Thesis (Ph. D.) in Chemical Engineering--University of Maine, 2001. / Includes vita. Advisory Committee: Edward V. Thompson, Prof. of Chemical Engineering, Advisor; Francois G. Amar, Prof. of Chemistry; Douglas W. Bousfield, Prof. of Chemical Engineering; Hemant P. Pendse, Prof. of Chemical Engineering; William N. Unertl, Prof. of Physics. Includes bibliographical references (leaves 121-126).

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Thesis (Ph. D.)--University of Wisconsin--Madison, 1923. / Cover title. Reprinted from the Journal of physical chemistry, v. 28, May, 1924. eContent provider-neutral record in process. Description based on print version record.

For 40% of currently discovered drugs which are poorly water soluble, engineering amorphous nanoparticles with rapid dissolution and enhanced solubility can improve their absorption. Antisolvent precipitation by mixing organic drug solutions with aqueous solutions produced sub-300 nm amorphous nanoparticle dispersions. Polymeric stabilizers increased the nucleation rate by lowering the interfacial tension and adsorbed to particle surfaces to inhibit growth by condensation and coagulation. An increase in the stabilizer concentration decreased the average particle size until reaching a threshold where the particles were < 300 nm for the poorly water soluble drug, itraconazole. The amorphous itraconazole nanoparticle dispersions dissolved at pH 1.2 to produce high supersaturation levels up to 90-times the equilibrium solubility. The supersaturation increased with particle curvature, as described qualitatively by the Kelvin equation. A thermodynamic analysis indicated the stabilizer maintained amorphous ITZ in the solid phase with a fugacity 90-times the crystalline value, while it did not influence the activity coefficient of ITZ in the aqueous phase. Recovery of the amorphous nanoparticles from water was achieved by adding salt to desolvate the polymeric stabilizers and flocculate the particles, which could then be rapidly filtered. The flocculation under constant particle volume fraction produced open flocs which were redispersible in water to their original ~300 nm size, after filtration and drying. Amorphous particles were preserved, as flocs were formed below the drug's glass transition temperature. After flocculation/filtration, medium surface area (2-5 m²/g) particles dissolved rapidly in pH 6.8 buffer with 0.17% surfactant to an unusually large supersaturation up to 17, comparable to that for high surface area (13-36 m²/g) particles. However, the decay in supersaturation was much slower for the medium surface area particles, as the smaller excess surface area of undissolved particles produced slower nucleation and growth from solution. In contrast, the maximum supersaturation was far lower for more conventional low surface area solid dispersions of drug in polymers, because of crystallization of undissolved solid during slow dissolution. The ability to design the particle morphology to manipulate the level in supersaturation in pH 6.8 media, offers new opportunities in raising bioavailability in gastrointestinal delivery. / text

Nucleation is the first step in the formation of a new phase in a thermodynamic system. The Classical Nucleation Theory (CNT) is the traditional theory used to describe this phenomenon. The object of this thesis is to investigate nucleation beyond one of the most significant limitations of the CNT: the assumption that the surface tension of a nucleating cluster of the new phase is independent of the cluster’s size and has the same value that it would have in the bulk of the new phase. In order to accomplish this, we consider a microscopic, two-dimensional Lattice Gas Automata (LGA) model of precipitate nucleation in a supersaturated system, with model input parameters Ess (solid particle-to-solid particle bonding energy), Esw (solid particle-to-water particle bonding energy), η (next-to-nearest neighbour bonding coeffiicent in solid phase), and Cin (initial solute concentration). The LGA method was chosen for its advantages of easy implementation, low memory requirements, and fast computation speed. Analytical results for the system’s concentration and the crystal radius as functions of time are derived and the former is fit to the simulation data in order to determine the system’s equilibrium concentration. A mean first-passage time (MFPT) technique is used to obtain the nucleation rate and critical nucleus size from the simulation data. The nucleation rate and supersaturation are evaluated using a modification to the CNT that incorporates a two-dimensional, radius-dependent surface tension term. The Tolman parameter, δ, which controls the radius-dependence of the surface tension, decreases (increases) as a function of the magnitude of Ess (Esw), at fixed values of η and Esw (Ess). On the other hand, δ increases as η increases while Ess and Esw are held constant. The constant surface tension term of the CNT, Σ0, increases (decreases) with increasing magnitudes of Ess (Esw) fixed values of Esw (Ess), and increases as η is increased. Together, these results indicate an increase in the radius-dependent surface tension, Σ, with respect to increasing magnitude of Ess relative to the magnitude of Esw. Σ0 increases linearly as a function of the change in energy during an attachment or detachment reaction, |ΔE|, however with a slope less than that predicted for a crystal that is uniformly packed at maximum density.

High-energy Si implantation into silicon creates a net defect distribution that is characterized by an excess of interstitials near the projected range and a simultaneous excess of vacancies closer to the surface. This defect distribution is due to the spatial separation between the distributions of interstitials and vacancies created by the forward momentum transferred from the implanted ion to the lattice atom. This dissertation investigates the evolution of the near-surface vacancy excess in MeV Si-implanted silicon both during implantation and post-implant annealing. Although previous investigations have identified a vacancy excess in MeV-implanted silicon, the investigations presented in this dissertation are unique in that they are designed to correlate the free-vacancy supersaturation with the vacancies in clusters. Free-vacancy (and interstitial) supersaturations were measured with Sb (B) dopant diffusion markers. Vacancies in clusters were profiled by Au labeling; a new technique based on the observation that Au atoms trap in the presence of open-volume defects. The experiments described in this dissertation are also unique in that they were designed to isolate the deep interstitial excess from interacting with the much shallower vacancy excess during post-implant thermal processing.

Nucleation is the first step in the formation of a new phase in a thermodynamic system. The Classical Nucleation Theory (CNT) is the traditional theory used to describe this phenomenon. The object of this thesis is to investigate nucleation beyond one of the most significant limitations of the CNT: the assumption that the surface tension of a nucleating cluster of the new phase is independent of the cluster’s size and has the same value that it would have in the bulk of the new phase. In order to accomplish this, we consider a microscopic, two-dimensional Lattice Gas Automata (LGA) model of precipitate nucleation in a supersaturated system, with model input parameters Ess (solid particle-to-solid particle bonding energy), Esw (solid particle-to-water particle bonding energy), η (next-to-nearest neighbour bonding coeffiicent in solid phase), and Cin (initial solute concentration). The LGA method was chosen for its advantages of easy implementation, low memory requirements, and fast computation speed. Analytical results for the system’s concentration and the crystal radius as functions of time are derived and the former is fit to the simulation data in order to determine the system’s equilibrium concentration. A mean first-passage time (MFPT) technique is used to obtain the nucleation rate and critical nucleus size from the simulation data. The nucleation rate and supersaturation are evaluated using a modification to the CNT that incorporates a two-dimensional, radius-dependent surface tension term. The Tolman parameter, δ, which controls the radius-dependence of the surface tension, decreases (increases) as a function of the magnitude of Ess (Esw), at fixed values of η and Esw (Ess). On the other hand, δ increases as η increases while Ess and Esw are held constant. The constant surface tension term of the CNT, Σ0, increases (decreases) with increasing magnitudes of Ess (Esw) fixed values of Esw (Ess), and increases as η is increased. Together, these results indicate an increase in the radius-dependent surface tension, Σ, with respect to increasing magnitude of Ess relative to the magnitude of Esw. Σ0 increases linearly as a function of the change in energy during an attachment or detachment reaction, |ΔE|, however with a slope less than that predicted for a crystal that is uniformly packed at maximum density.

Gas inFusion? is a novel remedial technology that dissolves CO₂ into water under pressure for NAPL recovery. As the supersaturated liquid flows through the porous medium gas evolution occurs in situ as the system returns to thermodynamic equilibrium. The evolution of gas bubbles leads to NAPL recovery by two mechanisms: 1) volatilization and 2) mobilization by the NAPL spreading in a film around the rising bubbles. Laboratory experiments by Li demonstrated that injecting the supersaturated water into a porous medium minimized the buoyancy driven flow of gas and the fingering phenomena that limit typical gas sparging. The distribution of carbon dioxide at partial pressures (p_CO2) above the applicable hydrostatic pressure and the evolved gas phase were determined in two field experiments conducted in the relatively homogeneous fine to medium sand at CFB Borden. First, CO₂-supersaturated water was injected into a single point located approximately 4 metres below ground surface. Then this injection was repeated with pumping of two nearby wells to see if the lateral distribution of CO₂ gas could be controlled hydraulically. Groundwater monitoring of p_CO2 above the hydrostatic pressure and geophysical surveys (neutron measurements, surface ground penetrating radar (GPR), and cross-borehole GPR) to find zones of induced gas content were supported by hydraulic monitoring and physical observations of gas bubble distribution at the water table. <br /><br /> Based on the results of these tests, enhanced CO₂ levels above the hydrostatic pressure were observed up to 5. 5-7. 0 m from the injection point and the gas phase up to ~5. 3 m. It was not possible to determine the impact hydraulic control had on the lateral distribution of CO₂ due to problems with the experiment. The distribution of the gas phase was heterogeneous with CO₂ gas pockets forming below low permeability layers, as evidenced by surface GPR, permeameter tests, and grain size analyses. These gas pockets accumulated until sufficient pressure built up to overcome the displacement pressure of these lower permeability layers. At this point there is evidence of CO₂ breakthrough in the cross-borehole GPR data and physical observations of gas bubbles at the water table. These observations are consistent with previous investigations, which indicate that although the Borden aquifer is homogeneous, distinct horizontal layering is present with sufficient variations in permeability/displacement pressure to trap and cause some lateral spreading of a gas phase. The evidence of channeling and the impact of heterogeneities on gas distribution are consistent with air sparging studies.