Master of Science / Department of Physics / Bruce M. Law / Interest in the properties of room-temperature ionic liquids is rapidly expanding. Although there have been numerous studies concerning their preparation, their use as a reaction medium and their physical properties, Ionic Liquids (ILs) are so new that many of their bulk physicochemical properties, optical properties, surface properties, toxicities etc. are unknown or only just beginning to be characterized. The highly polar nature of the ILs causes the surfaces of the liquids to become highly ordered in comparison with the surfaces of many other types of organic liquids. Surface structuring at the liquid-vapor interface of the imidazolium based ILs can be examined by using Brewster Angle Ellipsometry and Contact Angle Measurement. The preliminary observation of Ellipsometric measurement shows that there is an interfacial order-disorder transition at temperature Tc=385 K. This result is not analyzed yet but the initial thought behind this is an indication of a ferroelectric transition at the liquid-vapor interface of dipole moment of ILs. From the contact angle measurement it is shown that there is a remarkable change in the contact angle of the imidazolium based ILs over short interval of time ([similar to] ten minutes). Also study of the spreading of the ILs on hard surface shows that there is some definite structural dependence

Master of Science / Department of Physics / Christopher M. Sorensen / This work emphasizes advancements in Aerosol Gelation. We have attempted to expand the available materials used to synthesize Aerosol Gels by moving away from gas phase precursors toward liquid phase precursors and eventually reactants in the solid phase. The primary challenge was to efficiently administer the liquid fuels into the detonation chamber. After several attempts, it was concluded that the most efficient delivery technique was to heat the liquid fuel past the vapor point and evaporate it into the oxidizing gas for combustion. This method consistently yields soot with a density of 3.2 mg/cc approximately 10 minutes after the combustion. It was concluded that four criterion must be met to create an Aerosol Gel from a liquid: 1. The liquid must be as finely divided as possible 2. The energy of the spark must be large enough to cause a sustainable combustion 3. The fuel must have a Lower Explosive Limit above the necessary concentration to meet a volume fraction of 10[superscript]4 4. The fuel must have a relatively low boiling point

Doctor of Philosophy / Department of Physics / Christopher M. Sorensen / Nano-size materials are new materials in an intermediate state between the bulk and atomic or molecular states. Nanoparticles in colloidal solution and their assemblies have the great attention of researchers to investigate the novel fundamental properties and numerous applications. In this dissertation, we investigated the solubility phase transition behavior of gold nanoparticles in colloidal solution. We used the nearly monodisperse gold nanoparticles synthesized by either the inverse micelle or the solvated metal atom dispersion methods followed by digestive ripening. The gold nanoparticles were ligated with alkyl chains, which were dodecanethiol, decanethiol, or octanethiol for individual samples. They dispersed in toluene or t-butyl toluene like large molecules at room temperature. In analogy to molecular solutions, the colloidal solution had thermally reversible phase transitions between a dissolved phase of dispersed single nanoparticles and dispersed-aggregation co-existing phase. A more polar solvent, 2-butanone, was added to the colloidal solution for changing the solubility of gold nanoparticles and adjusting the phase transition temperatures to accessible temperatures. Superclusters formed by the nanoparticles when the colloidal solutions were quenched from a one-phase regime at high temperature to a two-phase regime at low temperature. Solubility phase diagrams were obtained for gold nanoparticles with different ligands in the mixtures of different ratios of 2-butanone and toluene or t-butyl toluene. The explanation from classical ideal solution theory gave the fusion enthalpy of superclusters. Temperature quenches from the one-phase to the two-phase regime yielded superclusters of the nanoparticle solid phase with sizes that depended on the quench depth. Classical nucleation theory was used to describe these sizes using a relative small value of the surface tension for the nanoparticle solid phase. This value is consistent with molecule size scaling of the surface tension. In total these results show that the solubility behavior of nanoparticles in colloidal solution is similar to the behavior in molecular solutions.

Master of Science / Department of Physics / Christopher M. Sorensen / We investigated the effect of shear on the structure and aggregation kinetics of unstable colloids using small angle light scattering. We used an aqueous suspension of 20 nm polystyrene latex microspheres and MgCl2 to induce aggregation. The sample was only sheared once for approximately 33 seconds at different times, typically 1 min., 5 min., or 15 min., after the onset of aggregation. The average shear rate was in the range of 0.13 - 3.56 s-1, which was in a laminar regime. The unsheared sample gelled after ca. 45 min. When the sample was sheared soon after the onset of aggregation, the aggregation followed DLCA kinetics to yield = 1.80 ± 0.04 aggregates unaffected by the shear. The gel time also remained the same as the unsheared gel. Shearing at later stages of aggregation shortened the gel time and enhanced the scattered light intensity significantly indicating rapid growth. Then, depending on the shear rate, there were three different behaviors. At high shear rates, the aggregate structure was inhomogeneous after the shear was stopped with a crossover in slope in the scattered light intensity vs. q, to imply hybrid superaggregates with two different fractal dimensions. At intermediate shear rates far from the gel point, there was a similar crossover after the shear was stopped; however, the fractal dimension regained 1.80 ± 0.04 at the gel point. At low shear rates, the aggregation rate was increased, but the aggregate structure was uniform, and the fractal dimension remained 1.75 ± 0.05.

Doctor of Philosophy / Department of Physics / Bruce M. Law / We have studied two critical binary liquid mixtures in the mixed phase regime with x-ray and neutron reflectometry to verify universal critical scaling at a non-critical interface. We compared our results with previous results obtained with ellipsometry. At a solid-liquid or liquid-vapor interface of an AB binary liquid mixture the component with the lower surface tension will dominate that interface. If the surface tension differential between the components of the mixture is large enough the composition of the surface layer will loose its dependence on. This case is referred to as strong adsorption. We study the case of strong adsorption for a binary liquid mixture at the critical composition with respect to the demixing phase transition. Sufficiently close to the critical temperature Tc the influence of bulk critical behavior is expected to dominate the way the surface adsorption layer decays with depth z from the surface composition to the bulk composition. The length scale of the decay profile is expected to be proportional to the composition correlation length, and is expected to scale with a universal composition scaling function. In a neutron reflectometry study of a critical mixture of D2O and 3-methylpyridine against a quartz substrate we verify universal critical scaling using a scaling function previously used to describe ellipsometry data. In an x-ray reflectometry study of the liquid-vapor interface of a critical mixture of n-dodecane and 1,1,2,2 tetrabromoethane, which had previously been studied with ellipsometry, we find that we are able to describe all data by using the same scaling function provided that we account for non-critical, system dependent surface structure as well. We are also able to simulate ellipsometry with our mathematical profile model and compare the simulation to the previous ellipsometry data.

Doctor of Philosophy / Department of Physics / Amitabha Chakrabarti / This work is the result of a wide range of computer simulation research into the aggregation behavior of dispersed colloidal and aerosol particles in a number of different environments from the continuum to the free-molecular. The goal of this research has been to provide a bridge between experimental and theoretical researchers in this field by simulating the aggregation process within a known model. To this end, a variety of interparticle interactions has been studied in the course of this research, focusing on the effect of these interactions on the aggregation mechanism and resulting aggregate structures. Both Monte Carlo and Brownian Dynamics codes have been used to achieve this goal. The morphologies of clusters that result from aggregation events in these systems have been thoroughly analyzed with a range of diverse techniques, and excellent agreement has been found with other researchers in this field. Morphologies of these clusters include fractal, gel, and crystalline forms, sometimes within the same structure at different length scales. This research has contributed to the fundamental understanding of aggregation rates and size distributions in many physical system, having allowed for the development of improved models of the aggregation and gelation process. Systems studied include DLCA and BLCA in two and three dimension, free-molecular diffusional (Epstein) system, selective aggregation in binary colloids, ssDNA mediated aggregation in colloidal systems, and several others.

Master of Science / Department of Physics / Bruce M. Law / With advances in micromechanical machining and nanotechnology, the sample volume needed for biological research and other analysis decreases. With small volume, sample-surface interactions including adsorption must be considered. These adsorption effects can be observed by analyzing light reflected from the solid-liquid interface, and the contact angle of a solution on the surface. Presented is the design and construction of an ellipsometer, a device used to analyze light reflected off of a solid-liquid interface to find interfacial properties, including thickness of a thin film formed by adsorption. The taq enzyme is shown to have a large change in contact angle from seventy degrees to about ten degrees over a short (ten minute) time period when placed on an SU-8 substrate, indicating a change in energy at the interface and a large amount of adsorption. Silane substrates are found to produce similar results. Ellipticity of a colloidal gold nanoparticle solution on a glass substrate is also observed, whose results are difficult to interpret due to bulk shifts in the sample. With the ellipsometer running correctly, it can be used for a number of experiments, including spectroscopic ellipsometry and Brewster angle microscopy, with some modifications.

Doctor of Philosophy / Department of Physics / Christopher M. Sorensen / We investigated the effect of shear on the structure and aggregation kinetics of unstable colloids using small angle light scattering. We used an aqueous suspension of 20 nm polystyrene latex microspheres and MgCl[subscript]2 to induce aggregation. The sample was only sheared once for approximately 33 sec at different times, typically 1 min, 5 min, or 15 min, after the onset of aggregation. The average shear rate was in the range of 0.13 - 3.56 sec[superscript]−1 , which was in a laminar regime. The unsheared sample gelled after ca. 45 min. When the sample was sheared soon after the onset of aggregation, the aggregation followed the diffusion limited cluster cluster aggregation (DLCA) kinetics to yield D[subscript f] = 1.80 [plus or minus] 0.04 aggregates unaffected by the shear. The gel time also remained the same as the unsheared gel. Shearing at later stages of aggregation shortened the gel time and enhanced the scattered light intensity significantly indicating rapid growth. Then, depending on the shear rate, there were three different behaviors. At high shear rates, the aggregate structure was inhomogeneous after the shear was stopped with a crossover in slope in the scattered light intensity versus q, to imply hybrid superaggregates with two different fractal dimensions. At intermediate shear rates far from the gel point, there was a similar crossover after the shear was stopped; however, the fractal dimension regained 1.80 [plus or minus] 0.04 at the gel point. At low shear rates, the aggregation rate was increased, but the aggregate structure was uniform, and the fractal dimension remained 1.75 [plus or minus] 0.05.

Doctor of Philosophy / Department of Physics / Christopher M. Sorensen / We demonstrated that an aerosol can gel. This gelation was then used for a one-step method to produce an ultralow density porous carbon or silica material. This material was named an aerosol gel because it was made via gelation of particles in the aerosol phase. The carbon and silica aerosol gels had high specific surface area (200 – 350 sq m/g for carbon and 300 – 500 sq m/g for silica) and an extremely low density (2.5 – 6.0 mg/cm[superscript3]), properties similar to conventional aerogels. Key aspects to form a gel from an aerosol are large volume fraction, ca. 10[superscript-4] or greater, and small primary particle size, 50 nm or smaller, so that the gel time is fast compared to other characteristic times. Next we report the results of a study of the cluster morphology and kinetics of a dense aggregating aerosol system using the small angle light scattering technique. The soot particles started as individual monomers, ca. 38 nm radius, grew to bigger clusters with time and finally stopped evolving after spanning a network across the whole system volume. This spanning is aerosol gelation. The gelled system showed a hybrid morphology with a lower fractal dimension at length scales of a micron or smaller and a higher fractal dimension at length scales greater than a micron. The study of the kinetics of the aggregating system showed that when the system gelled, the aggregation kernel homogeneity attained a value 0.4 or higher. The magnitude of the aggregation kernel showed an increase with increasing volume fraction. We also used image analysis technique to study the cluster morphology. From the digitized pictures of soot clusters the cluster morphology was determined by two different methods: structure factor and perimeter analysis. We find a hybrid, superaggregate morphology characterized by a fractal dimension of D[subscript f] nearly equal to 1.8 between the monomer size, ca. 50 nm, and 1 micron and D[subscript f] nearly equal to 2.6 at larger length scales up to [similar to] 10 micron. The superaggregate morphology is a consequence of late stage aggregation in a cluster dense regime near a gel point.

Master of Science / Department of Physics / Christopher M. Sorensen / Gold Nanoparticles (AuNP) 5nm in diameter, ligated with n-dodecanethiol, were dissolved in various hydrocarbon solvents including normal alkanes from n-hexane to n-hexadecane as well as two aromatics, toluene and para-xylene. These solutions were centrifuged at room temperature under 12000g acceleration for one hour to separate larger clusters from AuNP monomers dissolved in the supernatants. UV-Vis absorbance data were taken on the supernatants and were then converted to concentrations in moles of Au atoms/L. These concentrations correspond to the saturated concentration of dissolved AuNP monomers in equilibrium with a precipitate at room temperature. For the alkanes, we discovered a non-monotonic functionality of saturated concentration vs. solvent chain length with a maximum corresponding to n-dodecane. This agreed with predictions made of the ligands’ interactions with the solvents based on comparisons of solubility parameters where the n-dodecanethiol ligands were approximated as n-dodecane. The concentrations of AuNPs when dissolved in the aromatics did not follow the trend predicted by solubility parameters.