Chemistry & Biochemistry Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2752
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Item LOCAL MOLECULAR FIELD THEORY FOR NON-EQUILIBRIUM SYSTEMS(2019) Baker III, Edward Bigelow; Weeks, John D; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Local Molecular Field (LMF) theory is a framework for modeling the long range forces of a statistical system using a mimic system with a modified Hamiltonian that includes a self consistent molecular potential. This theory was formulated in the equilibrium context, being an extension of the Weeks Chandler Andersen (WCA) theory to inhomogeneous systems. This thesis extends the framework further into the nonequilibrium regime. It is first shown that the equilibrium derivation can be generalized readily by using a nonequilibrium ensemble average and its relevant equations of motion. Specifically, the equations of interest are fluid dynamics equations which can be generated as moments of the BBGKY hierarchy. Although this approach works well, for the application to simulations it is desirable to approximate the LMF potential dynamically during a single simulation, instead of a nonequilibrium ensemble. This goal was pursued with a variety of techniques, the most promising of which is a nonequilibrium force balance approach to dynamically approximate the relevant ensemble averages. This method views a quantity such as the particle density as a field, and uses the statistical equations of motion to propagate the field, with the forces in the equations computed from simulation. These results should help LMF theory become more useful in practice, in addition to furthering the theoretical understanding of near equilibrium molecular fluids.Item DEVELOPMENT OF A MICROSCALE ELECTROCHEMICAL PLATFORM FOR THE ANALYSIS OF THERMAL PROFILES OF IMMOBILIZED DNA SECONDARY STRUCTURES(2019) Robinson, Sarah; Lee, Sang Bok; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Understanding the thermal stability of DNA secondary structures is important to the pharmaceutical industry, as drug molecules that strongly bind will increase the stability of the structure, leading to higher measured melting temperatures. Development of an electronic platform that can measure the thermal profiles of small-volume samples with automation and methodology that is scalable for high-throughput screening (HTS) would represent an important asset for the drug discovery process. This thesis endeavored to produce and demonstrate the feasibility of such a technology. A microelectronic device has been fabricated in the configuration of a planar electrochemical platform with an embedded platinum thin film that can function as both a platinum resistance thermometer (PRT) and as a resistive microheater. The device assembly as well as automation of the temperature control and electrochemical methods have been instituted to increase measurement repeatability with the microscale device. The operational program was developed with a variety of features, including a PID controller, and has been demonstrated for a two-device array; functioning is scalable to larger device arrays with the addition of suitable electronics. A proof-of-concept methodology has been shown for monitoring the stabilization effects of ligand binding to duplex DNA. Results are presented for both refrigeration with resistive heating, and thermoelectric cooling and heating. The technology has also been adapted to examine other DNA secondary structures, such as G-quadruplexes, and the stabilization of these structures. The resulting analysis of such immobilized intramolecular secondary structures has demonstrated that the systems are more complicated and further fundamental studies are needed. With the future incorporation of microfluidics and larger-device arrays, a range of effects can be tested based on the demonstrated technology to understand binding events of relevance to drug discovery and the complexities of the surface chemistry effects on the analysis of thermal profiles.Item POLYMER ASSISTED ASSEMBLY OF INORGANIC MATERIALS FOR NEXT GENERATION BATTERIES(2019) Carter, Marcus; Rodriguez, Efrain; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Nanoscale materials have desirable electronic features (e.g. high surface areas, reduced mass and transport paths) that can be harnessed for a variety of technological applications. In most storage devices, there is a particular interest in nanostructured electrodes and solid-state electrolytes. A key challenge is the reproducible fabrication of these nanostructured materials. Polymers are nanoscale materials that could be used for nanoscale fabrication with improved reproducibility. In this thesis I explored two nanostructured systems using novel polymer assisted assembly methods. I fabricate a nano-structured MoS2 electrode and a nano-structured Li7La3Zr2O12 solid-state electrolyte with a garnet-type structure. A clear redox mechanism for MoS2 is currently being sought. Using our electrode, we propose a mechanism to understand the total or partial decomposition of the electrode and the formation of long soluble polysulfides. We complete a fundamental study to determine the peaks on a cyclic voltammetry curve of nanostructured MoS2. We resolve these peaks by building a novel but simple system of restacked MoS2 with a conformal polyaniline (PANI) coating. We propose that the novel coating functions by absorbing, capturing, and promoting charge transfer (oxidization and reduction) of sulfur atoms remaining at the surface. Our data suggests that PANI acts as redox mediator. Redox mediators can be molecules or solid surfaces that aid in the charge transfer to redox species, traditionally oxide species. Our findings suggest that sulfur behavior dominates the redox chemistry at 0.7 V even earlier than the proposed deep discharge. We propose that longer chain polysulfides are formed through surface mediated interactions with persistent lattice planes of MoS2. Solid-state electrolytes like cubic garnet type Li7La3Zr2O12 offer safety advantages over flammable liquid electrolytes, which is especially significant to the advancement of high energy density battery devices. Garnet however is unstable in air, suffers from low preparation efficiency and degradation into a two competitive phases, tetragonal type garnet and lithium carbonate phases, which have low conductivity. For two polymers systems, poly(styrene)-block-poly(acrylic acid), PS(0.3)-b-PAA(0.7) and PS(0.8)-b-PAA(0.2), we synthesize cubic Li7La3Zr2O12 garnet. We systematically investigate the effect of growth parameters, temperature and excess lithium content, to find the optimized synthesis conditions of 750 °C for ~5 h with 60 wt.% and 65 wt.% excess lithium salt, for the polymer systems.Item OXYGEN STORAGE PROPERTIES OF TERNARY METAL OXIDE SYSTEMS FOR CHEMICAL LOOPING REACTIONS(2019) Jayathilake, Rishvi Sewwandi; Rodriguez, Efrain E; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)We have studied the reversible uptake and release of oxygen in the layered metal oxide system AB2O4 to understand their suitability as oxygen storage materials. We examined their structures at their most reduced, oxidized, and intermediate phases of AFe2O4 for A= Lu, Yb, Y, and In, and studied their structures with high-resolution synchrotron X-ray diraction. Under simulated chemical looping conditions, we monitored their structures and reactivity towards H2 and O2 utilizing in-situ X-ray diraction, neutron diraction, and thermogravimetric analysis measurements. The nature of the trivalent A cation aects the oxidation kinetics, thermal cycling stability, and oxygen storage capacity (OSC). With the exception of the A = In analogue, these layered oxides underwent various phase transitions above 200 °C that included the creation of a superstructure as oxygen incorporates until a high temperature phase is established above 400 °C. To understand trends in the oxygen incorporation kinetics, we employed bond valence sum analysis of the Fe-O bonding across the series. The more underbonded the Fe cation, the more facile the oxygen insertion. During the cycling experiments all samples exhibited reversible oxygen insertion at 600 °C for this series, and displayed OSC values between 0.2-0.27 O2 mol/mol. The Y analogue displayed the fastest kinetics for oxidation, which may make it the most suitable for oxygen sensing applications. The structure of the oxidized phase was solved from with simulated annealing and Fourier dierence maps. Structural parameters were reported with combine neutron and X-ray Rietveld renement. PDF and XAS were used to conrm the nal structural model. As the nal steps experiments were carried out to explore the chemical looping reactivity of AB2O4 layered oxides, with A= Lu, Yb, Y and B=Mn, Fe. We reported the reactivity with methane of AB2O4 layered oxides for the rst time. The RT pristine structure was regenerated at 600 °C under methane. Mn substituted compounds exhibited faster kinetics and also higher oxygen storage capacities. We conclude that the layered, ternary metal oxide system, AB2O4, is a suitable candidate as an oxygen storage material for the potential application in chemical looping reactions.Item STRUCTURE AND PROPERTIES OF ALLOYED CHALCOGENIDES WITH THE ThCr2Si2 TYPE STRUCTURE(2019) Virtue, Austin; Rodriguez, Efrain E; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The ThCr2Si2-type structure has proven itself to be an incredibly robust structure type. Its ability to incorporate elements from the majority of the periodic table has earned it the moniker of \The perovskite of intermetallics". This layered structural motif has the nominal formula of AM2X2, where typically, A is an electropositive atom, M is a transition metal, and X is a main group element. They are ordered as a layered structure of layers of two-dimensional MX-n4 edge sharing tetrahedra separated by layers of An+ cations. The wide variety of different compounds that have been characterized with this structure has resulted in almost as wide a variety of properties, including superconductivity. This dissertation demonstrates the affects that having a mixed metal site has on the properties of these compounds. Powder and single crystal samples are prepared for a series of compounds so that these effects can be compared for different X atom chalcogenides. We demonstrate that increasing the bond distances through changing the X atom from sulfur to selenium has a pronounced effect on the magnetic and electrical properties. Possible magnetic structures for KCuMnS2 are proposed for the first time. Different methods at tuning the structure to obtain new compounds are discussed.Item UNDERSTANDING THE SURFACE CHEMISTRY OF GAS PHASE ORGANOPHOSPHORUS CHEMICAL WARFARE AGENTS WITH SORBENT MATERIALS(2019) Holdren, Scott; Zachariah, Michael R.; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Chemical warfare agents (CWAs) pose a serious threat to civilians and warfighters due to their high toxicity and usage in recent attacks. Although existing filtration media (ASZM-TEDA) used in gas mask devices absorbs and decomposes a wide spectrum of CWAs, the filtration performance of this material can be compromised in the battlefield due to poorly understood mechanisms. The high toxicity of CWAs remains a barrier for most research institutions to study these compounds experimentally which hinders the search for improved filtration materials. To overcome this issue, studies are performed using relatively benign simulant compounds that have similar adsorption and decomposition properties as toxic CWAs. In this work, a report of experimental findings will be presented regarding how dimethyl methylphosphonate (DMMP), an organophosphorus CWA simulant, will adsorb and decompose on components that makeup ASZM-TEDA. The work presented in this dissertation deconstructs the components that makeup ASZM-TEDA in order to identify the role of specific metal oxides and the carbon support. This approach was facilitated using different analytical techniques including TGA, FTIR spectroscopy, and DFT modeling to gain a molecular understanding of how DMMP interacts with porous carbon (Chapter 3) and metal oxide nanoparticles/surfaces (Chapters 4 and 5). Lastly, a new method is described (Chapter 6) that overcomes many of the difficulties encountered in conventional measurements that monitor gas phase DMMP adsorption/desorption processes on sorbent materials. This method can be used to obtain reliable quantitative measurements and parameters (e.g. adsorption capacities, ∆Hads, and kads) of low vapor pressure adsorbate/sorbent systems making it particularly useful for CWAs/CWA simulants and new filtration materials (e.g. DMMP and porous carbon).Item Solution Processing of Long Carbon Nanotubes: from Fundamentals to Applications(2019) Wang, Peng; Wang, YuHuang; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Single-walled carbon nanotubes (SWCNTs) are one of the most intensively studied nanomaterials due to their extraordinary mechanical, electrical, and optical properties. Attaining aqueous solutions of individual SWCNTs is the critical first step for harnessing their outstanding properties and applying them in many applications and further processing, such as sorting, imaging, and sensing. However, the current ultrasonication-then-ultracentrifugation approach inevitably introduces defects to SWCNTs and cuts the nanotubes into smaller pieces, compromising the electrical and mechanical properties of this otherwise remarkable material. In this dissertation, we introduce an unexpectedly simple approach that completely eliminates the need for ultrasonication, and nondestructively disperses SWCNTs in aqueous solution, so that the synthetic lengths of SWCNTs can be preserved. The dispersion is achieved by using surfactants to wrap and stabilize the protonated SWCNTs by simple acid-base neutralization reactions. The result is that the protons on SWCNTs are replaced by surfactants, and thus, we name this method “superacid-surfactant exchange (S2E).” In chapters 2-4, we demonstrate the length of dissolved SWCNTs by S2E can be 4-10 times longer than the sonicated controls, thereby significantly improving the optical, electrical and electromechanical properties. We further find that by tuning the concentrations of SWCNTs in this S2E process, short nanotubes can be selectively extracted out, allowing separation of the long carbon nanotubes (>10 µm). In chapter 5, we show that long SWCNTs can behave like mechanical reinforcing structures that enhance the mechanical strength of graphene through π-π interactions without sacrificing much of the outstanding transparency of graphene. This fact has enabled the fabrication of the mechanically strong yet ultrathin graphene/SWCNTs hybrid structure (G+T) for operando probing of the electrical double layer at the electrode-electrolyte interface by X-ray photoelectron. Finally, as a ramification result from the S2E process, chapter 6 describes the scalable synthesis of organic-color-center tailored SWCNTs.Item Towards the synthesis of PNAG crosslinkers to identify protein binding partners(2019) Mrugalski, Kevin R; Poulin, Myles; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Bacterial biofilms are an area of major concern in the medical field due to natural drug resistance. Many pathogenetic species of bacteria that infect humans including Escherichia coli, Staphylococcus aureus, Pseudomonas aeruginosa, and Vibrio cholera form biofilms and their associated infections are becoming harder to treat. Poly β-(1→6)-N-acetyl-D-glucosamine (PNAG) is a major component of biofilms across multiple species and has been found to play a key role in the early stages of the biofilm life-cycle. However, little information is known about what proteins interact with this important polysaccharide. Our goal is to create small PNAG analogues to covalently capture and identify PNAG binding partners in E. coli, an important model organism. PNAG analogues will contain photoaffinity groups, that when activated, covalently link associated proteins to the probe. Then, using a proteomics-mass spectrometry-based approach, we will identify PNAG binding partners. Here, we describe the efforts and challenges encountered synthesizing the final PNAG probes. New synthetic routes are proposed based on literature precedent that will enable synthesis of the desired compounds.Item PHOTOINDUCED ELECTRON TRANSFER FOR PROTECTING GROUPS AND POLYMER SYNTHESIS(2019) Thum, Matthew David; Falvey, Daniel E; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Using light to drive chemical reactions affords spatial and temporal control not typically displayed in thermal chemistry. For this reason, light induced transformations have been used in the manufacturing of polymers and plastics, and in the development of systems that require precise activation, such as drug delivery. The work presented in this dissertation will involve photoinduced electron transfer (PET) and its applications in protecting groups and polymer synthesis. Chapter 1 will discuss photoinduced electron transfer and its theory. Examples will be provided to demonstrate how it has been used as a trigger in photoremovable protecting groups, and as the mechanism of initiation in controlled radical polymerization. In Chapter 2, three different protecting groups triggered by PET will be analyzed. The analysis of key intermediates involved in the mechanism will be performed using nanosecond transient absorption spectroscopy. Chapter 3 will discuss the adaptation of an N-alkyl picoloinum protecting group to be activated by stepwise absorption of two photons of visible light. Chapter 4 will explore the photophysical properties of commonly used chain transfer agents for controlled radical polymerization. The behavior of the chain transfer agents under ultra violet and visible light photolysis, electron transfer, and energy transfer will be examined. Chapter 5 will discuss the role of the oxidation of dimethyl sulfoxide (DMSO) in the initiation of polymerization using photoredox catalysts. Our work demonstrates that, under highly oxidative conditions, an electron can be abstracted from DMSO and the resulting DMSO cation radical can degrade to for radicals capable of initiating polymerization. We explore this process for an anthraquinone-base photoredox catalyst, and apply it, along with a chain transfer agent, to the manufacturing of polymers with precise molecular weights and narrow molecular weight distributionsItem EXPLORING THE RELATIONSHIPS BETWEEN FUEL AND OXIDIZER REACTION OF BIOCIDAL ENERGETIC MATERIALS(2019) Wu, Tao; Zachariah, Michael R.; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Energetic materials are defined as a class of material with extremely high amount of stored chemical energy that can be released when ignited, along with intensive light emission and shock generation. Developing new energetic materials with high efficiency neutralization of biological warfare agents has gained increased attention due to the increased threat of bioterrorism. The objective of this dissertation is to develop new energetic materials with biocidal capabilities and apply them in various nanothermite systems to explore the relationships between fuel and oxidizer reactions. Aerosol techniques offer a convenient route and potentially direct route for preparation of small particles with high purity, and is a method proven to be amenable and economical to scale-up. Here I demonstrate the synthesis of various iodine oxides/iodic acids microparticles by a direct one-step aerosol method from iodic acid. A previously misidentified phase of I4O9 hydrate is in fact a new polymorph of HIO3 which crystalizes in the orthorhombic space group P212121. Various iodine oxides/iodic acids, including I2O5, HI3O8 and HIO3, were employed as oxidizers in thermite systems. Their decomposition behaviors were studied using a home-made time resolved temperature-jump/time-of-flight mass spectrometer (T-Jump/TOFMS). In addition, nano-aluminum (nAl), nano-tantalum and carbon black were adopted as the fuel or additive in order to fully understand how iodine containing oxidizers react with the fuel during ignition. The ignition and reaction process of those thermites were characterized with T-Jump/TOFMS. Carbon black was found to be able to lower both initiation and iodine release temperatures compared to those of Al/iodine oxides and Ta/iodine oxides thermites. Their combustion properties were evaluated in a constant-volume combustion cell and results show that nAl/a-HI3O8 has the highest pressurization rate and peak pressure and shortest burn time. However, an ignition delay was always present in their pressure profiles while combusting. To shorten or eliminate this ignition delay, a secondary oxidizer CuO is incorporated into Al/I2O5 system and four different Al/I2O5/CuO thermites by varying the mass ratio between two oxidizers are prepared and studied in a constant volume combustion cell. Significant enhancement is observed for all four thermites and their peak pressures and pressurization rates are much higher than that of Al/I2O5 or Al/CuO. Two other oxidizers also demonstrate similar effects as to CuO on promoting the combustion performance of Al/I2O5. A novel oxidizer AgFeO2 particles was prepared via a wet-chemistry method and evaluated as an oxidizer in aluminum-based thermite system. Its structure, morphologies and thermal behavior were investigated using X-ray diffraction, scanning electron microscopy, TGA/DSC, and T-Jump/TOFMS. The results indicate the decomposition pathways of AgFeO2 vary with heating rates from a two-step at low heating rate to a single step at high heating rate. Ignition of Al/AgFeO2 at a temperature just above the oxygen release temperature and is very similar to Al/CuO. However, with a pressurization rate three times of Al/CuO, Al/AgFeO2 yields a comparable result to Al/hollow-CuO or Al/KClO4/CuO, with a simpler preparation method. T-Jump/TOFMS was used to study the ignition and decomposition of boron-based thermites. The ignition behaviors of bare boron nanopowders and boron-based nanothermites at various gaseous oxygen pressure were investigated using the T-Jump method. High-heating rate transmission electron microscopy studies were performed on both B/CuO and B/Bi2O3 nanothermites to evaluate the ignition process. I propose a co-sintering effect between B2O3 and the oxidizer play an important role in the ignition process of boron-based nanothermites.