Theses and Dissertations from UMD
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New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM
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Item Nonlinear Optical Studies of Molecular Adsorption and Solvation at Solid/Liquid and Liquid/Liquid Interfaces(2011) Siler, Antonie Renee; Walker, Robert A; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Interfacial solvation is responsible for promoting biological phenomena in vivo including protein folding, solute transfer across membranes and enzymatic activity. The specific solvation interactions responsible for these and other processes can be both cooperative and complex. Because many cellular processes rely on interfacial effects, understanding how forces at an interface influence a solute will give insight into how molecules behave within these cellular bodies. The studies presented here are focused on isolating how these solvation interactions vary systematically with the identity of the solute and solvent at an interface. The interfaces probed in these experiments varied from weakly to strongly associating interfaces defined as such by the identity of the solvent used to form the silica/liquid interface. Findings from strongly associating interfaces gave rise to surprising results from both the silica/methanol and silica/ethanol interfaces. The silica/ethanol interface forms a very polar interface as probed by the solute p-nitroanisole (pNAs). At the silica/methanol interface, a very nonpolar region was probed by several solutes sensitive to solvent polarity. The findings from the silica/methanol interface, led us to the research completed in the final chapter of this thesis. Data obtained from these measurements described the interfacial solvation and adsorption behavior of two solutes, pNAs and p-nitrophenol (pNP). Several silica/liquid interfaces were used in this study including, water, dimethyl sulfoxide (DMSO), acetonitrile (ACN), n-hexane, decane, cyclohexane, and methyl-cyclohexane. The two solutes are sensitive to solvent polarity and show similar solvatochromic behavior in bulk solvents. The solutes sample different interfacial polarities at the same silica/liquid interfaces according to SHG spectra obtained. pNAs is shown to be more sensitive to solvent identity at an interface than pNP, but less surface active. The sensitivity of pNAs to solvent identity at a silica/liquid interface is attributed to the solute's higher solubility in the solvents than pNP's solubility in the same solvents. On average, pNP has ~10 kJ/mol more adsorption energy at the measured interfaces than pNAs, and this too can be attributed to the inability of pNP to sufficiently solvate in many of the alkane solvents, forcing the solute out of solution and into the interface.Item PROBING ATMOSPHERIC AEROSOL AND GAS PROPERTIES WITH PHOTOACOUSTIC SPECTROSCOPY(2011) Bueno, Pedro Antonio; Zachariah, Michael R; Dickerson, Russell R; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Absorption by atmospheric aerosols is the wild card for global climate change. Issues regarding atmospheric gases and aerosols have been at the forefront and the work presented within is directed at those issues. Specifically, work has been performed in order to help understand the issue of absorption in the atmosphere and whether this contributes towards positive forcing or warming of the atmosphere. In the process of conducting this research a custom, first-principles photoacoustic spectrometer was improved, calibrated and used extensively in order to obtain knowledge of the interaction of light with atmospherically relevant gases and make the first measurements of absorbing aerosols. The absorption cross-section of uncoated and coated soot was measured and quantified and found to be consistent with other work where amplifications on the order of nearly 100% were observed with uncertainty levels much lower than previously reported. Soot was also found to be optically thin where the total mass of the soot contributes to the absorption. Consequential to the soot work, the photoacoustic spectrometer developed to measure the absorption was utilized as a high precision greenhouse gas sensor. The photoacoustic spectrometer was found to produce results on the absorption of CO2 to within 3% of the theoretically predicted line profile Moreover, the photoacoustic spectrometer was used to determine measurable coating thicknesses of less than 10 nanometers on 100 nm soot particles.Item Modeling and simulation of organic molecular clusters and overlayers on solid surfaces(2011) Liu, Qiang; Weeks, John D; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Driven by the rapid development of experimental methods and technology, nano scale physics and chemistry has become more and more important and practicable to study. Monolayers of organic molecules have been studied a lot recently because of many potential applications, such as organic photovoltaic devices (OPV) or organic Liquid Electric Diodes (OLED). It is important to understand and interpret these new experimental advances. At molecular scales, Monte Carlo (MC) simulations and molecular dynamics (MD) are two important methods in computational chemistry and materials science. This dissertation will use these simulation methods along with statistical mechanical theory to study the behavior of single monolayers of organic molecules on solid surfaces. First we give a brief introduction to two dimensional molecular systems. Different from bulk system or single molecules, 2D systems have many unique properties, and attract much experimental and theoretical research attention. Some common methods in experimental and theoretical studies are reviewed. After introducing the properties and experimental results of ACA/Ag(111), we build a lattice gas model and run Monte Carlo simulations to help interpret the experiments. The Pair approximation, a generalization of mean-field theory, is used to calculate the global phase diagrams and put our model into the more general class of spin-1 Ising models. The pair approximation can be used for modeling various monolayer organic molecular systems which correspond to different regions of the parameter space. Then we studied the C60/ZnPc/Ag(111) system, using molecular dynamic simulations. The C60 molecules form unusual chain structures instead of the close packed islands seen on metal surfaces, and we try to provide a theoretical explanation. Finally we use a density functional theory software to calculate the electronic structures of the C60/ZnPc/Ag(111) systems. This calculation predicts a 0.4e charge transfer from substrate to C60 molecule, which we believe is important for the C60 interactions on these surfaces. In general this thesis studies the behavior of organic monolayers and bilayers on metal substrates. This basic work could help us to understand general 2-D system dynamics and electronic properties, and may help us to find new interesting systems with special properties and applications.Item Solvation, Structure and Organization at Liquid Surfaces(2009) Brindza, Michael Ross; Walker, Robert A; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation presents the results of nonlinear spectroscopic studies whose goal is to understand how the asymmetric nature of interfaces and intermolecular interactions give rise to interfacial solvation properties and solvent structure. The first part of this thesis uses resonance enhanced second harmonic generation to examine the polarity and hydrogen bonding opportunities at interfaces formed between hydrophilic silica and both weakly and strongly associating organic liquids. Measuring interfacial electronic spectra of probe molecules that exhibit solvatochromic sensitivity to polarity and hydrogen bonding, we saw that small changes in solvent structure affect interfacial polarity, and strongly associating alcohols solvents create a region of heterogeneous polarity at the interface. Silica appears to donate hydrogen bonds to adsorbates no matter what solvent (protic or aprotic) was chosen. The second part of this dissertation uses another nonlinear spectroscopic technique, vibrational sum frequency generation, to determine the structure and orientation of solvent molecules adsorbed to silica/vapor, silica/liquid, and neat liquid/vapor interfaces. By comparing spectral features appearing under different experimental polarization conditions, we have determined average solvent orientations and degree of organization. Our initial studies of alkanes adsorbed to the silica/vapor interface show that despite strong substrate-adsorbate interactions, molecules at the interface show some degree of long range order and organization. In order to examine how the strength of intermolecular forces between adsorbates and either the substrate or neighboring molecules affect interfacial organization, we measured vibrational spectra of octanol isomers as well as different functional group containing n-alkyl molecules at silica/vapor and silica/liquid interfaces. The octanol studies show that strongly associating molecules form ordered monolayers at the silica/vapor interface, but that strength of lateral interactions is important for preserving that order when the liquid is brought into contact. Branched isomers appeared very disordered at solid/liquid interfaces. Further examining this change in order between solvents at silica/vapor and silica/liquid interfaces using equal length but different functional group containing solvents, we see that the energetics of adsorption and solvation are likely to be responsible for the degree of order both at the solid/vapor surface (adsorption) and solid/liquid interface (both adsorption and solvation).