incubation. DTPA-extractable Cd was performed after the incubation period. FC degradation was described by a two-stage equation of chemical kinetics. FC degradation rates were between 15 and 33%. Total CO2 emitted from FC declined with increasing degree in Cd-contamination and the DTPA-extractable Cd declined with FC rates.

REDUÇÃO DE VARIÁVEIS DE ENTRADA DE REDES NEURAIS ARTIFICIAIS A PARTIR DE DADOS DE ANÁLISE DE COMPONENTES PRINCIPAIS NA MODELAGEM DE OXIGÊNIO DISSOLVIDO

The objective of this work is to demonstrate the efficient utilization of the Principal Components Analysis (PCA) as a method to pre-process the original multivariate data, that is rewrite in a new matrix with principal components sorted by it's accumulated variance. The Artificial Neural Network (ANN) with backpropagation algorithm is trained, using this pre-processed data set derived from the PCA method, representing 90.02% of accumulated variance of the original data, as input. The training goal is modeling Dissolved Oxygen using information of other physical and chemical parameters. The water samples used in the experiments are gathered from the Paraíba do Sul River in São Paulo State, Brazil. The smallest Mean Square Errors (MSE) is used to compare the results of the different architectures and choose the best. The utilization of this method allowed the reduction of more than 20% of the input data, which contributed directly for the shorting time and computational effort in the ANN training.

Influência de adições de prata na cinética de precipitação da liga Cu-5%Al

A influência de adições de 2%, 4%, 6%, 8% e 10% de prata, em massa, na cinética de formação e precipitação de precipitados ricos em prata, na liga Cu-5% Al foi estudada através de medidas de variação da microdureza com a temperatura e o tempo. Os resultados obtidos indicaram que a presença da prata aumenta consideravelmente a dureza da liga e também interfere na cinética de precipitação, com uma mudança mecanística no processo que está relacionada com o teor de prata das ligas.

Influência de adições de prata na cinética de recristalização da liga CU-5%Al

A influência de adições de 2, 4, 6 e 8% de prata, em peso, na cinética de recristalização da liga Cu-5%Al com taxas de deformação de 40, 70 e 90%, foi estudada utilizando-se metalografia por microscopia óptica e medidas de variação do tamanho de grão com o tempo e a temperatura de envelhecimento. Os resultados obtidos indicaram uma diminuição na velocidade de crescimento dos grãos e na energia de ativação correspondente, com o aumento da concentração de prata.

Cinética de dissolução da superfície de aço SAE-AISI 1005 em meio ácido

O setor automotivo é um dos mais competitivos e um grande consumidor de materiais ferrosos, que exigem uma eficiente proteção contra a corrosão. Esta proteção é alcançada aplicando revestimentos adequados. Para se aplicar revestimentos se requer uma cuidadosa preparação da superfície que pode ser feita por ataque químico. Neste trabalho, determinou-se a velocidade de reação do aço extra-doce, SAE-AISI 1005, laminado a frio (CFF) utilizando várias soluções ácidas, de baixo custo, baixa agressividade ao meio ambiente e que geram pouco hidrogênio. O estudo cinético foi realizado medindo-se a perda de massa (deltam/mg cm-2) de chapas de aço CFF em função do tempo (s) de reação nas diferentes soluções, com e sem agitação, a 30ºC. Os resultados foram interpretados considerando-se os efeitos de transporte de massa, o efeito oxidante dos íons Me3+ e inibidor dos íons Fe2+. Dentre as várias soluções estudadas a que apresentou maior velocidade de reação foi a solução de H2SO4 2 M + MeCl3 1 M, v = 15,44 x 10-6 g cm-2s-1 para tempos até 1800 s.

Estudo da cinética de consumo do estabilizante 2NDPA no propelente base dupla

Este trabalho tem como propósito avaliar a cinética de consumo do estabilizante 2NDPA presente em uma amostra de propelente tipo base-dupla (BD). Tópicos relativos ao método analítico empregado e, a interferências na cinética de consumo do estabilizante pela nitroglicerina e o salicilato de chumbo presente no propelente, também são abordados. Foram obtidos, ainda, os parâmetros cinéticos de consumo do 2NDPA. Para isto, foi empregada a técnica da Cromatografia Líquida de Alta Eficiência (CLAE). Os dados experimentais foram bem ajustados para uma cinética de pseudoprimeira ordem. A energia de ativação (Ea) e o fator pré-exponencial (A) determinados foram, respectivamente, (1,22 ± 0,42) 10² kJ mol-1 e (10(16) ±10(6)) dia-1.