5 Naturwissenschaften und Mathematik
"The New Chemistry" - Sustainable Catalysis with Alcohols
- Subject of the thesis are new iridium complexes stabilized by anionic P,N- or P,N,P-ligands. These complexes were used in homogeneous catalysis. Furthermore, mechanistic studies were performed to provide an insight into the catalytic cycles. Synthesis protocols for a multitude of different product classes have been developed.
The iridium complex 1, stabilized by a neutral P,N-ligand, reacts under basic conditions with 2-aminopyridines. By elimination of dipyridylamine the new catalyst species 2a was formed, which is more stable than catalyst 1.
Based on this finding eight new anionic P,N-ligands and the resulting iridium complexes were synthesized.
After optimization of the reaction conditions (solvent, base, temperature and catalyst loading) these catalysts were used in BH (borrowing hydrogen)/HA (hydrogen autotransfer) reactions. The selective monoalkylation of anilines with primary alcohols was investigated. In comparative experiments the superiority of the new class of catalysts versus the original catalyst 1 was clearly shown. Under mild reaction conditions (70 °C) the selectivity profile with respect to the monoalkylation has been preserved.
The catalytic protocol was subsequently extended to the alkylation of aromatic diamines. Therefore various diaminobenzenes were used as substrates. Also Dapsone®, an important drug in treatment of leprosy could be used as starting material. We succeeded in both symmetric and unsymmetric monoalkylations of diamines. Due to the selectivity profile of the catalyst regarding aromatic amines, also unprotected amino alcohols could be used as alkylating reagents.
By the use of tridentate P,N,P-ligands, a novel class of more stable catalysts compared to complexes 2a-9a, could be developed. Due to sealing the synthesis reactor with a semipermeable membrane, the retransfer of the “borrowed” hydrogen could be prevented and H2 is released. Dehydrogenation and condensation steps are now possible instead of BH/HA. By reacting secondary alcohols with β-amino alcohols, pyrroles were accessible.
After adapting the synthesis protocol to this new class of products the tolerance of functional groups was tested. Diversely functionalized alcohols were used. Under mild reaction conditions (90 °C) and very low catalyst loadings (down to 0.03 mol% iridium), a large number of novel pyrroles was accessible. Using this protocol 21 differently substituted α,α-pyrroles, 12 bicyclic pyrroles, symmetrically as well as non-symmetrically substituted oligopyrroles and three β-aminopyrroles were synthesized. The catalyst resting state was identified by NMR experiments and X-ray structure analysis to be an iridium trihydride. This trihydride is formed under catalytic conditions, by treatment of the pre-catalyst with alcohols or in hydrogen atmosphere.
In the final part of the work, a catalytic pyridine synthesis was developed. In this so far unknown heterocycle synthesis up to four different substituents could be introduced within a single reaction step. 2,6-, 2,5-, 2,4- and 2,3-substituted pyridines were synthesized selectively by using variously substituted primary or secondary alcohols and γ-amino alcohols. Furthermore, both the synthesis of bicyclic pyridines as well as the synthesis of pyridines that bear chiral substituents is possible.
Budget and fluxes of nitrogen in mountainous agroecosystems in a summer monsoonal climate under intensive land use
- A balanced nitrogen (N) cycle in intensively managed ecosystems is necessary as it underpins other ecosystem services. This study evaluated the agricultural practices in a typical mountainous catchment in South Korea in respect to N dynamics and their potential effect on water quality with the aim to develop options for a more sustainable catchment management.
In the 1st study, we used two approaches to calculate N budgets for the 5 key crops of the basin at the field scale. The gross and net N budgets for all crop types were found to be positive. Based on the small differences between the results of the two approaches we identified fertilizer N as well as soil Nmin as the dominant N input sources. As fertilizer N application was the major N input source (>50%), its reduction is the major scope of action for N savings at the field scale. A closely linked action is the synchronization of fertilizer N with soil Nmin. The large amount of fertilizer that is applied prior to planting (>60%) at the beginning of the monsoon season revealed that split applications could help reducing the fertilizer N additions and increase the low N use efficiencies (NUE). Based on the significant differences between gross and net N surplus for rice and bean fields, we identified the high amount of plant residues remaining after harvesting (>100 kg N ha-1) as a further factor for potential N savings. The 5 main crops accounted for over 80% of the total catchment N surplus (>400 Mg), even though their contribution to the area was only around 20%. A land use shift to perennial crops with lower N inputs was therefore found to be a possible but spatially limited chance to reduce N surpluses at the catchment scale. The comparison of catchment N surplus with stream N export revealed that 73-86% of the agricultural N surpluses were transported to water bodies in the catchment by either leaching or surface runoff.
In the 2nd study, we followed the fate of fertilizer N in a ridge and furrow (R/F) cultivation with polyethylene (PE) mulch by using 15N tracer. N leaching was simulated with Hydrus 2D. The comparison of 4 N fertilization levels (0, 150, 250 and 350 kg N ha-1) revealed that already 150 kg N ha-1 is sufficient to reach the maximal yield of radishes. Based on the low results of fertilizer N use efficiency (FNUE), we recommend two applications during the first 25 days of growth and a further application around day 50. These split applications adjusted to the plants’ needs increase the FNUE of the radish and decrease the fertilizer N losses during the growing season. However, split applications might be impractical in plastic covered R/F cultivations because mechanical equipment to apply fertilizer under the PE mulch is required. Based on the finding that 15N retention in soil and nitrate concentration in seepage water decreased similarly for ridges and furrows during the entire growing season, we conclude that the PE mulch had no significant effect on 15N retention in soil and on nitrate concentration in seepage water and did therefore not effectively protect the fertilizer in the ridges from percolation. Based on the simulation results, we found that the ridges and furrows contributed approximately an equal amount of leached N to the total amount. We therefore conclude that the PE mulch provided little protection for the fertilizer N in the ridges during heavy rainfall. N leaching amounts were further found to increase linearly with an increase in N addition rate as it is well known for R/F cultivations without PE mulch. The PE mulch did therefore not prevent the linear increase in leaching with an increase in fertilizer N addition. We summarize that without the use of additional measures such as split applications of fertilizer, the application of PE mulch in a summer monsoon climate with heavy rainfall events does not positively influence the N leaching rates.
In the 3rd study, we monitored soil water dynamics in the field and used this data set to simulate the influence of PE mulch on water fluxes with Hydrus 2D. We simulated soil water dynamics in 1) conventional flat tillage (CT); 2) R/F cultivation without PE mulched ridges (RT); and 3) R/F cultivation with PE mulched ridges (RTpm). The comparison of the simulated pressure heads during dry and wet periods revealed that the PE mulch induced significant soil moisture patterns only during the dry periods. During monsoon events, the effect of the PE mulch was dependent on the soil texture and the hydraulic conductivity. Summarizing the advantages and disadvantages of the R/F cultivation with PE mulch on sloped fields, the practice was observed to have the lowest amount of drainage water, the lowest evaporation rates but also the highest surface runoff rates. Hence, PE mulching might be assessed as a tool to reduce percolating water, but it concurrently increases water contribution to the river network by surface runoff.
Coarse-grained Modeling of Protein Dynamics using Elastic Network Models
- Dynamics is crucial for the functioning of biological macromolecules. Because of severe limitations in studying protein dynamics experimentally or with all-atom simulations, coarse-grained methods, especially elastic network models (ENMs), are frequently employed. In the last years, studies on various proteins showed that ENMs reliably reproduce experimental data, despite the simplified protein representation and the purely harmonic potential function. This work on two proteins with very different dynamical properties highlights the remarkable success of ENMs and shows which kind of questions can be answered using coarse-grained methods.
The allosteric enzyme aminoglycoside phosphotransferase(3')-IIIa (APH), which confers resistance against a broad range of aminoglycoside antibiotics to pathogenic bacteria, drastically changes its flexibility upon binding of substrates, but without changing its average conformation. In contrast, the homotrimeric vesicular stomatitis virus glycoprotein G (VSV-G), which triggers the pH-dependent fusion of viral and host membrane, undergoes a large structural rearrangement. A striking difference between the two proteins is their shape. VSV-G contains weakly constrained protein segments, the fusion loops, which can undergo large-scale motions at low energetic cost, whereas APH is not obviously arranged into different protein segments. Nevertheless, ENM calculations show that also APH consists of independently moving segments with correlated internal motion, so-called dynamic domains. The concept of dynamic domains can explain the differential effects of ligand binding on the dynamics of APH.
The first chapter of this thesis describes how experimental evidence for the importance of dynamics successively replaced the former idea of static proteins, and explains the dynamic basis of ligand binding, allostery and conformational changes. In the second chapter, theoretical methods for the analysis of protein dynamics are introduced, with emphasis on the ENM-based methods used in my studies. The studies are summarized in the third chapter. In the study on APH, I employ the Gaussian network model to analyze the ligand-dependent dynamics, the broad substrate specificity and the perturbation-sensitivity of the ligand binding sites. In a second study, ENM-based as well as all-atom molecular dynamics simulations are used to analyze the conformational change of VSV-G. Both approaches detect the fusion loops of VSV-G as most flexible parts of the protein, and thus as most likely starting point for the structural rearrangement, but only the all-atom model can generate deviations from the average structure at low pH. The last study describes the implementation and application of a dynamic domain assignment method, called CovarDom, which is based on covariances of residue fluctuations. Calculation of dynamic domains for a large protein set demonstrates the general applicability of CovarDom.
Crystalline-core micelles based on triblock terpolymers with polyethylene middle blocks
- This thesis is focused on the crystallization-induced structure formation of polyethylene containing triblock terpolymers in organic solvents to surface-compartmentalized worm-like crystalline-core micelles (wCCMs). Obtaining profound knowledge of the parameters controlling the self-assembly process allowed the production of a variety of complex one-dimensional micellar architectures with many potential applications, such as adaptive surfactants.
At first, the basic parameters that control the crystallization-induced self-assembly were explored using symmetric polystyrene-block-polyethylene-block-poly(methyl methacrylate) (PS-b-PE-b-PMMA) triblock terpolymers and a PS-b-PE-b-PS triblock copolymer. In good solvents for the PE block, e.g. THF and toluene, the selective formation of wCCMs was observed over a wide range of concentration, applied crystallization temperature and polymer composition. Whereas wCCMs produced by PS-b-PE-b-PS showed a homogeneous PS corona, a patch-like compartmentalization of the corona was observed if the micelles were formed by PS-b-PE-b-PMMA. As THF shows equal solvent quality for both corona blocks, wCCMs with almost alternating PS and PMMA compartments of about 15 nm were observed in this solvent. However, if structure formation was conducted in bad solvents for PE, such as dioxane or dimethylacetamide, spherical micelles with amorphous PE cores were formed already before crystallization. Hence, the subsequent crystallization of PE resulted in spherical CCMs with a patchy or a homogeneous corona depending on the used triblock. These findings allow the highly selective production of stable spherical or worm-like CCMs from the same polymer.
As the corona structure of the patchy micelles self-assembled from triblock terpolymers was mainly deduced from transmission electron microscopy (TEM) performed on dried samples, a small-angle neutron scattering (SANS) study was performed in order to elucidate the morphology in solution. Therefore a partly deuterated triblock terpolymer was synthesized and measured at different contrasts to allow the selective detection of the different corona compartments. The resulting SANS curves could be interpreted using a form factor model for core-shell cylinders with alternating PS and PMMA hemishells including interparticle interactions, thus validating the TEM observations. Notably, Janus-type and patchy cylinders can be clearly distinguished using the applied form factor model.
Moreover, the controlled formation of wCCMs with tunable corona composition and structure was achieved using the cocrystallization of different triblock copolymers. Via random cocrystallization of PS-b-PE-b-PMMA and PS-b-PE-b-PS the corona morphology could be tuned continuously from a mixed corona at low PMMA content over spherical PMMA patches of increasing number and size to alternating PS and PMMA patches. This approach allows to manufacture wCCMs with predefined corona structure omitting the need to synthesize a new tailor-made triblock terpolymer for every desired morphology.
By establishing the controlled crystallization-driven self-assembly of triblock terpolymers with PE middle blocks, it was further possible to prepare wCCMs with predefined average lengths up to 500 nm and length polydispersities as low as Lw/Ln = 1.1. Here, self-assembled spherical CCMs of PS-b-PE-b-PS were used as seeds for the controlled growth of PS-b-PE-b-PS unimers. Upon further addition of PS-b-PE-b-PMMA unimers these grew epitaxially onto the preexisting wCCMs, resulting in triblock co-micelles that consisted of middle blocks with a homogeneous PS corona and outer blocks with alternating PS/PMMA compartments. These structures represent not only the first block co-micelles including blocks with a patchy corona, but also the first ones produced from purely organic block copolymers.
In view of application, the ability of patchy wCCMs formed by PS-b-PE-b-PMMA to stabilize interfaces was investigated using pendant-drop tensiometry. The observed reduction of the interfacial tension at the toluene/water interface was significantly higher than that of comparable triblock terpolymer single chains and that of wCCMs with a homogeneous PS corona. Interestingly, the obtained equilibrium interfacial tension equaled that of Janus cylinders with similar dimensions. To explain this unexpected finding the corona chains were proposed to adapt to the interface via selective collapse and shielding of the incompatible part of the corona chains. Studying wCCMs formed by several triblock terpolymers with different compositions, the interfacial activity was found to increase with increasing overall length of the corona chains, and to a certain extent with the molar fraction of PS units in the corona.
Design of Robust Heterogeneous Catalysts for Sustainable Chemistry
- Robust heterogeneous catalysts based on polymer derived non-oxide ceramics (SiC/SiCN) with integrated late transition metal (Ni, Pd) nanoparticles (NPs) were designed, characterized and tested for their catalytic potential. For the decoration of SiCN support with metal NPs, a molecular approach was applied which makes the use of amido metal complexes for the chemical modification of the preceramic polymer (commercially known as HTT 1800, Clariant Advanced Materials GmbH, Sulzbach, Germany) by the transfer of metal to its nitrogen functions. The modified polymer was transformed to an amorphous SiCN material containing metal NPs by controlled pyrolysis under an inert atmosphere. By following aforementioned approach, Cu@SiCN materials (non-porous) have already been developed in our research group.
In present thesis, attempts were made to fabricate porous SiCN/SiC materials with good accessibility of metal (Ni and Pd) NPs. Firstly, the synthesis of palladium containg SiCN materials by the pyrolysis of palladium modified polysilazane was attempted. It was found that during polymer to ceramic transformation, palladium reacts with silicon to form intermetallic palladium silicide particles. The materials were characterized (TGA, FT-IR, NMR, PXRD and TEM) and applied in catalytic hydrogenation reactions. The activity of the catalysts was found low because of the silicidation of metal and non-porous SiCN support.
The issues of the low porosity of SiCN ceramics and the formation of metal silicides were addressed in the synthesis of Ni@SiCN materials. Controlled pyroylsis of Ni modified polysilazane at 600°C provided microporous Ni@SiCN materials, whereas at 1100°C the formation of nickel silicides in non-porous SiCN matrix was observed. Ni@SiCN materials were characterized by solid state NMR, FT-IR, PXRD, TGA, N2-physisorption and TEM. Hyperpolarized 129Xe NMR was used to study the effect of nickel loading and annealing time on pore structure. The microporous materials showed good thermal stability and activity in selective hydrogenation of aryl acetylenes but as polymer to ceramic conversion was not complete at 600°C, the materials possessed low hydrothermal stability at higher temperatures due to the hydrolysis of Si-N bonds.
In order to replace Si-N bonds with more stable Si-C bonds, a commercial polycarbosilane was used to fabricate SiC materials with integrated Ni NPs by the pyrolysis of nanostructured polycarbosilane-block-polyethylene (PCS-b-PE) polymer. PCS-b-PE was synthesized by the reaction of PCS with PE-OH catalyzed by an amido nickel complex followed by its nanostructuring via microphase separation technique. The length of PE block affected the type of pores generated in SiC material and the synthesis of highly porous materials with micro-, meso- and hierarchical porosity was achieved. Ni@SiC materials were characterized by TGA, PXRD, N2-physisorption and TEM. Hierarchically porous Ni@SiC selectively cleaves aromatic C-O bond of aryl ethers in water and was found a reusable catalyst.
Lastly, controlled pyrolysis of palladium modified polysilzane provided nanoporous SiCN materials with very small and highly accessible Pd NPs. This shows the role of SiCN support whose nitrogen atoms have a stabilizing effect on small metal nanoparticles. Moreover, SiCN support ineracts nicely with the NPs leading to the prevention of their leaching. In general, NPs have agglomeration tendencies with increasing metal loading but in the case of Pd@SiCN, size of metal NPs remained small even with a palladium loading of 14 wt%. Pulse titration with hydrogen showed high accessibility of Pd. Pd@SiCN materials showed better activity than commercial Pd/C catalyst in oxidation of alcohols in the absence of air or oxygen with the evolution of hydrogen.
M@SiCN/SiC materials may find applications in sustainable production of chemicals and fuels from renewable sources, for instance, from lignocellulosic biomass.
Direct Force Measurements on the Colloidal Scale: From Modified Electrodes to Particle Manipulation
- In this thesis the interfacial surface forces and mechanical properties of thin films have been studied by the colloidal probe technique. One central point is the combination of direct force measurements with an electrochemical setup in order to tune interfacial properties of an electrode modified with an organic layer. In particular the adhesion and ion adsorption have been studied, which are ubiquitous phenomena in the colloid science, electrochemistry, and biology. Moreover, a novel technique has been developed to fabricate chemically and mechanically stable colloidal probes for atomic force microscopy (AFM). Additionally, the elastic properties of polyelectrolyte multilayer films were locally resolved under controlled humidity.
The adhesive behaviour of colloidal particles on modified electrodes has been studied by direct force measurements with a micrometre-sized silica probe attached to an AFM-cantilever. By controlling the external potential applied to the modified electrode by means of a potentiostat, separate adhesion contributions at the modification layers in electrolyte solution were quantified. In particular, to determine the influence of the terminating functional groups, gold electrodes modified with self-assembled monolayers (SAMs) terminated in non-ionizable groups were used. It has been demonstrated that electrostatic double-layer forces dominate the adhesion of colloidal particles on hydrophobic and hydrophilic interfaces. In contrast to hydrophilic interface, for hydrophobic one forces due to the solvent exclusion play a significant role and leads to an offset in the adhesive force, which otherwise can be compensated by the external potential. However, the electrocapillarity is of minor importance and can be neglected.
To quantify the ion adsorption at organic interfaces a novel approach was followed, which is based on direct force measurements with silica colloidal probes on SAM-modified electrodes in electrolyte solutions. By variation of applied potential and concentration of specifically adsorbed ions, given by the solution’s pH, the charging behaviour of hydrophilic SAM-OH and hydrophobic SAM-CH3 has been determined. In difference to electrokinetic techniques, direct force measurements allow to probe the full range of the diffuse layer. The analysis of the diffuse layer potential as a function of externally applied potential provides important information. In particular, the shift of the potential of zero charge (pzc) indicates on the specific ion adsorption in the Stern layer as it alters the charging behaviour of the electrode’s interface. It has been demonstrated that hydronium and hydroxide ions adsorb on both the hydrophobic and hydrophilic interfaces. However, the presence of the background electrolyte (KCl) does not shift pzc and thus its ions have no specific affinity towards the interfacial adsorption. The adsorption of hydronium and hydroxide ions is stronger on hydrophobic, than on hydrophilic interface. This is in agreement with theoretical studies. The simple three-capacitor model based on a Langmuir-type adsorption isotherm provides semi-quantitative description of observed dependence of the diffuse double layer potential on applied potential.
A new technique for colloidal probe preparation was developed. A great challenge for the force measurements with the AFM is to ensure the cleanliness, chemical and mechanical stability of the used probes. The approach is based on high-temperature sintering of micrometer-sized silica particles to AFM cantilever with enhanced contact area. Due to a “neck” formed by nanometer-sized particles the increased mechanical stability of colloidal probes was achieved, which has been quantitatively determined by lateral force spectroscopy. The implementation of sintering procedure for silica colloids allowed the development of the highly stable colloidal probes, whose surface properties could be renewed by heating.
Finally, the mechanical properties of polyelectrolyte multilayer films have been determined by nanoindentation as a function of relative humidity. For these series of measurements again a colloidal probe has been used. It has been demonstrated that films containing polyglutamic acid have Young’s modulus, which depends on humidity. The change of stiffness with ambient humidity has reversible character.
Dual-Responsive Polymer and Hybrid Systems: Applications for Gene Delivery and Hydrogels
- This thesis focused on the synthesis of functional materials based on water-soluble and responsive polymers, in particular poly((2-dimethylamino)ethyl methacrylate) (PDMAEMA). The dual-responsive behavior and polycationic character at physiological pH of PDMAEMA lead to outstanding properties and thus, to a versatile component for water-based applications. The main concept of the thesis was to combine the ability for gene delivery of PDMAEMA with the magnetic properties of iron oxide nanoparticles to enable an activity of the gene vector in an applied external magnetic field. Another point was to apply the dual-responsive behavior of PDMAEMA (temperature and pH) for physically cross-linked hydrogels.
Initial studies on magnetic dual-responsive gene vectors revealed a facile synthesis of PDMAEMA-grafted iron oxide nanoparticles utilizing dopamine as physically binding anchor group for the polymer chains. Here, a dopamine-based ATRP initiator was applied for the surface modification of the nanoparticles, which enabled a controlled polymerization technique via the “grafting-from” approach. Gene transfection experiments with CHO-K1 cells show that the transfection efficiency is significantly higher than for poly(ethyleneimine) (PEI), which is regarded as the “gold standard” among the polycationic gene vectors. Although the hybrid particles show a considerably high molecular weight (4.3 MDa), which should lead to a significant increase of the cytotoxicity as observed for linear PDMAEMA their cytotoxicity is remarkably low, lower than that of PEI. Thus, the excellent performance in gene delivery experiments can be attributed to the star-like architecture of the PDMAEMA. Moreover, the uptake of our superparamagnetic gene vector into the cells enables a magnetic cell separation by applying an external magnetic field.
However, due to the non-covalent bonds of dopamine to the iron oxide nanoparticles, the PDMAEMA chains undergo a detachment with time from the nanoparticle surface. This led to the synthesis of PDMAEMA-based magnetic core-shell-corona nanoparticles. Here, the iron oxide nanoparticles were covered with a thin silica shell in order to link the PDMAEMA chains covalently to the inorganic core via silane chemistry. This approach revealed stable dual-responsive hybrid nanoparticles with irreversible binding of the polymer chains and a high long-term stability in aqueous media. These hybrid star-like particles also show excellent gene delivery. The inter-polyelectrolyte complex formation between the PDMAEMA corona of the core-shell-corona particles and pDNA showed that the pDNA molecules are individually complexed with single nanoparticles at N/P ratios (polymer nitrogen / pDNA phosphorous) where the best transfection results are obtained. The magnetic cell separation was further improved by using a Magnetic Activated Cell Sorting system (MACSTM). The magnetically separated cells maintain a high transfection efficiency as well as viability and could even be further cultivated.
Another aspect of this thesis was to include PDMAEMA as stimuli-responsive block in a double switchable block copolymer-based hydrogel. For this purpose, we chose a physically cross-linked ABCBA pentablock terpolymer system, which was polymerized via sequential ATRP and consist of a water-soluble PEO middle block, two dual-responsive (temperature/pH) PDMAEMA B-blocks as well as two thermo-responsive poly(di(ethylene glycol) methyl ether methacrylate) (PDEGMA) A-blocks (PDEGMA-b-PDMAEMA-b-PEO-b-PDMAEMA-b-PDEGMA). The aggregation behavior in dilute solution was investigated via temperature-dependent Dynamic Light Scattering (DLS) revealing that both stimuli-responsive blocks can be triggered separately and the coil-to-globule transition temperatures of the stimuli-responsive blocks were found to be strongly dependent on the block lengths for low molecular weights. In concentrated solutions, however, rheology studies did not show a further change in the mechanical properties after gelation for the investigated ABCBA pentablock terpolymer compositions. As a result, the principle of our complex system points towards a successful synthesis of a dual-responsive ABCBA pentablock terpolymer hydrogel system, which may show two distinct phase transition even for the gel state, if longer block lengths of the outer A- and B-blocks would be applied.
Dynamic Self-Assembly of Magnetic Colloidal Particles
Dynamic self-assembly represent one of the most powerful tools in Nature to spontaneously organize a system on a hierarchy of different scales.
Most of the processes at the nano/micro scale occur at very low Reynold’s number where inertia can be neglected. Creeping flow magnetic systems can be characterized by the Mason number.
The Mason number measures the ratio between the viscous and the magnetic torque and is the main parameter governing the behavior of paramagnetic colloids investigated in this thesis.
The work presented in this thesis explores new dynamic regimes of colloidal dynamics which occur when suddenly switching to high Mason numbers.
In a static magnetic field the equilibrium structure of paramagnetic colloids are chains. At high Mason number in a rotating magnetic field the time averaged equilibrium conformation is a two dimensional cluster.
By switching from a static to a rotating magnetic external field, we cause a transient dynamics from a static to the dynamic equilibrium state.
The first question addressed in this thesis is: what is the physics that determines the transient folding pathway from one to the other equilibrium state?
Dynamic magnetic fields were used by others to propel top down DNA-linked chains of paramagnetic colloids in a liquid.
The second question asked is whether we can dynamically self-assemble swimmers taking a fully bottom up approach?
The third question is: is it possible to assemble more complex dynamic patterns that lead to motion of the swimmers governed by more collective coupled hydrodynamics that goes beyond slender body theory of the linked chains?
This thesis answers the three questions and contributes to the understanding of colloidal dynamics and self-assembly in dynamic magnetic fields in the regime of high Mason numbers.
We explore two aspects of the dynamic self-assembly i.e. the transient kinetics between two dynamic self-assembled equilibria and the dynamically self-assembled propulsion of magnetic swimmers beyond slender body hydrodynamics.
The thesis therefore aims at achieving magnetic control over the assembly of complex dynamic colloidal structures.
Energy-domain synchrotron radiation Mössbauer source for physics under extreme conditions
- Iron is one of the most abundant elements on Earth, and it is an important component in minerals. Electronic and magnetic properties of iron-bearing materials significantly affect processes occurring in the deep interior of the Earth. In the materials that make up the Earth’s lower mantle iron may exist in different valence, spin states and crystallographic environments. Most of the existing experimental techniques either do not allow to separately follow evolution of different iron sites or are not suitable for measurements under high-pressure/high-temperature conditions. This makes studies of iron electronic structure under such conditions extremely challenging.
The current Ph.D. thesis is divided into two major parts. The first part is dedicated to the development of a Synchrotron Mössbauer Source (SMS). This device allows energy domain Mossbauer spectroscopy to be performed on a sample under pressures above 100 GPa in laser heated diamond anvil cells. The second part is dedicated to studying the behavior of iron in iron/alumina-bearing silicate perovskite under conditions of the Earth’s lower mantle.
1. Synchrotron Mössbauer Source
There are several techniques that allow magnetic and electronic properties of materials under extreme conditions to be probed: X-ray Emission Spectroscopy (XES), X-ray absorption near edge structure (XANES), Nuclear Resonance Spectroscopes, etc. For elements in which observation of Mössbauer effect is possible the most mature, sensitive, and suitable technique for studies of magnetic and electronic properties is energy-domain Mössbauer spectroscopy.
However, due to low brilliance of utilized radioactive sources and low natural abundance of iron in lower mantle minerals measurements using conventional energy- resolved Mössbauer spectroscopy require very long time and usually are limited to moderate pressures. The problem can be solved by combining the outstanding properties of synchrotron radiation (high brilliance, possibility for extreme focusing) with the energy-resolved approach. In brief, what is needed is a synchrotron source of Mössbauer radiation. Construction of such source was the primary task of my PhD work. The possibility to develop such a source was demonstrated at the Nuclear Resonance beamline ID18 at the European Synchrotron Radiation Facility (ESRF) by Smirnov et al. (1997). The source is based on pure nuclear reflections existing in antiferromagnetic 57FeBO3 crystals.
￼The major technical goals of my PhD work were to (a) construct a device that would be permanently ready for operation, and (b) optimize it to have the best possible resolution and highest possible intensity. In order to achieve these goals theoretical calculations were first conducted to understand how to best improve the performance. Second, several experiments were performed to confirm the theoretically predicted results. Third, several optical schemes of the SMS were tested in order to determine the optimal setup.
As a result of the research and development program a powerful Synchrotron Mössbauer Source (SMS) for high-pressure applications was constructed at the Nuclear Resonance beamline (ID18) of the ESRF. Using results obtained in the combined theoretical/experimental study of angular dependence of energy and temporal distributions of the pure nuclear reflections of iron borate crystal, the SMS was optimized for the highest possible intensity and best possible resolution. The bandwidth of radiation provided by the SMS is between 10-15 neV (2-3 Γ0, where Γ0 is a natural linewidth of Mössbauer resonance for Iron), the intensity is ~2.5×104 photons/s and the typical scanning velocity range is about ±12 mm/s (±0.6 μeV). In contrast to conventional radioactive sources, the SMS gives the possibility to focus the beam to tens of microns. SMS is the in-line monochromator, permanently located in the optics hutch and operational immediately after moving it into the incident beam position. The source can be used with all existing sample environments in the experimental hutches downstream of the beamline.
The implementation of this device opens the possibility for studying systems with complex hyperfine structure utilizing energy-resolved approach under various extreme conditions, for example at high-pressure. Furthermore, the SMS allows for very short collection times of only a few minutes, which enables data to be collected during laser heating. Several high-pressure and high-pressure/high-temperature studies that have already been performed are described in the second part of this Ph.D. thesis. The almost 100% recoilless resonant radiation delivered by the source and its high brightness allow a broad field of SMS applications. The SMS can be utilized in any mode of synchrotron storage ring operation.
2. Study of the spin state of Fe3+ ions in perovskite
Silicate perovskite (Mg,Fe)(Si,Al)O3 is the most abundant phase in the Earth’s lower mantle. Knowledge of its properties is indispensable for understanding lower mantle behavior. Dynamic, thermodynamic, and transport properties of silicate perovskite can be significantly affected by the valence and spin state of iron. Silicate perovskite with 5-10 mol% of Fe (where Fe3+/ΣFe ~50–75% (McCammon et al., ￼1997)) and Al, is dominant phase in Earth lower mantle (~75 vol%) (Zhang et al., 2006; Stackhouse et al., 2007). The behavior of Fe electronic properties under conditions close to those of the Earths lower mantle remains strongly controversial.
The second part of my Ph.D. work is dedicated to an investigation of the spin state of iron in Fe3+- rich silicate perovskite at high pressure. Four different silicate perovskite samples with different stoichiometry were studied using the Synchrotron Mössbauer Source. SMS spectra were collected at room temperature and pressures up to 122 GPa using diamond anvil cells, with or without laser annealing of the samples.
The hyperfine parameters, i.e., centre shift and quadrupole splitting, for the same phases, which were extracted from measured spectra for all perovskite samples studied in this work, are the same at each pressure within experimental error. Moreover, there is no change in Fe3+/ΣFe for individual samples over the entire pressure range of the experiment. The hyperfine parameters of the Fe3+ doublet are consistent with the high-spin state (Gütlich et al., 2011), and their smooth variation with pressure indicates that Fe3+ does not undergo spin crossover within the entire pressure range. All observed changes in the spectra are associated with abrupt changes in the electronic state of Fe2+. The hyperfine parameters of the low QS Fe2+ doublet correspond to the high-spin state (McCammon et al., 2008), while the doublet with high quadrupole splitting, whose intensity grows with pressure at the expense of the Fe2+ high-spin state, corresponds either to intermediate-spin (IS) Fe2+ (McCammon et al., 2008) or a distortion of the site occupied by high-spin Fe2+ (Hsu et al., 2010). Based on results presented in a work of Narygina (2010), we indentify changes in Fe2+ electronic structure as high-spin to intermediate spin transition. Irrespective of the interpretation of the Fe2+ spin state, conclusions regarding the absence of spin crossover in Fe3+ remain valid.
These results show that the previously reported spin crossover of Fe3+ ions does not occur when Fe3+ occupies the A-site. In both alumina-containing and alumina-free silicate perovskites Fe3+ ions remain in the high-spin state up to at least 122 GPa, i.e., almost up to the pressure corresponding to the lower mantle - outer core boundary. The results also indicate that Fe3+ ions do not diffuse from the A-site to the B-site in perovskite after high-temperature annealing at high pressure, Mössbauer spectra of before and after annealing are identical. There is also no evidence for high-spin to low-spin crossover of Fe3+ ions due to site change. In contrast, the results confirm that Fe2+ ions undergo a transition from a high-spin to an intermediate spin state, without reaching a low-spin state within the studied pressure range at room temperature. These results suggest that the seismic velocity anomalies in the lower mantle cannot be attributed to spin crossover in Fe3+.
Impact of extreme hydrological conditions on belowground carbon cycling and redox dynamics in peat soils from a northern temperate fen
Cristian Estop Aragonés
- Peatlands have an important role in the global carbon cycle and constitute the largest pool of carbon stored in terrestrial ecosystems due to their disproportionally high areal soil carbon density. This globally relevant carbon stock is the result of a process mostly initiated after the last glaciation period. A key factor for this long term carbon accumulation is the relative low decomposition of organic matter in these predominantly water logged ecosystems. Hydrological conditions play thus a fundamental role in peatlands and the feedback of carbon cycling in these ecosystems in response to climate change is under debate. Peatlands are important CO2 sinks but also constitute global sources of CH4. The atmospheric exchange and production rates of these greenhouse gases are strongly influenced by the hydrological regime. An increased frequency of extreme meteorological conditions resulting in drying and flooding events is predicted to occur in the future.
The major issue regarding the climate change debate at the global scale is how rapid these greenhouse gases are being released to the atmosphere. Despite the general consensus regarding the broad effects of drying and flooding on CO2 and CH4 exchange, belowground processes producing such greenhouse gases and their response to water table dynamics is underrepresented and usually simplified or overgeneralized. Temperature, moisture, oxygen content and nutrient content are among the major environmental controls for organic matter decomposition rates in peat soils. Another important and intrinsic control is peat quality or humification degree of organic matter. The interrelation and relevance of all these factors vary among sites and with hydrological condition in a temporal and spatial scale.
This work presents investigations focusing on belowground redox processes aiming to witness the dynamic interrelation of soil physical and chemical (soil gas and pore water chemistry) variables, and evaluates the relevance of some controls of organic matter decomposition during a wide range of hydrological conditions. Most of this work shows information under in situ conditions and complementary laboratory experiments were performed minding the in situ observations. The findings contribute to general knowledge by providing raw data in fen peats under fluctuating and contrasting water table conditions in a relatively high spatiotemporal resolved scale. Dryings led to increased air filled porosity, O2 intrusion, CO2 degassing, inhibition of methanogenesis and renewal of electron acceptors. The opposite trend occurred upon rewetting with pulses of iron and sulphate reduction and delayed methane production to a variable extent. Upon flooding, continued anaerobic conditions stimulated the accumulation of reduced products, methanogenic precursors (acetate and hydrogen) and CH4.
The general assumption that the water table directly controls the oxygen content in peat was relativized. This work shows that such relation is greatly influenced by peat physical properties, which partially control the changes in moisture. Based on these findings, the mineral content and the degree of compaction in organic soils can be implemented to more accurately represent the dynamics of aeration in peats upon water table changes. Another general assumption is that drying events, i.e. temporary decline of water table below mean position, lead to increased CO2 production and emission from peat soils to the atmosphere. Such statement was also relativized and must account for the depth distribution of respiration rates in relation to the mean water table of the peat deposit. Based on these findings, the high relative contribution of upper peat layers already exposed above the water table mask the effects of increased CO2 production in deeper peat upon water table drop. Additionally, the role of moisture was shown to be little for aerobic respiration. This work also evaluates the importance of drought severity by accounting for the post drought effects on methane production. More intense and prolonged drying events led to a greater regeneration of electron acceptors in peat soil, which broadly suppressed or limited methane production upon rewetting. This relation was not simple and several factors such as water table position, post drought water table fluctuations, temperature and organic matter content contributed to the recovery of methane production after drying. The provision of substrates by fermentation processes limited peat respiration during shallow water table and drying. In contrast, accumulation of acetate and hydrogen was observed during flooding indicating a decoupling of fermentation from terminal metabolism and favouring the co-occurrence of iron reduction, sulphate reduction and methanogenesis.