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 From Structure to Thermodynamics with Local Molecular Field Theory(2013) Remsing, Richard Charles; Weeks, John D; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A fundamental goal of statistical mechanics is to connect a description of the intermolecular interactions and the accompanying microscopic structural details of a molecular system to its macroscopic thermodynamic properties. When the interactions between molecular components are treated with sufficient simplicity, as in an ideal gas or a hard sphere fluid for example, the link between structure and thermodynamics can be apparent. In contrast, when both local and non-local interactions are present in the system, competition between the various short and long ranged forces can lead to surprising thermodynamic behaviors as exemplified by the complexities of liquid water. Local molecular field (LMF) theory provides a physically motivated formalism for systematically decomposing the structure and thermodynamics of molecular systems into portions arising from local and non-local interactions. In this thesis, LMF theory is employed to examine the structure and thermodynamics of molecular systems, with a focus on aqueous solutions. LMF-motivated truncations of classical water models are first developed as analysis tools to explore the roles of the local hydrogen bond network, dispersion interactions, and long ranged multipolar interactions in the determination of sev- eral anomalous thermodynamic properties of bulk water. This type of analysis is then extended to the study the relative importance of hydrogen bonding and inter- facial unbalancing potentials in hydrophobic effects. The underlying ideas of LMF theory are then utilized to study local and non-local interactions in ion solvation. Modifications to classical dielectric continuum theories are explored with a focus on determining the electrostatic potentials inside ionic cores. LMF ideas are then used to develop the concept of a Gaussian test charge. We then argue that this type of test charge is the appropriate generalization of a classical point test charge to probe the dielectric response of molecularly detailed systems and develop an accurate for- malism for the description of the dielectric response to such probes. Finally, a LMF theoretic foundation for performing free energy calculations is developed and tested before concluding the thesis with a discussion of future work involving LMF theory.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).