Liquids oscillating reactions

Since the development of nonequilibrium thermodynamics in the late 1940s, initiated by the work of Prigogine (7), numerous reports have appeared dealing with the possibility of oscillations in reaction systems far from equilibrium. Initially the main focus of these studies was the Belouzov-Zhabotinskii liquid-phase reaction (2), but since the discovery of oscillating reactions in heterogeneous catalysis in the late 1960s (3-7), over 300 publications have described research in this field as well. This review focuses on this emerging and important area of research. 

In 1921, Bray published the first description of an oscillating reaction in the liquid phase, the catalytic decomposition of hydrogen peroxide under the influence of iodate ion. Amazingly, the initial response of the chemical community, instead of undertaking a normal study of the reaction, was to try to prove that the cause of the oscillations was some unknown heterogeneous impurity. 

Several homogeneous gas- and liquid-phase reactions are now also known to exhibit self oscillations and it is clear that many living organisms depend on coupled oscillatory reactions catalysed by enzymes to control biological functions.However, only heterogeneous oxidation reactions catalysed by noble metals are reviewed here. Experimental studies are first described, followed by a discussion of kinetic analyses which have been put forward to account for them. Particular attention is given to the most extensively studied system to date, the oxidation of CO over Pt catalysts. 

Chemical processes Oscillating reactions, chain reaction of catalysis, molecular self-assembly, ordering in liquid crystals, monolayers self-assembly, molecular-phase transition, Langmuir-Blodgett films, etc. 

Figure 12.2 Schematic diagram of an apparatus consisting of two CSTRs for studying physically coupled oscillating reactions. A needle valve controls the flow between the reactors. Inputs to the reactors are independently controlled. Drop detectors ensure that liquid flows out of the two reactors at the same rate so that there is no net mass transfer from one to the other. Reprinted, in part, with permission from Crowley, M. F. Epstein, I. R. 1989. Experimental and Theoretical Studies of a Coupled Chemical Oscillator Phase Death, Multistability, and In-Phase and Out-Of-Phase Entrainment, J. Phys. Chem. 93, 2496-2502. CC 1989 American Chemical Society.)...

The oscillations observed with artificial membranes, such as thick liquid membranes, lipid-doped filter, or bilayer lipid membranes indicate that the oscillation can occur even in the absence of the channel protein. The oscillations at artificial membranes are expected to provide fundamental information useful in elucidating the oscillation processes in living membrane systems. Since the oscillations may be attributed to the coupling occurring among interfacial charge transfer, interfacial adsorption, mass transfer, and chemical reactions, the processes are presumed to be simpler than the oscillation in biomembranes. Even in artificial oscillation systems, elementary reactions for the oscillation which have been verified experimentally are very few. 

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 sulfoxidation of alkanes occurs with heat evolution. This is the basis for rate of oscillation of rapid sulfoxidation at a relatively high pressure when the feedback arises between reaction rate, diffusion of reactants into liquid phase, and heat evolution [27],... 

Figure 2. The elastic (G7) and viscous modulus (G") as a function of the frequency of oscillation at three different UV exposure times. At t = 1660 s, G7 and G" are parallel, indicating the sample has reached its gel point. Note that G is initially smaller than G" throughout the frequency range (t = 1280 s), but becomes larger than G" and independent of frequency towards the end of the reaction (t = 2780 s), indicating that the sample has been transformed from a liquid to a highly cross-linked network.

Oscillations have also been reported for 02/02 redox reaction in quinoline media on HMDE, liquid Hg, and solid Hg-coated electrodes [56]. 

Houcine et al. (64) used a non-intrusive laser-induced fluorescence method to study the mechanisms of mixing in a 20 dm CSTR with removable baffles, a conical bottom, a mechanical stirrer, and two incoming liquid jet streams. Under certain conditions, they observed an interaction between the flow induced by the stirrer and the incoming jets, which led to oscillations of the jet stream with a period of several seconds and corresponding switching of the recirculation flow between several metastable macroscopic patterns. These jet feedstream oscillations or intermittencies could strongly influence the kinetics of fast reactions, such as precipitation. The authors used dimensional analysis to demonstrate that the intermittence phenomenon would be less problematic in larger CSTRs. 

Kotronarou et al. (1991) detected temperatures on the order of 2000 K at the gas/liquid interfacial region. Sonochemical reactions are characterized by the simultaneous occurrence of pyrolysis and radical reactions, especially at high solute concentrations. Any volatile solute will participate in the former reactions because of its presence inside the bubbles during the oscillations or collapse of the cavities. In the solvent layer surrounding the hot bubble, both combustion and free radical reactions are possible. 

---