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Title:Dynamics of molecular liquids under conditions of high pressure and geometric confinement
Author(s):Wallen, Scott L.
Doctoral Committee Chair(s):Jonas, Jiri
Department / Program:Chemistry
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Chemistry, Analytical
Chemistry, Physical
Abstract:Raman spectroscopy is utilized to further the understanding of the molecular dynamics of liquids. The fluids are studied under conditions of high pressure and confined to spatial geometries whose dimensions approach the molecular diameters of the fluids. The dynamic processes studied include intermolecular and intramolecular energy transfer; vibrational and reorientational relaxation mechanisms; and the influence of attractive and repulsive molecular interactions.
Typically when one changes the temperature of a system the kinetic energy of the system is altered, however, equally important changes in the molecular packing or volume occur. High pressure techniques can be utilized to separate effects on the fluid molecular dynamics due to changes in the packing and those due to changes in the kinetic energy. This is accomplished by examining trends of the molecular parameters as temperature is varied at constant density and as density is varied under isothermal conditions. The liquid systems examined in this manner are furan, propylene carbonate, chloroethylene carbonate and dichloroethylene carbonate. In the first system, a previously reported nonmonotonic temperature dependence of the bandwidth is shown to be due to density changes. Also, the results of frequency and bandwidth changes as a function of density and temperature are interpreted in terms of available theoretical models. In the carbonate systems the noncoincidence effect, the peak frequency and the isotropic bandwidths are investigated as a function of density and temperature.
Another area investigated is the fluid molecular dynamics when the fluid's motion is restricted to spaces approaching its own molecular diameter. This is accomplished by studying the fluid imbibed in porous, sol-gel prepared xerogels with narrow pore size distributions. The local order of the confined systems are very different from those in the bulk fluid as are various relaxation mechanisms that the molecules undergo. The observed differences have a significant impact on the understanding of chromatographic and membrane separation processes and the mechanisms of reaction in confined media. The effect of geometric confinement on the symmetric stretching modes of methyl iodide is investigated with particular emphasis placed on the resonant intermolecular vibrational coupling of the $\nu\sb2$ vibrational mode. The results indicate that the liquid has greater orientational order and slower resonant energy transfer when confined to the pores of the xerogel hosts. The noncoincidence effect of confined acetone, N,N-dimethylformamide and carbon disulfide is investigated and it is determined that confinement does not alter the noncoincidence value but specific interactions between the confined liquid and the silanol groups on the xerogel surface cause a decrease in the noncoincidence value. The effect of confinement on the liquid molecular dynamics of carbon disulfide are examined. The reorientational correlation times are observed to increase significantly upon confinement and the results are interpreted in terms of a model which predicts a steric hindrance of molecular motions at and near the surface. The vibrational dephasing times are shown to decrease as a function of decreasing pore size while the intramolecular Fermi resonance coupling strength decreases with decreasing pore size as does the anharmonic force constant. Lastly, the gauche/anti population ratio of 1,2-dichloroethane is investigated and the results show that specific surface interactions between the liquid and the xerogel's silanol groups cause a change in the population ratio, however, geometric confinement has no effect within the experimental error. (Abstract shortened by UMI.)
Issue Date:1994
Rights Information:Copyright 1994 Wallen, Scott L.
Date Available in IDEALS:2011-05-07
Identifier in Online Catalog:AAI9503343
OCLC Identifier:(UMI)AAI9503343

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