Chemically Oscillating Reactions during the Diagenetic Formation of Ediacaran Siliceous and Carbonate Botryoids

Chemically oscillating reactions are abiotic reactions that produce characteristic, periodic patterns during the oxidation of carboxylic acids. They have been proposed to occur during the early diagenesis of sediments that contain organic matter and to partly explain the patterns of some enigmatic spheroids in malachite, phosphorite, jasper chert, and stromatolitic chert from the rock record. In this work, circularly concentric self-similar patterns are shown to form in new chemically oscillating reaction experiments with variable mixtures of carboxylic acids and colloidal silica. This is carried out to best simulate in vitro the diagenetic formation of botryoidal quartz and carbonate in two Ediacaran-age geological formations deposited after the Marinoan–Nantuo snowball Earth event in South China. Experiments performed with alkaline colloidal silica (pH of 12) show that this compound directly participates in pattern formation, whereas those with humic acid particles did not. These experiments are particularly noteworthy since they show that pattern formation is not inhibited by strong pH gradients, since the classical Belousov–Zhabotinsky reaction occurs in solution with a pH around 2. Our documentation of hundreds of classical Belousov–Zhabotinsky experiments yields a number of self-similar patterns akin to those in concretionary structures after the Marinoan–Nantuo snowball Earth event. Morphological, compositional, and size dimensional comparisons are thus established between patterns from these experiments and in botryoidal quartz and carbonate from the Doushantuo and Denying formations. Selected specimens exhibit circularly concentric layers and disseminations of organic matter in quartz and carbonate, which also occurs in association with sub-micron-size pyrite and sub-millimetre iron oxides within these patterns. X-ray absorption near edge structure (XANES) analyses of organic matter extracted from dolomite concretions in slightly younger, early Cambrian Niutitang Formation reveal the presence of carboxylic and N-bearing molecular functional groups. Such mineral assemblages, patterns, and compositions collectively suggest that diagenetic redox reactions take place during the abiotic decay of biomass, and that they involve Fe, sulphate, and organic matter, similarly to the pattern-forming experiments. It is concluded that chemically oscillating reactions are at least partly responsible for the formation of diagenetic siliceous spheroids and concretionary carbonate, which can relate to various other persistent problems in Earth and planetary sciences.

Oscillating Reactions Meet Polymers at Interfaces

Chemo-mechanical phenomena, including oscillations and peristaltic motions, are widespread in nature—just think of heartbeats—thanks to the ability of living organisms to convert directly chemical energy into mechanical work. Their imitation with artificial systems is still an open challenge. Chemical clocks and oscillators (such as the popular Belousov–Zhabotinsky (BZ) reaction) are reaction networks characterized by the emergence of peculiar spatiotemporal dynamics. Their application to polymers at interfaces (grafted chains, layer-by-layer assemblies, and polymer brushes) offers great opportunities for developing novel smart biomimetic materials. Despite the wide field of potential applications, limited research has been carried out so far. Here, we aim to showcase the state-of-the-art of this fascinating field of investigation, highlighting the potential for future developments and providing a personal outlook.

Investigation of CO2 capture systems, Lewis acid-base pairs, and oscillating reactions with electronic structure theory and kinetics-based approaches

Equilibrium is a key theme in chemistry education. Starting in high school and continuing in freshman general chemistry courses, STEM students have to learn the foundations of equilibria. What is the key concept of an equilibrium? How can we describe an equilibrium? The concept of an equilibrium constant K is introduced, and its relation to the Gibbs enthalpy [delta] G [superscript 0] is noted. The equilibrium constant K also is related in a straightforward manner to the forward and backward reaction rate constants. Usually, a few simple applications are discussed, primarily in the area of acid-base chemistry. The topic is revisited in organic chemistry and clarified conceptually with reaction energy diagrams. To study equilibrium as a student is one thing, and to study equilibrium problems as a researcher is quite another. How does one determine equilibrium constants and how does one determine reaction rate constants? What do we know about the accuracy of the experimental quantities reported in the literature? How does one deal with multi-equilibria? How does one account for non-ideal conditions and concentrated solutions? Over the last six years, I have learned how to approach and solve all of these issues. One of the most stunning insights was the realization that even so-called non-linear reactions can in fact be described in some cases by application of complicated systems of equilibrium reactions. The Glaser group very strongly believes that the interplay between experimental and theoretical work is vitally important to really understand a problem. This combination builds a strong focus on quantitative aspects and it often also leads to new insights that might not be attainable from experimentation or modeling alone. The five chapters presented here show that this two-pronged approach is widely applicable to several areas of chemistry. The two main topics of our studies have been carbon dioxide capture from air and reaction mechanisms of oscillating chemical systems. All of the chapters in my dissertation do have a very strong connection between theory and experimentation. I studied both aspects in most cases. Only in one case (Chapter 5) did I not perform the experiments, but even in this case, a very deep engagement with the experimental literature was required to solve a decades-long discrepancy. Chapter 1 is about the study of equilibria between different conformations of substrates and products and an evaluation of their effects on the overall reaction energy. Specifically, we studied the capture of CO2 by small alkylamines. The quality of that discussion was tested directly with the work described in Chapter 2. The work that led to Chapter 2 was an enormous learning experience; it was amazing to see all the pieces of the complicated multi-equilibrium system come together to determine the [delta] G [superscript 0] of the carbamylation of butylamine in aqueous solution. The interest in equilibria actually began with the quest of the non-linear dynamics group to understand oscillating chemical reactions. From the outset, this quest was pursued as an interdisciplinary project between chemistry and mathematics. My work with the dynamics group resulted in Chapters 3 and 4 of the present dissertation. Chapter 3 is a re-evaluation of the video-based kinetic analysis with high temporal resolution and over long timescales. The colorimetric studies revealed unexpected "hysteresis loops" in cerium-catalyzed Belousov-Zhabotinsky oscillating reactions. We studied the reaction progress in RGB space because we wanted to learn under what conditions the video-based analysis would allow for quantitative concentration determinations. The desire to assess the quality of the video-based analysis in RGB space, led to the serendipitous discovery of hysteresis loops. The origins of Chapter 4 had to do with the question as to whether accounting for ionic strength would be essential to obtain accurate simulations of BZ reactions. The goal of my work on phosphate buffers was an evaluation of the usefulness of Debye-Huckel theory to electrolyte solutions with highly-charged ions present in significant concentrations. The phosphate buffer systems are widely in use and outstanding experimental sets of pH values were available to really test the performance of the solution models. Many years of studies of the Lewis acid-base pair F3B[arrow]PH3 illustrate in a beautiful fashion what can go wrong when expertalists interpret their data based on inaccurate theory and when computational chemists do not seek consistency with existing experimental data published in the literature. A careful read of the literature clearly showed early on that experimental and theoretical reports on F3B[arrow]PH3 are entirely inconsistent. It took years to explain what was actually measured, namely the compound F2B-PH2, and to explain why many theoretical reports predicted the wrong dative-bonding geometry.

Chemically oscillating reactions in the formation of botryoidal malachite

The origin of banding patterns in malachite [Cu2CO3(OH)2] is an enduring problem in geology. While the bright green, vivid colors of this mineral have been attributed to the presence of Cu, no specific process has been proposed that can explain the perfect circularly concentric banding and geometrical shapes in botryoidal malachite. These patterns of concentric equidistant laminations are comparable to those arising from chemically oscillating experiments using the classical reactants of the Belousov-Zhabotinsky (B-Z) reaction. Through optical microscopy and micro-Raman imaging, this contribution documents that the geometric centers of the self-similar geometric patterns are often composed of organic matter. Carbon isotopes and trace elements further suggest that non-biological decarboxylation reactions of biological organic matter took place during diagenesis. Hence, the morphological and chemical characteristics of chemically oscillating reactions offer a plausible explanation for the formation of botryoidal malachite and abiotic environmental decarboxylation reactions.

0D–2D heterostructures as nanocatalysts for self-oscillating reactions: an investigation into chemical kinetics

Ceria-decorated graphene nanocomposites as an efficient catalyst for the oscillatory Belousov–Zhabotinsky reaction.