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 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).Item OPTICAL KERR EFFECT SPECTROSCOPY OF SIMPLE LIQUIDS(2009) Zhong, Qin; Fourkas, John T; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Optical Kerr effect (OKE) spectroscopy has become a prominent nonlinear optical technique for studying liquids, which allows for the direct, time-resolved probing of collective orientational diffusion as well as Raman-active intermolecular and intramolecular modes. The temperature-dependent orientational dynamics of 1, n-dicyano n-alkane liquids ranging from dicyanomethane to 1,8-dicyanooctane has been investigated by ultrafast OKE spectroscopy. The dependence of the reorientational times on temperature and viscosity is consistent with the molecules adopting a largely extended structure in the liquid state, with a preference for gauche conformations at the methylenes bonded to the cyanide groups. The data are also suggestive of temperature-dependent, collective structural rearrangements in these liquids. Ultrafast OKE spectroscopy has also been used to study the intermolecular dynamics of aromatic liquids. A model that links the differences in the OKE spectra to corresponding differences in the local ordering of the liquids has been proposed previously based on the temperature-dependent OKE study of five aromatic liquids. The spectra of some other aromatic liquids such as pyridine, pyridine-d5, 2,4,6-trifluoropyridine and 1,3,5-tris(trifluoromethyl) benzene has been obtained to test this model, and the relative importance of molecular shape and electrostatic forces in determining the form of the OKE reduced spectral density for such liquids has been realized. It has recently been shown that liquid tetrahydrofuran (THF) has an unusual structure that features voids of significant dimension. Such voids should affect other observable properties of this liquid. Temperature-dependent, optical Kerr effect spectra for THF and a number of related liquids (furan, cyclopentane, tetrahydropyran, cyclohexane, diethyl ether, hexamethylphosphoramide and n-pentane) has been obtained to test whether the shape of the spectra can be used to reveal the presence of sizeable voids in liquids. Liquid under tension is another interesting system to study: A method based on Berthelot tube technique has been developed to hold benzene and acetonitrile under tension successfully. A new scheme for measuring different tensor elements of the OKE response is developed. A dual-ring, polarization dependent Sagnac interferometer is used to create two co-propagating probe pulses that arrive at the sample at different times but that reach the detector simultaneously and collinearly. The tensor element of the response that is measured is determined by the polarization of the pump pulse. By controlling the relative timing of the probe pulses it is also possible to perform optical subtraction of two different tensor elements of the response at two different times, a strategy that can be used to enhance or suppress particular contributions to the OKE response.