Chemical Waves and Patterns (Understanding Chemical Reactivity)
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By Philip Ball 20 February Chemical reactions proceeding far from their equilibrium state can generate complex spatial patterns in which concentrations of the reacting and diffusing ingredients vary from place to place.
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So-called reaction-diffusion processes like this are thought to underlie many natural patterns, from animal markings to coloured bands in minerals. Now, a team working in France and Japan have used strands of DNA to control the parameters of the molecular interactions so that the patterns they make might be tailored to order 1. As well as helping fundamental investigations of these chemical reactions, the method might be used to fix useful arrangements of molecules in new devices and materials. The key to these pattern-forming systems is autocatalysis: some of the reactants catalyse their own formation, creating a positive-feedback process that enables small differences in concentration to blossom into large ones.
A balance between runaway feedback that consumes the ingredients and molecular diffusion that replenishes them can lead to oscillations, radiating chemical waves and stationary patterns.
Here autocatalysis comes from the growth of an nucleotide strand A into a doubled nucleotide strand on a template with a complementary base sequence T , catalysed by a DNA polymerase enzyme. A second enzyme then nicks the mer in half, releasing two molecules of A. Co-author Yannick Rondelez of the University of Tokyo and his colleagues previously used this DNA-based reaction-diffusion scheme to make a molecular system with reaction dynamics mimicking that of a model of the interactions between populations of predators and their prey 2 , for which parallels with oscillating chemical reactions were first drawn over a century ago.
To control these patterns, one must be able to vary the two key parameters of the process: the rates of reaction and of diffusion. The former is quite straightforward: the rate of autocatalysis of A can be varied simply by changing the concentration of the template strand T. Controlling the diffusion is more challenging.
The researchers do it by slowing down T with a cumbersome appendage. They attach a cholesteryl group to the end, which, being fatty and hydrophobic, accumulates a shell of surfactants when these are added to the mixture. This reduces the rate of diffusion by a factor of up to 2. About one-third of the students had neither any notion of how the atoms in the copper II carbonate lattice interacted and were rearranged in the reaction nor any concept of bond-breaking and reformation in a chemical reaction.
Lesson Chemical Reactions Un-Notes | BetterLesson
Lagrangian descriptors of driven chemical reaction manifolds. The persistence of a transition state structure in systems driven by time-dependent environments allows the application of modern reaction rate theories to solution-phase and nonequilibrium chemical reactions. However, identifying this structure is problematic in driven systems and has been limited by theories built on series expansion about a saddle point.
Recently, it has been shown that to obtain formally exact rates for reactions in thermal environments, a transition state trajectory must be constructed. Here, using optimized Lagrangian descriptors [G. Craven and R. Hernandez, Phys. In particular, we demonstrate that this is exact for harmonic barriers in one dimension and this verification gives impetus to the application of Lagrangian descriptor-based methods in diverse classes of chemical reactions.
The development of these objects is paramount in the theory of reaction dynamics as the transition state structure and its underlying network of manifolds directly dictate reactivity and selectivity. In a recent paper it was shown that, for chemical reaction networks possessing a subtle structural property called concordance, dynamical behavior of a very circumscribed and largely stable kind is enforced, so long as the kinetics lies within the very broad and natural weakly monotonic class.
In particular, multiple equilibria are precluded, as are degenerate positive equilibria. Moreover, under certain circumstances, also related to concordance, all real eigenvalues associated with a positive equilibrium are negative. These conditions are weaker than earlier ones invoked to establish kinetic system injectivity, which, in turn, is just one ramification of network concordance. Because the Species- Reaction Graph resembles pathway depictions often drawn by biochemists, results here expand the possibility of inferring significant dynamical information directly from standard biochemical reaction diagrams.
Chemical reactions in reverse micelle systems. This invention is directed to conducting chemical reactions in reverse micelle or microemulsion systems comprising a substantially discontinuous phase including a polar fluid, typically an aqueous fluid, and a microemulsion promoter, typically a surfactant, for facilitating the formation of reverse micelles in the system.
The system further includes a substantially continuous phase including a non-polar or low-polarity fluid material which is a gas under standard temperature and pressure and has a critical density, and which is generally a water-insoluble fluid in a near critical or supercritical state. Thus, the microemulsion system is maintained at a pressure and temperature such that the density of the non-polar or low-polarity fluid exceeds the critical density thereof.
The method of carrying out chemical reactions generally comprises forming a first reverse micelle system including an aqueous fluid including reverse micelles in a water-insoluble fluid in the supercritical state. Then, a first reactant is introduced into the first reverse micelle system, and a chemical reaction is carried out with the first reactant to form a reaction product. In general, the first reactant can be incorporated into, and the product formed in, the reverse micelles.
A second reactant can also be incorporated in the first reverse micelle system which is capable of reacting with the first reactant to form a product. Chemical computing with reaction -diffusion processes. Chemical reactions are responsible for information processing in living organisms. It is believed that the basic features of biological computing activity are reflected by a reaction -diffusion medium. We illustrate the ideas of chemical information processing considering the Belousov-Zhabotinsky BZ reaction and its photosensitive variant.
The computational universality of information processing is demonstrated. For different methods of information coding constructions of the simplest signal processing devices are described.
The function performed by a particular device is determined by the geometrical structure of oscillatory or of excitable and non-excitable regions of the medium. In a living organism, the brain is created as a self-grown structure of interacting nonlinear elements and reaches its functionality as the result of learning. We discuss whether such a strategy can be adopted for generation of chemical information processing devices.
Recent studies have shown that lipid-covered droplets containing solution of reagents of BZ reaction can be transported by a flowing oil. Therefore, structures of droplets can be spontaneously formed at specific non-equilibrium conditions, for example forced by flows in a microfluidic reactor.
We describe how to introduce information to a droplet structure, track the information flow inside it and optimize medium evolution to achieve the maximum reliability. Applications of droplet structures for classification tasks are discussed. All rights reserved. Minimum Energy Pathways for Chemical Reactions. Computed potential energy surfaces are often required for computation of such parameters as rate constants as a function of temperature, product branching ratios, and other detailed properties. The talk will focus on a number of applications to reactions leading to NOx and soot formation in hydrocarbon combustion.
MRI of chemical reactions and processes. As magnetic resonance imaging MRI can spatially resolve a wealth of molecular information available from nuclear magnetic resonance NMR , it is able to non-invasively visualise the composition, properties and reactions of a broad range of spatially-heterogeneous molecular systems. Hence, MRI is increasingly finding applications in the study of chemical reactions and processes in a diverse range of environments and technologies.
This article will explain the basic principles of MRI and how it can be used to visualise chemical composition and molecular properties, providing an overview of the variety of information available. Examples are drawn from the disciplines of chemistry, chemical engineering, environmental science, physics, electrochemistry and materials science. The review introduces a range of techniques used to produce image contrast, along with the chemical and molecular insight accessible through them.
Methods for mapping the distribution of chemical species, using chemical shift imaging or spatially-resolved spectroscopy, are reviewed, as well as methods for visualising physical state, temperature, current density, flow velocities and molecular diffusion. Strategies for imaging materials with low signal intensity, such as those containing gases or low sensitivity nuclei, using compressed sensing, para-hydrogen or polarisation transfer, are discussed.
Systems are presented which encapsulate the diversity of chemical and physical parameters observable by MRI, including one- and two-phase flow in porous media, chemical pattern formation, phase transformations and hydrodynamic fingering instabilities.