Daniel Barragán

Understanding the induction period of the Belousov-Zhabotinsky reaction

In this paper we present the dependence of the induction time of the Belousov-Zhabotinsky reaction (BZ) on the initial concentrations of malonic acid, bromate and cerium. The experimental results show that the induction time gets larger with bromate increasing and this behaviour does not agree with the mechanistic explanations based on the models proposed for the BZ reaction. We propose that a kinetic competition between the bromination of malonic acid and the oxidation of bromomalonic and malonic acids is a way to understand this behaviour. Model calculations using the GTF and MBM models support the propose explanation.

Bursting in the Belousov-Zhabotinsky reaction added with phenol in a batch reactor

4+ to Ce 3+ and in the removal of molecular bromine of the reaction mixture. The oscillating reaction of two substrates exhibited burst firing and an oscillatory period of long duration. Analysis of experimental data shows an increasing of the bursting phenomenon, with a greater spiking in the burst firing and with a longer quiescent state, as a function of the initial phenol concentration increase. It was hypothesized that the bursting phenomenon can be explained introducing a redox cycle between the reduced phenolic species (hydroxyphenols) and the oxidized ones (quinones). The hypothesis was experimentally and numerically tested and from the results it is possible to conclude that the bursting phenomenon exhibited by the oscillating reaction of two substrates is mainly driven by a p-di-hydroxy-benzene/p-benzoquinone redox cycle.

Additive effects of methyl ketones in the Belousov-Zhabotinsky reactions

The role of the methyl ketones in oscillating reactions of the Belousov-Zhabotinsky (BZ) type is to eliminate bromine through an enolization process, but its importance in the dynamic of the reaction depends upon the organic compound that is used as substrate. This work shows that in a binary mixture of methyl ketones every ketone acts independently, but with an additive effect in the dynamics of the BZ reaction. The results obtained are supported by numerical simulations based upon the mechanisms for both reactions.

Induction period in the BrO3-, Ce(III), H2SO4, oxalic acid and ketone oscillating reaction

Contrary to what has been thought, systems classified as bromine-hydrolysis-controlled (BHC) show an induction period. In studies on the effect of a family of ketones upon this type of oscillators, it was found that an induction period appears in an interval of concentrations of the ketone, or in an interval of the value of the enolization constant. Simulations of the processes involved and a corresponding explanation are shown in this paper.

Estudio cinético de la descomposición catalizada de peróxido de hidrógeno sobre carbón activado

The kinetic study of decomposition of hydrogen peroxide catalyzed by activated carbon was carried out. The effect of concentrations of reactants and temperature were experimentally studied. Kinetic data were evaluated using differential method of initial rates of reaction. When a typical kinetic law for reactions in homogeneous phase is used, first order of reaction is obtained for hydrogen peroxide and activated carbon, and activation energy of 27 kJ mol-1 for the reaction was estimated. Experimentally was observed that surface of activated carbon is chemically modified during decomposition of hydrogen peroxide, based on this result a scheme of reaction was proposed and evaluated. Experimental data fits very well to a Langmuir- Hinshelwood kinetic model and activation energy of 40 kJ mol-1 was estimated for reaction in heterogeneous phase.

CALORIMETRIC STUDY OF THE COMPONENT STEPS OF OSCILLATING CHEMICAL REACTIONS

The complexity of oscillating chemical reactions makes difficult a direct calorimetric study of them. It is more advantageous to carry out studies of the component steps and then try to put the parts together. Here, a mass-flow heat conduction calorimeter was used to study component reactions of two of the principal chemical oscillators. The studied reactions were: the net reaction of the inorganic set of the Belousov-Zhabotinsky reaction BrO3-+4Ce3++5H+D4≤;8805;Ce4++HOBr+2H2O), and the Dushman reaction IO3-+5I-+6H+≤;8805;3I2+3H2O), which is a component of the Bray-Liebhafsky oscillator. The experimental values of the enthalpies of these two reactions are reported in this work.

Adsorción física sobre sólidos: aspectos termodinámicos

A thermodynamic formalism based on the Gibbs Dividing Surface (GDS) for the description of a solid-fluid interface is presented, so that the adsorption layer is understand as a phase and the adsorption process as the transference of components between a 3-dimensional phase and a 2-dimensional one. Using a state equation derived from the Henrys Law, we shall show how the Langmuir isotherm is deduced from de Gibbs isotherm. The GDS is useful also for understanding the release of heat by a system as the adsorption occurs.

Enzymatic evolution driven by entropy production

We show that the structural evolution of enzymes is largely influenced by the entropy produced in the enzymatic process. We have computed this quantity for the case in which the process has unstable and metastable intermediate states. By assuming that the kinetics takes place along a potential barrier, we have found that the behavior of the total entropy produced is a non-monotonic function of the intermediate state energy. By diminishing the number of metastable intermediate states, the total entropy produced decreases and consequently the enzyme kinetics and the thermodynamic efficiency are enhanced. Minimizing locally the total entropy produced for an enzymatic process with metastable intermediate states, the kinetics and the thermodynamic efficiency are raised. In contrast, in the absence of metastable intermediate states, a maximum of the entropy produced results in an improvement of the kinetic performance although the thermodynamic efficiency diminishes. Our results show that the enzymatic evolution proceeds not only to enhance the kinetics but also to optimize the total entropy produced.

Transition from Thermokinetic to Chemical Instabilities in a Modified Sal’nikov Model

Traditionally, thermokinetic and chemical oscillations have been studied independently, but in cellular media recent studies have shown that cell’s temperature is not uniform. Thus, on this context it is possible to inquire about the influence of thermal effects on chemical oscillatory behavior and vice versa. To this end, in this paper we propose a dynamical model based on a modified Sal’nikov oscillator that can address both kinds of oscillatory behavior (thermokinetic and chemical). We found that the system modeled can jump from thermal oscillations to mixed chemical-thermal oscillations through a transition or bifurcation parameter. Thermokinetic oscillations are well defined in a limit cycle, while chemical-thermal oscillations appear in the form of a burst. The model could be useful in explaining biochemical energy recovery under cellular stress conditions.

Understanding Gelation as a Nonequilibrium Self-Assembly Process

Gel formation is described by a nonequilibrium self-assembly (SA) mechanism which considers the presence of precursors. Assuming that nonequilibrium structures appear and are maintained by entropy production, we developed a mesoscopic nonequilibrium thermodynamic model that describes the dynamic assembly of the structures. In the model, the evolution of the structures from the initially inactivated building blocks to the final agglomerates is governed by kinetic equations of the Fokker-Planck type. From these equations, we get the probability densities which enable one to know the measurable quantities such as the concentrations of the different components and the dynamic structure factor obtained in light-scattering experiments. Our results obtained are in very good agreement with the experiments. The model proposed can in general be used to analyze the kinetics of formation of nonequilibrium SA structures usually found in biomedicine and advanced materials.