In-situ examination of diffusion and precipitation processes during the evolution of chemical garden systems

“Chemical” or “silicate gardens” are a well known example for the spontaneous formation of a complex and structured system from ordinary educts. Simply by addition of soluble metal salt crystals to alkaline silica sols, dissolution of the metal salt and subsequent solidification initiate a self-organization process, which not only produces two separated compartments with drastically different chemical conditions by precipitation of a closed and tubular membrane but also produces a variety of stunning structures reminiscent of living forms such as trees or aquatic plants. Although a considerable number of scientific studies was dedicated to chemical gardens and related phenomena during the last more than 300 years, current literature is still lacking in central aspects of chemical garden growth. Especially due to the fact that most of the studies concentrated on ex-situ characterizations of these stunning architectures, only rare information is available to date on the evolution of dynamic processes occurring during their growth. The present thesis therefore mainly focuses on the time-resolved evaluation of crucial parameters in the course of chemical garden growth to contribute to the amplification of the knowledge on these long since discovered phenomena. Several strategies are developed in this work, aiming on the establishment of suitable in-situ examination techniques allowing for the direct observation of critical parameters in both generated compartments during chemical garden evolution. Implementation of a novel preparation procedure – involving the slow addition of sodium silicate solution to tablets of pressed metal salts (instead of small seed crystals) – yields in the formation of uniform and macroscopic tubular membranes with one end open to the atmosphere. This modification of the classical preparation procedure paves the way for directly accessing the heretofore caged interior compartment and therefore allows a continuous analysis by different techniques. X-ray absorption spectroscopy (XAS) and atomic emission spectroscopy (AES) techniques are applied to analyze the temporal evolution of ion species concentrations and their distribution in the outer and interior compartments of the generated silicate garden tubes. Together with continuous measurements of the pH in the interior and exterior compartments, these techniques are used for the detection of occurring diffusion processes across the precipitated tubular membrane and their temporal evolution. Presented results from these measurements show that chemical gardens are a complex system operating far from equilibrium due to a spontaneous separation of two solutions with drastic concentration differences by a porous membrane during the early stages of formation. It is demonstrated that the evolution of the system and its return to thermodynamic equilibrium are not at all completed once macroscopic growth of the well-known tubular structures is terminated. Instead, a series of diffusion and (coupled) precipitation processes occur over timeframes of up to days after preparation, gradually relieving the initially generated concentration gradients. The results of this work further illustrate that the walls of silica gardens allow bidirectional and non-specific ion transport, and thus fundamentally challenge the currently accepted model of a semi-permeable membrane. Observed concentration and pH gradients across the tube wall implicate the existence of appreciable potential differences between the two compartments, which were directly measured in this work. From results of long-term electrochemical potential measurements, it is deduced that the overall detected cell potential can be explained by a superposition of diffusion, membrane and pH induced potentials prevailing across the silicate garden walls. Different in-situ and ex-situ XRD techniques are used to identify the existence of crystalline material in the precipitated tube walls. Recorded data from independently obtained XRD analyses, together with results from AES and XAS measurements, are used to establish a model that describes the kinetics of precipitation and crystallization processes during chemical garden growth as those of an irreversible consecutive reaction. The kinetic model of silicate garden growth is found to be universally applicable and might therefore probably be transferable to other systems, in which a combination of dissolution, precipitation and crystallization processes play an important role, e.g. in the progress of Portland cement hydration. Ex-situ analyses of isolated membrane tubes on their structural and chemical composition, using scanning electron microscopy (SEM) in combination with energy dispersive X-ray spectroscopy (EDX), reveal a partial layering of the precipitated membrane tubes, exhibiting a silica-rich exterior and an interior surface mainly consisting of pure metal hydroxide. Furthermore, different kinds of stunning sub-structurings of the exterior and interior surfaces of the tubular precipitates are observed, ranging from periodical waviness on the exterior silica skin via interwoven fiber-networks up to clusters of metal oxide hydroxide rosettes or isolated crystallites. Therefore, this structural diversity directly mirrors the influence of the preparation technique as well as of precipitation and crystallization processes, occuring during the growth of chemical gardens, on their resulting structure

Diffusion and Precipitation Processes in Iron-Based Silica Gardens

Silica gardens are tubular structures that form along the interface of multivalent metal salts and alk. solns. of sodium silicate, driven by a complex interplay of osmotic and buoyant forces together with chem. reaction. They display peculiar plant-​like morphologies and thus can be considered as one of the few examples for the spontaneous biomimetic self-​ordering of purely inorg. materials. Recently, we could show that silica gardens moreover are highly dynamic systems that remain far from equil. for considerable periods of time long after macroscopic growth is completed. Due to initial compartmentalization, drastic concn. gradients were found to exist across the tube walls, which give rise to noticeable electrochem. potential differences and decay only slowly in a series of coupled diffusion and pptn. processes. The effect of the nature of the metal cations on the dynamic behavior of the system has been studied. The authors have grown single macroscopic silica garden tubes by controlled addn. of sodium silicate sol to pellets of iron(II) and iron(III) chloride. In the following, the concns. of ionic species were measured as a function of time on both sides of the formed membranes, while electrochem. potentials and pH were monitored online by immersing the corresponding sensors into the two sepd. soln. reservoirs. At the end of the expts., the solid tube material was furthermore characterized with respect to compn. and microstructure by a combination of ex situ techniques. The collected data are compared to the previously reported case of cobalt-​based silica gardens and used to shed light on ion diffusion through the inorg. membranes as well as progressive mineralization at both surfaces of the tube walls. These results reveal important differences in the dynamics of the three studied systems, which can be explained based on the acidity of the metal cations and the porosity of the membranes, leading to substantially dissimilar time-​dependent soln. chem. as well as distinct final mineral structures. The insight gained in this work may help to better understand the diffusion properties and pptn. patterns in tubular iron (hydr)​oxide​/silicate structures obsd. in geol. environments and during steel corrosion

PROPHECY—a yeast phenome database, update 2006

Connecting genotype to phenotype is fundamental in biomedical research and in our understanding of disease. Phenomics—the large-scale quantitative phenotypic analysis of genotypes on a genome-wide scale—connects automated data generation with the development of novel tools for phenotype data integration, mining and visualization. Our yeast phenomics database PROPHECY is available at . Via phenotyping of 984 heterozygous diploids for all essential genes the genotypes analysed and presented in PROPHECY have been extended and now include all genes in the yeast genome. Further, phenotypic data from gene overexpression of 574 membrane spanning proteins has recently been included. To facilitate the interpretation of quantitative phenotypic data we have developed a new phenotype display option, the Comparative Growth Curve Display, where growth curve differences for a large number of mutants compared with the wild type are easily revealed. In addition, PROPHECY now offers a more informative and intuitive first-sight display of its phenotypic data via its new summary page. We have also extended the arsenal of data analysis tools to include dynamic visualization of phenotypes along individual chromosomes. PROPHECY is an initiative to enhance the growing field of phenome bioinformatics

Precipitation and Crystallization Kinetics in Silica Gardens

Silica gardens are extraordinary plant-like structures resulting from the complex interplay of relatively simple inorganic components. Recent work has highlighted that macroscopic self-assembly is accompanied by the spontaneous formation of considerable chemical gradients, which induce a cascade of coupled dissolution, diffusion, and precipitation processes occurring over timescales as long as several days. In the present study, this dynamic behavior was investigated for silica gardens based on iron and cobalt chloride by means of two synchrotron- based techniques, which allow the determination of concentration profiles and time-resolved monitoring of diffraction patterns, thus giving direct insight into the progress of dissolution and crystallization phenomena in the system. On the basis of the collected data, a kinetic model is proposed to describe the relevant reactions on a fundamental physicochemical level. The results show that the choice of the metal cations (as well as their counterions) is crucial for the development of silica gardens in both the short and long term (i. e. during tube formation and upon subsequent slow equilibration), and provide important clues for understanding the properties of related structures in geochemical and industrial environments

Simulations on transient absorption spectroscopy of energy and charge transfer systems

Anregungsinduzierte Ladungstransferprozesse gemischtvalenter Verbindungen in einem, bzw. zwei Vibrationsfreiheitsgraden werden mithilfe vibronischer Modellsysteme untersucht. Anhand transienter und linearer Absorptionsspektren werden die berechneten mit experimentell bestimmten Daten verglichen. Eine detailliertere theoretische Analyse erfolgt unter den Gesichtspunkten der Populations- und Wellenpaketdynamik. Darüber hinaus wird der Prozess der Exziton-Exziton-Annihilierung mithilfe eines elektronischen Modellsystems untersucht. Zu diesem Zweck werden, zusätzlich zu den oben genannten Methoden, spektroskopische Signale unterschiedlicher Emissionsrichtungen zum Vergleich herangezogen.Optically induced charge transfer processes of mixed-valence compounds in one or two vibrational degrees of freedom respectively are studied using vibronic model systems. Calculated and experimentally determined data are compared based on transient as well as linear absorptions spectra. By means of population and wave-packet dynamics a more detailed theoretical analysis is performed. Furthermore, the process of exciton-exciton annihilation is studied using an electronic model system. Therefore, in addition to the methods mentioned above, spectroscopic signals in different directions of emission are compared

Additive-induced morphological tuning of self-assembled silica-barium carbonate crystal aggregates

Crystallisation of barium carbonate from alkaline silica solutions results in the formation of extraordinary micron-scale architectures exhibiting non-crystallographic curved shapes, such as helical filaments and worm-like braids. These so-called "silic

The effect of silica on polymorphic precipitation of calcium carbonate: an on-line energy-dispersive X-ray diffraction (EDXRD) study

Calcium carbonate is the most abundant biomineral and a compd. of great industrial importance. Its pptn. from soln. has been studied extensively and was often shown to proceed via distinct intermediate phases, which undergo sequential transformations before eventually yielding the stable cryst. polymorph, calcite. In the present work, we have investigated the crystn. of calcium carbonate in a time-resolved and non-invasive manner by means of energy-dispersive X-ray diffraction (EDXRD) using synchrotron radiation. In particular, the role of silica as a sol. additive during the crystn. process was examd. Measurements were carried out at different temps. (20, 50 and 80 °C) and various silica concns. Expts. conducted in the absence of silica reflect the continuous conversion of kinetically formed metastable polymorphs (vaterite and aragonite) to calcite and allow for quantifying the progress of transformation. Addn. of silica induced remarkable changes in the temporal evolution of polymorphic fractions existing in the system. Essentially, the formation of calcite was found to be accelerated at 20 °C, whereas marked retardation or complete inhibition of phase transitions was obsd. at higher temps. These findings are explained in terms of a competition between the promotional effect of silica on calcite growth rates and kinetic stabilization of vaterite and aragonite due to adsorption (or pptn.) of silica on their surfaces, along with temp.-dependent variations of silica condensation rates. Data collected at high silica concns. indicate the presence of an amorphous phase over extended frames of time, suggesting that initially generated ACC particles are progressively stabilized by silica. Our results may have important implications for CaCO3 pptn. scenarios in both geochem. and industrial settings, where soln. silicate is omnipresent, as well as for CO2 sequestration technologies. [on SciFinder(R)

Additive-induced morphological tuning of self-assembled silica-barium carbonate crystal aggregates

Crystn. of barium carbonate from alk. silica solns. results in the formation of extraordinary micron-scale architectures exhibiting non-crystallog. curved shapes, such as helical filaments and worm-like braids. These so-called "silica biomorphs" consist of a textured assembly of uniform elongated witherite nanocrystallites, which is occasionally sheathed by a skin of amorphous silica. Although great efforts have been devoted to clarifying the phys. origin of these fascinating materials, to date little is known about the processes underlying the obsd. self-organization. Herein, we describe the effect of two selected additives, a cationic surfactant and a cationic polymer, on the morphol. of the forming crystal aggregates, and relate changes to expts. conducted in the absence of additives. Minor amts. of both substances are shown to exert a significant influence on the growth process, leading to the formation of predominantly flower-like spherulitic aggregates. The obsd. effects are discussed in terms of feasible morphogenesis pathways. Based on the assumption of a template membrane steering biomorph formation, it is proposed that the two additives are capable of performing specific bridging functions promoting the aggregation of colloidal silica which constitutes the membrane. Morphol. changes are tentatively ascribed to varying colloid coordination effecting distinct membrane curvatures

Stabilization of amorphous calcium carbonate in inorganic silica-rich environments

In biomineralization, living organisms carefully control the crystallization of calcium carbonate to create functional materials and thereby often take advantage of polymorphism by stabilizing a specific phase that is most suitable for a given demand. In particular, the lifetime of usually transient amorphous calcium carbonate (ACC) seems to be thoroughly regulated by the organic matrix, so as to use it either as an intermediate storage depot or directly as a structural element in a permanently stable state. In the present study, we show that the temporal stability of ACC can be influenced in a deliberate manner also in much simpler purely abiotic systems. To illustrate this, we have monitored the progress of calcium carbonate precipitation at high pH from solutions containing different amounts of sodium silicate. It was found that growing ACC particles provoke spontaneous polymerization of silica in their vicinity, which is proposed to result from a local decrease of pH nearby the surface. This leads to the deposition of hydrated amorphous silica layers on the ACC grains, which arrest growth and alter the size of the particles. Depending on the silica concentration, these skins have different thicknesses and exhibit distinct degrees of porosity, therefore impeding to varying extents the dissolution of ACC and energetically favored transformation to calcite. Under the given conditions, crystallization of calcium carbonate was slowed down over tunable periods or completely prevented on time scales of years, even when ACC coexisted side by side with calcite in solution

Dynamic diffusion and precipitation processes across calcium silicate membranes

Hypothesis: Chemical gardens are tubular inorganic structures exhibiting complex morphologies and interesting dynamic properties upon ageing, with coupled diffusion and precipitation processes keeping the systems out of equilibrium for extended periods of time. Calcium-based silica gardens should comprise membranes that mimic the microstructures occurring in ordinary Portland cement and/or silicate gel layers observed around highly reactive siliceous aggregates in concrete. Experiments: Single macroscopic silica garden tubes were prepared using pellets of calcium chloride and sodium silicate solution. The composition of the mineralized tubes was characterized by means of various ex-situ techniques, while time-dependent monitoring of the solutions enclosed by and surrounding the membrane gives insight into the spatiotemporal distribution of the different ionic species. The latter data reflect transport properties and precipitation reactions in the system, thus allowing its complex dynamic behavior to be resolved. Findings: The results show that in contrast to the previously studied cases of iron- and cobalt-based silica gardens, the formed calcium silicate membrane is homogeneous and ultimately becomes impermeable to all species except water, hydroxide and sodium ions, resulting in the permanent conservation of considerable concentration gradients across the membrane. The insights gained in this work may help elucidate the nature and mechanisms of ion diffusion in Portland cements and concrete, especially those occurring during initial hydration of calcium silicates and the so-called alkali-silica reaction (ASR), one of the major concrete deterioration mechanisms causing serious problems with respect to the durability of concrete and the restricted use of many potential sources of raw materials. (c) 2022 Elsevier Inc. All rights reserved