UMD Theses and Dissertations
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Item Effect of Free Chlorine Oxidation on the Deposition Kinetics of Bacteriophage MS2 on a Silica Surface(2015) Stephens, Heungkook Noriomoral; Mi, Baoxia; Civil Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Understanding the transport kinetics of water contaminates such as pathogenic viruses around solid surfaces is important in controlling groundwater contaminant plumes and optimizing contaminant removal in sand filtration units. The effect of increasing oxidative stress, in the form of small doses of free chlorine, on the deposition behavior of bacteriophage MS2 onto a silica surface was examined using a quartz crystal microbalance with dissipation (QCM-D). MS2 deposition rates were analyzed by reaction time and inactivation level. A statistically significant increase in the deposition rate was identified between MS2 test solutions not exposed to free chlorine and most test solutions that were exposed. However, as exposure to free chlorine was increased, no relationship was able to be deduced from the collected deposition data. Potential explanations based on previous work were discussed. Observations also indicated that more comprehensive purification procedures in comparison to previous studies were necessary to obtain accurate QCM-D data.Item Applications of 2,3-Diketoesters in Organic Synthesis and Stereoselective Transformations(2014) Truong, Phong Minh; Doyle, Michael P; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Dimethyldioxyrane oxidation of δ-hydroxy-α-diazo-β-ketoesters that are prepared by zinc triflate catalyzed Mukaiyama-aldol condensation of methyl diazoacetoacetates with aldehydes, occurred in quantitative yield to form dihydrofuranols that undergo acid catalyzed dehydration under mild conditions to generate 3-methoxyfuran-2-carboxylates in good yield. Oxidation of ζ-keto-α-diazo-β-ketoesters that are formed by zinc triflate catalyzed Mukaiyama-Michael condensation of methyl diazoacetoacetate enones procduced their 2,3,7-diketoester derivative in quantitative yield. The intramolecular acid catalyzed aldol cyclization of 2,3,7-triketoesters provides highly functionalized cyclopentanones with good diastereoselectivity in high overall yields via kinetically controlled and stereodivergent catalytic processes. Lewis acid catalysis gives high selectivity for the 1,3-anti tetrasubstituted cyclopentanones, whereas Brønsted acid catalysis produces the corresponding 1,3-syn diastereomer. The first enantioselective transformation of 2,3-diketoesters was demonstrated in carbonyl-ene reactions catalyzed by [Cu((S,S)-tert-Bu-box)](SbF6)2 generating chiral α-functionalized-α-hydroxy-β-ketoesters in up to 94% yield and 97% ee. The suggested mode of activation is bi-dentate coordination between copper and the oxygen atoms of the two keto-carbonyl groups. The 2,3-diketoesters are conveniently accessed from the corresponding α-diazo-β-ketoester, and catalyst loading as low as 1.0 mol % is achieved.Item The Novel Use of Nitroxide Antioxidants as Free Radical Scavengers in Ultra-High Molecular Weight Polyethylene (UHMWPE) for Total Joint Replacements(2010) Chumakov, Marina Konstantinovna; Al-Sheikhly, Mohamad; Bioengineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Ultra-high molecular weight polyethylene (UHMWPE) has been the standard load-bearing material used in total joint replacements since the 1960s. However, oxidative degradation can lead to premature aging and wear of UHMWPE, requiring implant revision. The novel use of nitroxide antioxidants to prevent oxidation in UHMWPE was proposed and the resulting structure and property changes were evaluated in this work. Standard sterilization and crosslinking methods of Co-60 gamma or high energy electron beam radiation produce alkyl free radicals on the polymer chain. Alkyl radicals react to form bimolecular crosslinks and long-lived allyl radicals at high dose rates; at low dose rates they tend to react with oxygen to form peroxyl radicals. The peroxyl radicals further interact with the polymer chain producing hydroperoxides and more free radicals, leading to oxidative degradation. As an alternative to post-irradiation remelting, which allows radical recombination but reduces fatigue strength, antioxidants can be introduced into UHMWPE to scavenge residual radicals. Nitroxides are stable organic compounds that have a strong paramagnetic signal and are very efficient in preventing lipid peroxidation and in providing radioprotection in biological tissues. The nitroxides used are 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) and 4-hydroxy-TEMPO (Tempol). Through radical-radical interactions, radiolytically-produced alkyl radicals in UHMWPE are scavenged by the nitroxide radical. This is demonstrated through Electron Paramagnetic Resonance (EPR) spectroscopy where the paramagnetic nitroxide signal decays as it interacts with carbon-centered radicals in UHMWPE. Pulse radiolysis kinetics studies also show that alkyl radicals in UHMWPE preferentially react with nitroxides in the absence of oxygen. Controlled infiltration of UHMWPE with nitroxides is also observed using EPR. The resulting crosslink densities were investigated using Thermomechanical Analysis. It was observed that the addition of nitroxides after irradiation does not alter the crosslink density. Tensile testing of crosslinked and nitroxide-doped UHMWPE demonstrates increased ultimate tensile strength and toughness and the material exhibits an increase in crystallinity. Additionally, accelerated aging of specimens containing trace levels of nitroxide show very low oxidation levels when placed in an aggressive oxygen environment. Consequently, low concentrations of nitroxides diffused into UHMWPE after crosslinking produce an oxidation-resistant and highly crosslinked material for improved implant performance.Item MOLECULAR DYNAMICS STUDIES OF METALLIC NANOPARTICLES(2009) Henz, Brian John; Zachariah, Michael R; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Metal nanoparticles have many desirable electrical, magnetic, optical, chemi-cal, and physical properties. In order to utilize these properties effectively it is neces-sary to be able to accurately predict their size-dependent properties. One common method used to predict these properties is with numerical simulation. The numerical simulation technique used throughout this effort is the molecular dynamics (MD) si-mulation method. Using MD simulations I have investigated various metallic nano-particle systems including gold nanoparticles coated with an organic self-assembled monolayer (SAM), the self-propagating high-temperature synthesis (SHS) reaction of nickel and aluminum nanoparticles, and the mechano-chemical behavior of oxide coated aluminum nanoparticles. The model definition, boundary conditions, and re-sults of these simulations are presented in the following dissertation. In the first material system investigated MD simulations are used to probe the structure and stability of alkanethiolate self-assembled monolayers (SAMs) on gold nanoparticles. Numerous results and observations from this parametric study are pre-sented here. By analyzing the mechanical and chemical properties of gold nanopar-ticles at temperatures below the melting point of gold, with different SAM chain lengths and surface coverage properties, we have determined that the material system is metastable. The model and computational results that provide support for this hy-pothesis are presented. The second material system investigated, namely sintering of aluminum and nickel, is explored in chapter 4. In this chapter MD simulations are used to simulate the kinetic reaction of Ni and Al particles at the nanometer scale. The affect of par-ticle size on reaction time and temperature for separate nanoparticles has been consi-dered as a model system for a powder metallurgy process. Coated nanoparticles in the form of Ni-coated Al nanoparticles and Al-coated Ni nanoparticles are also analyzed as a model for nanoparticles of one material embedded within a matrix of the second. Simulation results show that the sintering time for separate and coated nanoparticles is dependent upon the number of atoms or volume of the sintering nanoparticles and their surface area. We have also found that nanoparticle size and surface energy is an important factor in determining the adiabatic reaction temperature for both systems, coated and separate, at nanoparticle sizes of less than 10nm in diameter. The final material system investigated in chapters 5 and 6 is the oxide coated aluminum nanoparticle. This material system is simulated using the reactive force field (ReaxFF) potential which is capable of considering the charge transfer that occurs during oxidation. The oxidation process of oxide coated aluminum nanoparticles has been observed to occur at a lower temperature and a faster rate than micron sized nanoparticles, suggesting a different oxidation mechanism. From this effort we have discovered that the oxidation process for nanometer sized oxide coated aluminum particles is the result of an enhanced transport due to a built-in electric field induced by the oxide shell. In contrast to the currently assumed pressure driven diffusion process the results presented here demonstrate that the high temperature oxidation process is driven by the electric field present in the oxide layer. This electric field ac-counts for over 90% of the mass flux of aluminum ions through the oxide shell. The computed electric fields show good agreement with published theoretical and experi-mental results. The final chapter includes some important conclusions from this work and highlights some future work in these areas. Future work that is outlined includes ef-forts that are currently underway to analyze the interactions of multiple alkanethiolate coated gold nanoparticles in vacuum and in solvent. Other future efforts are farther out over the horizon and include using advanced computing techniques such as gen-eral purpose graphical processing units (GPGPU) to expand simulation sizes and physical details over what it is currently possible to simulate.Item Oxidation-Reduction Transformations of Chromium in Aerobic Soils and the Role of Electron-Shuttling Quinones in Chemical and Microbiological Pathways(2008-05-05) Brose, Dominic; James, Bruce R; Plant Science and Landscape Architecture (PSLA); Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Soils from three toposequences in Maryland with minimal heavy metal contamination were sampled to investigate oxidation-reduction transformations of chromium in whole soil samples. Chromium (VI) reduction to Cr(III) was observed in all 18 samples, and 11 demonstrated enhanced reduction with the electron shuttle anthraquinone-2,6-disulfonate (AQDS). Oxidation of Cr(III) to Cr(VI) was observed in 12 samples, and 7 samples demonstrated diminished oxidation with AQDS. Lactate was added to the Watchung series to enhance biological activity, and high salt concentration was added to inhibit it. Both treatments reduced Cr(VI) to below detection limits by 11 d, suggesting abiotic reduction. The control treatment demonstrated reduction of Cr(VI) without soil. To further investigate, increasing lactate concentrations were added to Cr(VI) and AQDS. Reduction increased with increasing concentration; 60 mM lactate reduced all Cr(VI) within 1 hr. Other organic acids were tested for similar interactions; tartrate and citrate reduced Cr(VI), which was enhanced with AQDS.