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Nanoparticles are regarded as exciting new building blocks for functional materials due to their fascinating physical properties because of the nano-confinement. Organizing nanoparticles into ordered hierarchical structures are highly desired for constructing novel optical and electrical artificial materials that are different from their isolated state or thermodynamics random ensembles. My research integrates the surface chemistry of nanoparticles, interfacial assembly and lithography techniques to construct nanoparticle based functional structures. We designed and synthesized tailor-made ligands for gold, semiconductor and magnetic nanoparticle, to modulate the assembly process and collective properties of the assembled structures, by controlling the key parameters such as particle-interface interaction, dielectric environments and inter-particle coupling etc. Top-down technologies such as micro contact printing, photolithography and nanoimprint lithography are used to guide the assembly into arbitrarily predesigned structures for potential device applications.

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<p>  </p> <p>In the practical applications of colloidal dispersions and suspensions, such as inks, paints, and food industry, the suspended particles must be stabilized, and remain well-dispersed for long times. Particles which are more dense than the suspending media may sediment rapidly, even with no agglomeration occurring, under many conditions of size and density difference. Then, a dispersant would be necessary for stabilization of particle suspensions against both agglomeration and sedimentation, while the suspensions should remain flowable in many applications. Moreover, when many aqueous suspension media may contain salts, the dispersant also needs to be an effective stabilizer against sedimentation under the specific salinity conditions of that application.</p> <p>DDAB (didodecyldimethylammonium bromide) , a cationic double-chain surfactant, forms lamellar liquid crystal phases when dispersed in water. It also easily forms aqueous vesicle dispersions (unilamellar closed particles with an internal solvent compartment) and liposomes (multilamellar vesicles, MLVs, or lamellar liquid crystallites) at relatively low DDAB weight fractions, <em>w</em>_D. To better understand the phase/dispersion behavior of DDAB and the corresponding optical properties, new analytical solutions of the spherical particles have been obtained for the light scattering theory in the Rayleigh (R) and the Rayleigh-Debye-Gans (RDG) regimes, for single and independent scattering. Moreover, the specific Rayleigh ratio <em>R</em>_q** and the specific turbidity <em>t</em>** were derived analytically for both scattering regimes. Spectroturbidimetry (ST) data at 25 °C for DDAB were compared to the <em>t</em>** predictions. <em>t</em>** data for DDAB vesicles are consistent with the RDG predictions, which are also used to estimate the vesicle sizes.</p> <p>For a better understanding of the effect of the preparation method and salinity on the formation of DDAB vesicles, spectroturbidimetry was used to measure the average radius of the unilamellar DDAB vesicles, which were prepared via two different methods in water and in NaBr salt solutions. The radius was ~24 nm after sonication (SS method) and ~74 nm after extrusion/ultrafiltration (SE method). The radii were larger when the vesicles were produced in 10 mM NaBr, ~65 nm for the SS method and ~280 nm for the SE method. The <em>t</em>** values of these vesicular dispersions increased with decreasing <em>w</em>_D values, until a constant value was reached at <em>w</em>_D*, which depends on the preparation method and the dispersion medium. The constant values of <em>t</em>** are indicative of single and independent scattering, and were used to estimate vesicle radii by solving the <em>t</em>** equations derived for the RDG regime. Estimates of the average distances between the vesicles and their corresponding Debye lengths were obtained to evaluate the importance of inter-vesicle electrostatic interactions, which could lead to dependent scattering at higher weight fractions.</p> <p>DDAB prepared with magnetic stirring of multilamellar liposomes, followed by ultrasonication to generate unilamellar vesicles, were found to have very high viscosities at very low shear stresses at DDAB weight fractions <em>w</em>D from 0.025 to 0.027. The vesicles had average diameters ranging from 68 to 80 nm, as previously determined from spectroturbidimetry. These vesicle dispersions stabilized suspensions of monodisperse spherical amorphous silica particles with diameters of <em>d</em>_sed = 454 nm, 691 nm, and 826 nm against sedimentation, at least for several weeks. Similar results were obtained for suspensions, in DDAB vesicle dispersions, of polydisperse, nonspherical, crystalline titania particles with sizes ranging from ca. 96 nm to 156 nm. At the relatively low values of <em>w</em>_D = 0.009 and 0.018, the effective viscosities,<em> h</em>eff, of the DDAB dispersions, determined from the sedimentation velocities, ranged from 1.35 to 1.87 cP and from 4.34 to 5.57 cP, respectively. At <em>w</em>_D = 0.027 for the silica particles with <em>d</em>_sed = 454 nm, or at <em>w</em>_D = 0.025 for all other particles considered, <em>h</em>_eff was essentially infinite, and each vesicle dispersion behaved as a gel at low shear stresses. At higher shear stresses, however, the dispersions were highly shear-thinning, and flowable in a capillary tube under gravity. This behavior is critical for the practical applications of such dispersions for paints and inkjet printing. To further understand the feasibility of the vesicle stabilization mechanism at various NaBr concentrations, <em>w</em>NaBr, the salinity effects on the stabilization of silica particles against sedimentation were also examined. It was found that at <em>w</em>_NaBr < 0.0020 and at <em>w</em>D > 0.060, the DDAB dispersion could stabilize silica particles against sedimentation for at least two weeks. The relationship of the phase and dispersion behavior of DDAB/aqueous NaBr solutions to their stabilizing effectiveness will be further studied.</p> <p>A first discovery of iridescent liquid-like aqueous vesicle dispersions formed from the DDAB is also reported. Although iridescence arises from some solid crystallites, thin films, and colloidal crystals, it had never been observed in systems that are liquid-like. Visual observations and ST at wavelengths of 350 nm to 700 nm were used to determine vesicle sizes and microstructure formation in dispersions for DDAB weight fractions <em>w</em>D between 0.020 to 0.030. The DDAB vesicle dispersions exhibited iridescent colors for <em>w</em>D = 0.023 to 0.027, due to the formation of “soft” crystallites formed by self-assembled vesicles. Effective vesicle radii from 30 to 60 nm were inferred from the ST measurements. The volume fractions of the vesicles <em>f</em>v and their effective volume fractions <em>f</em>v*, which account for the electrostatic double layers around a vesicle, were also estimated. The high values of <em>f</em>v* for the iridescent dispersions indicate that they contain neighboring vesicles with highly overlapping electrostatic double layers, even though their values of <em>f</em>v remain relatively low. Hence, strong electrostatic repulsive interactions arise between the vesicles. These interactions probably drive the formation of the “soft” crystallites, and thus the observed iridescence. Nevertheless, these “soft” crystallites, which could be easily broken up but were quick to reform, remain suspended. Consequently, these vesicle dispersions still flowed as a bulk dispersion with a high viscosity; the dispersion as a whole remained liquid-like or as a “liquid gem”, in contrast to what occurs to the other colloidal crystals made of rigid colloids. Beside their beautiful appearances, these DDAB vesicle dispersions also act as effective stabilizers of dense silica suspensions against sedimentation even at relatively low values of <em>w</em>D. </p>

The focus of the thesis research was to investigate the surface reactivity of three different minerals, pyrite (FeS2), an ordered form of ferrihydrite (an iron oxyhydroxide phase), and birnessite (MnO2), toward environmentally relevant aqueous reactants. In particular, research was carried out with the goals of 1) understanding the redox chemistry of nitrite (NO2-) and nitrate (NO3-) on pyrite and 2) understanding the redox (photo) chemistry of arsenite (AsO2-, As(III)) on ordered ferrihydrite and birnessite. A motivation for all these studies stemmed in part from the recognition that NO2-, NO3-, and As(III) are all environmental pollutants when they are present at sufficiently high concentration in the environment. The removal of these species or conversion of each of them on mineral surfaces to more benign chemical species is of importance in the realm of environmental chemistry. In the case of NO2- and NO3- on pyrite, an additional and primary motivation for the research was that it has been hypothesized in the "origin-of-life" community that the reaction of NO2- and NO3- with iron sulfide (e.g., pyrite) may have played a role in the production of ammonia (NH3) on early Earth. Such prebiotic chemistry had been hypothesized to an essential step in the production of biomolecules that included proteins. With regard to the NO2- reaction with pyrite, results detailed in this thesis showed that ammonia in µmol/kg quantities could be produced by reacting NO2- in the presence of pyrite under anaerobic conditions. The concentration of NH3 (detected as ammonium, NH4+, in solution) was a strong function of the reaction temperature. At the lower temperatures studied (22oC and 70oC), a small amount of NH4+ was formed, but µmol.kg-1 amounts of NH4+ were formed at a reaction temperature of 120oC. Only about 5% of the initial NO2- concentration was converted to NH4+. In the NO3-/pyrite system, the NO3- reactant concentration remained unchanged at all the three reaction temperatures studied, consistent with the low amounts of NH4+ formed in these experiments. Finally, it was shown using in situ infrared spectroscopy that surface-bound NO formed on pyrite during the conversion of the nitrogen oxides to ammonia. Overall, it was shown that the kinetics of NH4+ formation was slower for NO3- than that observed for NO2-. Studies presented in this thesis that focused on the surface reactivity of As(III) on ordered ferrihydrite and birnessite nano particles showed that As(III) could be oxidized to arsenate (referred to as As(V)) in the presence of simulated solar radiation. In the ordered ferrihydrite circumstance the adsorption of As(III) and photo-induced oxidation to As(V) was compared to the same reaction on the more disordered and smaller ferrihydrite particles (known as "2-line" ferrihydrite). A comparison of the adsorption rate of As(III) on the two surfaces in the presence of light after normalizing for differences in surface area showed that the ordered ferrihydrite exhibited a higher arsenic adsorption rate. Also, the oxidation rate of As(III) to As(V) in the presence of light on the ordered ferrihydrite showed a strong dependence on the amount of dissolved oxygen in solution while the oxidation rate on the more disordered form showed no such dependence. It was proposed that differences in the rates of the heterogeneous oxidation rate of ferrous iron with dissolved oxygen on the two surfaces were the reason for this behavior. Finally, the photo-induced oxidation of As(III) to As(V) on Na- and K-birnessite at solution pHs of 5.0 and 7.4 was investigated. It was shown that the oxidation rate of As(III) to As(V) occurred at a faster rate on birnessite in the presence of light when compared to the same system in the dark. Mn(II) formed during the reductive dissolution of birnessite during the oxidation of As(III) was experimentally observed at pH 5.0, but not at pH 7.4. Experiments were also conducted that investigated the reductive dissolution of Na- and K-birnessite (having different sizes and average oxidation states) by As(III) under more alkaline conditions. These experiments were conducted at pH 8.5 and the post-reaction samples were analyzed with X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). It was shown under these alkaline conditions using X-ray diffraction that structural changes occurred in/on both the Na- and K-birnessite during the As(III) oxidation reaction. / Chemistry

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This paper presents an approach to modeling a binary mixture near a surf ace. The approach used is based on statistical mechanics. The Cluster Variation (CV) method is used to solve an Ising-like model with a cluster size of two (neighboring lattice points). The free energy of the system is expressed in terms of the probabilities of occurrences of particular clusters. The CV method is first developed for a homogeneous system which leads to a set of three non-linear partial differential equations. The surface boundary is then introduced and the CV method is developed for this system which leads to a set of seven non-linear partial differential equations for each layer. In the calculations, the Natural Iteration (NI) method is introduced as a method of solving these non-linear partial differential equations. The steps involved in implementing the NI method are outlined. The number of layers is chosen to be 32. Concentration profiles are calculated for the binary system using the NI method and the results are discussed.

Call number: LD2668 .T4 1985 S25 / Master of Science