This dissertation describes results of extensive Fast Thermal Desorption Spectroscopy (FTDS) studies of vaporization kinetics, molecular transport, reactions, and phase transitions in polycrystalline ice near its melting point. Incentives for fundamental studies of ice, the latest achievements, and historical trends in the field of physical chemistry of ice, and the summary of important unresolved problems are given in Chapter 1. Chapter 2 provides in depth descriptions of the experimental apparatus and procedures, and reports results of several critical experimental tests designed to demonstrate the basic capabilities of the approach. Chapter 3, 4, and 5 report the essential findings of the research.
In Chapter Three, FTDS studies of vaporization kinetics of thin polycrystalline ice near it melting point are reported. FTDS was used to investigate the vaporization kinetics of thin (50-100 nm) H2O18 and HDO tracer layers from 2-5 μm thick polycrystalline H2O16 ice films at temperatures ranging from -15 to -2 degrees C. The isothermal desorption spectra of tracer species demonstrate two distinct peaks, alpha and beta, which we attribute to the vaporization of H2O 18 initially trapped at or near the grain boundaries and in the crystallites of the polycrystalline ice, respectively. The data show that the diffusive transport of the H2O18 and HDO tracer molecules in the bulk of the H2O16 film is slow as compared to the film vaporization. Thus, the two peaks in the isothermal spectra are due to unequal vaporization rates of H2O18 from grain boundary grooves and from the crystallites and, therefore, can be used to determine independently the vaporization rate of the single crystal part of the film and rate of thermal etching of the film. Theoretical analysis of the tracer vaporization kinetics demonstrates that the vaporization coefficient of single crystal ice is significantly greater than those predicted by the classical vaporization mechanism at temperatures near ice melting point. Surface morphological dynamics and the bulk transport phenomena in single crystal and polycrystalline ice near 0 °C are also discussed.
FTDS studies of H/D isotopic exchange reaction in polycrystalline ice near its melting point are reported in Chapter Four. FTDS was used to investigate the kinetics of H/D isotopic exchange in 3 μm thick polycrystalline H 2O ice films containing D2O layers at thicknesses ranging from 10 to 300 nm at a temperature of -2.0 ± 1.5 degrees C. According to experimental results over the duration of a typical fast thermal desorption experiment (3-4 ms), the isotopic exchange is confined to a 50 ± 10 nm wide reaction zone located at the boundary between polycrystalline H 2O and D2O ice. Combining these data with a theoretical analysis of the diffusion in polycrystalline medium, the range of possible values for water self-diffusion coefficients and the grain boundary widths characteristic of our ice samples is suggested. The analysis shows that for the grain boundary width on the order of a few nanometers, the diffusivity of D2O along the grain boundaries must be at least two orders of magnitude lower than that in bulk water at the same temperature. Based on these results, it was suggested that, in the limit of low concentrations of impurities, polycrystalline ice does not undergo grain boundary premelting at temperatures up to -2 degrees C.
In Chapter Five, the results of a fast thermal desorption spectroscopy study of the H/D isotopic exchange kinetics in a few micrometer thick, pure polycrystalline ice film and in ice films doped with HCl are presented. The isotopic exchange reaction was used as a probe of molecular transport in ice to determine the effective water-self diffusion coefficients, D eff, in pure and doped polycrystalline ice at temperatures ranging from -20 to -1 °C. In the case of pure polycrystalline ice, D eff demonstrates an Arrhenius dependence on temperature with an effective activation energy of 69 ± 3 kJ·mol-1 and a pre-exponential of 109±0.5 m2·ms-1 up to -2 °C. According to theoretical analysis, the self-diffusion coefficient of water at the grain boundaries also shows an Arrhenius dependence on temperature with an activation energy of 69 ± 3 kJ·mol-1 and pre-exponential of 1011±1 m2·ms -1. However, upon the addition of 0.04% of HCl into the polycrystalline ice results in a marked deviation of Deff from Arrhenius law at -8 °C, which is caused by premelting and takes place at intersections of grain boundaries. Structure and transport properties of condensed aqueous phases along grain boundaries of polycrystalline ice at various temperatures are also discussed.
|Commitee:||Cahill, Christopher, Lewis, John, Ramaker, David, Wagner, Michael|
|School:||The George Washington University|
|School Location:||United States -- District of Columbia|
|Source:||DAI-B 69/07, Dissertation Abstracts International|
|Keywords:||Melting point, Molecular transport, Polycrystalline ice, Premelting, Vaporization|
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