Spontaneous oscillating reactions

D. Spontaneous Oscillating Reactions 1. Studies on Supported Metals... 

As mentioned in the introduction, it is difficult to explain the characteristics of the oscillation based on the mechanisms which have been proposed so far for the potential oscillations with systems similar to Eq. (16) [4,7,35-38]. A precise investigation on individual ion transfers and adsorptions at two interfaces is necessary for the elucidation of the oscillation mechanism, although the spontaneous oscillation might be realized by the combination of a much larger number of ion transfer reactions and adsorptions than those in the case of the oscillation under the applied potential or current. 

Today it comes as no surprise that chemical reactions can oscillate spontaneously—such reactions have become a standard demonstration in chemistry classes, and you may have seen one yourself. (For recipes, see Winfree (1980).) But in Belousov s day, his discovery was so radical that he couldn t get his work published. It was thought that all solutions of chemical reagents must go monoton-ically to equilibrium, because of the laws of thermodynamics. Belousov s paper was rejected by one journal after another. According to Winfree (1987b, p. 161), one editor even added a snide remark about Belousov s supposedly discovered discovery to the rejection letter. 

The observation of oscillations in heterogeneous catalytic reactions is an indication of the complexity of catalyst kinetics and makes considerable demands on the theories of the rates of surface processes. In experimental studies the observed fluctuations may be in catalyst temperature, surface species concentrations, or most commonly because of its accessibility, in the time variation of the concentrations of reactants and products in contact with the catalyst. It is now clear that spontaneous oscillations are primarily due to non-linearities associated with the rates of surface reactions as influenced by adsorbed reactants and products, and the large number of experimental studies of the last decade have stimulated a considerable amount of theoretical kinetic modelling to attempt to account for the wide range of oscillatory behaviour observed. 

By introduction of reaction-difiusion mechanism, self-organization, and self-regulation perceptions in the oscillating reactions, its applications have grown substantially in recent years. The reaction-difiusion mechanism is found to be veiy usual in diverse kinds of natural phenomenon that employed to assemble and fabricate the stmctures on the length scales. On the other hand, self-organization is treated as a fantastic phenomenon by which a spontaneous dissipative pattern could be possible by input of energy and matter in non-equilibrium conditions. 

Expanding target patterns in the Belousov-Zhabotinsky reaction were first discovered by Zaikin and Zhabotinsky (1970). In their experiment, they used a spontaneously oscillating thin layer of solution which was contained in a Petri dish of diameter 100 mm. The reagent contained bromate, bromomalonic acid and ferroin. With this prescription, one may observe periodic alternation of oxidized and reduced forms of the catalyst through a dramatic color change of the solution between red (reduced state) and blue (oxidized state). Some features of the pattern observed by them and by later experimenters are the following . ... 

The most curious process in which Ru(bpy)3 " plays the role of LES is an oscillating reaction. It is well established that certain types of chemical reactions, under appropriate experimental conditions, organize themselves spontaneously to give rise to regular spacial patterns or to periodic rate fluctuations (see, e.g. ref. 59). The best studied among the oscillating homogeneous processes is the classical Belousov-Zhabotinskii (BZ) reaction[60], in which a crucial role is played by a redox catalyst. The usual catalyst of the BZ reaction is the couple, but polypyridine... 

The urea-urease reaction follows Michaelis-Menten kinetics and has a bellshaped rate-pH curve with maximum at pH 7 (see Fig. 1). In non-buffered condition, the rate-pH curve can be exploited to obtain feedback-driven behaviour, for instance, through external stimuli, e.g. by delivering an acid or a base to the solution a reaction acceleration or inhibition can be obtained. Therefore, the conditions for spontaneous oscillations between two pH states can be achieved. This reaction has also the distinct advantages of high solubility and stability of substrate and enzyme in water making it suitable for experiments in vitro. 

Stanley R J and Boxer S G 1995 Oscillations in the spontaneous fluorescence from photosynthetic reaction centers J. Phys. Chem. 99 859-63... 

In the Belousov-Zhabotinskii reaction, beautiful regular patterns form spontaneously as the result of the oscillating concentrations of reactants and products due to competing reactions. 

In this chapter, novel oscillations observed with liquid membrane systems by the present authors [22-25] will be introduced, and the mechanisms for the oscillation are clarified by using VITIES, taking into consideration ion transfer reactions and adsorptions at two aqueous-membrane interfaces. The mechanism of the spontaneous potential oscillation in a liquid membrane system proposed by Yoshikawa et al. is also discussed briefly. 

The three best-known examples of biochemical oscillations were found during the decade 1965-1975 [40,41]. These include the peroxidase reaction, glycolytic oscillations in yeast and muscle, and the pulsatile release of cAMP signals in Dictyostelium amoebae (see Section V). Another decade passed before the development of Ca " " fluorescent probes led to the discovery of oscillations in intracellular Ca +. Oscillations in cytosolic Ca " " have since been found in a variety of cells where they can arise spontaneously, or after stimulation by hormones or neurotransmitters. Their period can range from seconds to minutes, depending on the cell type [56]. The oscillations are often accompanied by propagation of intracellular or intercellular Ca " " waves. The importance of Ca + oscillations and waves stems from the major role played by this ion in the control of many key cellular processes—for example, gene expression or neurotransmitter secretion. 

If, however, we actually integrate the reaction rate equations numerically using the rate constants in Table 1.1 we find that the system does not always stick to, or even stay close to, these pseudo-steady loci. The actual behaviour is shown in Fig. 1.10. There is a short initial period during which d and b grow from zero to their appropriate pseudo-steady values. After this the evolution of the intermediate concentrations is well approximated by (1.41) and (1.42), but only for a while. After a certain time, the system moves spontaneously away from the pseudo-steady curves and oscillatory behaviour develops. We may think of the. steady state as being unstable or, in some sense repulsive , during this period in contrast to its stability or attractiveness beforehand. Thus we have met a bifurcation to oscillatory responses . The oscillations... 

Chemical reactions with autocatalytic or thermal feedback can combine with the diffusive transport of molecules to create a striking set of spatial or temporal patterns. A reactor with permeable wall across which fresh reactants can diffuse in and products diffuse out is an open system and so can support multiple stationary states and sustained oscillations. The diffusion processes mean that the stationary-state concentrations will vary with position in the reactor, giving a profile , which may show distinct banding (Fig. 1.16). Similar patterns are also predicted in some circumstances in closed vessels if stirring ceases. Then the spatial dependence can develop spontaneously from an initially uniform state, but uniformity must always return eventually as the system approaches equilibrium. 

There are many systems of different complexity ranging from diatomics to biomolecules (the sodium dimer, oxazine dye molecules, the reaction center of purple bacteria, the photoactive yellow protein, etc.) for which coherent oscillatory responses have been observed in the time and frequency gated (TFG) spontaneous emission (SE) spectra (see, e.g., [1] and references therein). In most cases, these oscillations are characterized by a single well-defined vibrational frequency, It is therefore logical to anticipate that a single optically active mode is responsible for these features, so that the description in terms of few-electronic-states-single-vibrational-mode system Hamiltonian may be appropriate. 

The major characteristics of excitable media, such as oscillating chemical reactions, and some important concepts necessary for understanding their behaviour have been discussed. The capacity of Belousov-Zhabotinsky (BZ) reactions for spontaneous spatiotemporal auto-organization is described 289... 

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