Browsing by Subject "Supercritical fluids"

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Chemical equilibria and nanocrystal synthesis in high temperature supercritical solutions
(2001) Ziegler, Kirk Jeremy; Johnston, Keith P., 1955-; Korgel, Brian Allan
In supercritical water (SCW), reaction equilibrium constants may differ by several orders of magnitude from room temperature values due to changes in the solvent dielectric constant. While these changes have been well characterized for ionic acid-base reactions, non-ionic polar reaction equilibria in SCW have received little attention. Here, we have shown for several oxidation states of NOx species that polarity differences from reactants to products result in significant changes in equilibrium constants that scale with density. In addition, linear extrapolation of these equilibrium constants to zero density results in values which are very close to independent gas phase values. Nitrate reaction equilibria provide the framework for the synthesis of copper nanocrystals in SCW. Aqueous copper nitrate (Cu(NO3)2) solutions when taken to supercritical conditions, hydrolyze to form large copper (II) oxide particles. Because of the low dielectric environment, SCW is a suitable solvent to employ organic capping ligands to control and stabilize the synthesis of nanocrystals. The presence of 1-hexanethiol results in reduction of copper (II) and produces copper nanocrystals approx. 7 nm in diameter. A proposed mechanism for sterically stabilized nanocrystal growth in SCW describes competing pathways of hydrolysis to large oxidized copper particles versus ligand exchange and arrested growth by thiols to produce small monodisperse Cu nanoparticles. A new synthetic method was developed to produce organic-monolayer passivated silicon and germanium nanocrystals in solvents under supercritical conditions. By thermally degrading organosilane or organogermane precursors at high temperatures and pressures, sterically-stabilized nanocrystals could be obtained using octanol as a capping ligand. During the reaction, octanol binds to the nanocrystal surface through an alkoxide linkage to provide steric stabilization through the hydrocarbon chain. The absorbance and luminescence spectra of the nanocrystals exhibit significant size-dependent blue shifts in optical properties from bulk luminescence due to quantum confinement effects. High luminescence quantum yields are observed. The smallest nanocrystals exhibit discrete optical transitions, characteristic of quantum confinement effects for crystalline nanocrystals with a narrow size distribution.
Controlled synthesis and characterization of silicon nanocrystals
(2004) Pell, Lindsay Erin; Korgel, Brian Allan
In response to the demand for shrinking feature sizes and faster electronics, many resources have been dedicated to the research of nanotechnology. At present, silicon is undoubtedly the building block and key to the microelectronics world. In its bulk form, silicon is an inefficient emitter in the infrared, but as its dimensions shrink to the nanoscale, silicon exhibits unique optical and electrical properties such as size tunable photoluminescence. The more successful methods of the synthesis of silicon nanocrystals include laser ablation and silane pyrolysis; however these methods offer little in the way of particle stabilization which would prevent oxidation and allow for manipulation through dispersion in organic solvents. A novel supercritical fluid synthesis is investigated with respect to various silicon precursors such as diphenylsilane, silicon tetrachloride and trisilane. The electrochemical and luminescent properties of silicon nanocrystals, synthesized via the thermal decomposition of diphenylsilane, were studied. Differential pulse voltammetry of silicon nanocrystals in DMF and acetonitrile exhibit quantized double layer charging as previously reported for Au and CdS nanocrystals. Additionally, electron transfer reactions between positively and negatively charged nanocrystals (or between charged nanocrystals and molecular redoxactive coreactants) occurred that led to electron and hole annihilation, producing visible light. The electrogenerated chemiluminescence spectra exhibited a peak red shifted from the photoluminescence maximum. Single nanocrystal photoluminescence was investigated via Argon laser excitation and confocal microscopy. The single nanocrystals demonstrate stochastic single-step “blinking” behavior and size-dependent PL spectra with line widths approximately only three times greater than those measured for CdSe nanocrystals at room temperature. Investigation of trisilane as a viable silicon precursor in a supercritical fluid synthesis led to the formation of well formed, sub-micron, amorphous silicon colloids in high yield. Manipulation of temperature, pressure and precursor concentration allowed for the synthesis of amorphous silicon particles 60-400 nm in diameter. Polydisperse samples exhibited two dimensional, longviii range orientational order in the absence of translational order which has been compared to the Reverse Brazil Nut Effect. Additionally, metal induced crystallization was observed in amorphous silicon particles annealed in a vacuum evaporator.
The effect of supercritical fluid on polymer systems
(2007) Wang, Xiaochu, 1979-; Sanchez, Isaac C., 1941-
Great interest has been directed toward the study of polymer thin films recently due to their emerging applications, and appreciable deviated properties and phenomena as compared with bulk polymers. Carbon dioxide (CO₂) has received attention as an environmentally benign alternative to hazardous industrial solvents. Unlike conventional liquid solvents, the density and hence the solvent strength of supercritical CO₂ can be tuned by small variations in pressure, temperature or both. The objective of this work is to study the interaction between high pressure CO₂ and polymer systems. We introduced the methodology used in this dissertation. The combination of gradient theory of inhomogeneous systems and Sanchez-Lacombe Equation of State is used to calculate the interfacial properties, such as interfacial density profile, interfacial tension and interfacial thickness. We first investigated the adsorption of supercritical fluid on polymer surfaces. We showed analytically that surface adsorption of high pressure fluid on an attractive surface is proportional to the compressibility of the fluid. We have also investigated numerically the sorption of supercritical CO₂ on poly(dimethylsiloxane) (PDMS) and polyisobutylene (PIB), and supercritical 1,1-difluorethane on PS. By calculating the Gibbs adsorption and adsorption layer thickness of the supercritical fluids, we found in all cases that maximum adsorption occurred when the supercritical fluid was near its compressibility maximum. We then examined the compatibilization effect of supercritical fluid on two incompatible polymers. We calculated the interfacial density profile, interfacial thickness and interfacial tension between the two polymers with and without the supercritical fluid. We found that the interfacial tension was decreased and the interfacial thickness was increased with high pressure super-critical fluid for the ternary systems we have investigated. No enhancement or deleterious effects on compatibilization were observed as the critical point was approached and the compressibility became large. We also examined the morphological structures of asymmetric poly(ethylene oxide)-b-poly(1,1'-dihydroperflurooctyl methacrylate) (PEO-b-PFOMA) thin films upon annealing in supercritical CO₂. The strong affinity between PFOMA and CO₂ was found to induce phase segregation when annealing PEO-b-PFOMA films as compared with vacuum at the same temperature.
Emulsions and microemulsions of water and carbon dioxide: novel surfactants and stabilization mechanisms
(2005) Ryoo, Won Sun; Johnston, Keith P., 1955-
During the last two decades colloid and interface science in the field of supercritical fluid technology has brought enormous potentials in the utilization of supercritical carbon dioxide as an environmentally benign solvent. Liquid or supercritical CO2 exhibits solvent properties that are tunable with pressure, and is essentially nontoxic and nonflammable. Emulsions and microemulsions of water and CO2, whether in the form of water-in-CO2 (w/c) or CO2-in-water (c/w), offer new possibilities for separations on the basis of polarity, and as media for reactions between polar and nonpolar molecules. For the first time, formation of thermodynamically stable c/w microemulsions was characterized by dynamic light scattering (DLS) technique. High-pressure carbon dioxide swells potassium carboxylate perfluoropolyether (PFPE-K) cylindrical micelles in water, elongating the micelles significantly from 20 up to 80 nm. As the micelles swell to form microemulsions, the solubility of pyrene increases by a factor of ca. 10. It was demonstrated w/c microemulsions may be formed with nonionic hydrocarbon surfactant. Methylated branched tail of the surfactant enhances formation of stable w/c microemulsions as it raises surfactant solubility in CO2, shifts the curvature towards bending about water, and weakens interdroplet interactions by reducing overlap between surfactant tails. As a novel medium for reactions, w/c microemulsions with low water content are utilized for the synthesis of TiO2 nanoparticles via the controlled hydrolysis of titanium tetraisopropoxide. The size of particles could be controlled by adjusting the water-to-surfactant ratio (wo). Based on DLS measurements, the size of TiO2 particles was comparable to that of the microemulsion droplets indicating steric stabilization was sufficient during the rapid hydrolysis. Finally, electrostatic repulsion between water droplets of w/c emulsion was explored as an alternative to the steric stabilization mechanism. Negative zeta-potentials as high as 70 mV are measured for emulsion droplets by microelectrophoresis. Unprecedented crystalline structure of the droplet array with a spacing of several droplet-diameters is identified by microscopy, and investigated in terms of a balance between long-range electrostatic repulsions acting through the low dielectric medium (εr = 1.5 for high pressure CO2) and the gravitational force which tends to decrease inter-droplet distances.
Nanocrystal stabilization, synthesis and assembly using supercritical fluids
(2003) Shah, Parag Suresh; Johnston, Keith P., 1955-; Korgel, Brian Allan
Supercritical and compressed solvents provide a unique medium for nanocrystal synthesis and assembly as their tunable solvation strength and favorable wetting characteristics have the potential to overcome current processing limitations. Here we examine nanocrystal dispersibility, separation, synthesis and organization with compressed solvents. Gold and silver nanocrystals were dispersed in carbon dioxide and ethane by using the appropriate capping ligands. Larger nanocrystals, which exhibit stronger core attractions, required better solvent conditions (higher densities) than smaller nanocrystals in order to form a dispersion. Lowering the solvent density precipitated the largest nanocrystals demonstrating density tunable colloidal separations in supercritical fluids. Silver, iridium and platinum nanocrystals were synthesized in supercritical CO2 by reducing a miscible organometallic precursor. By reducing the precursor in the presence of a thiol, particle growth was quenched and the nanocrystals could be collected, cleaned and redispersed in compatible solvents. Tuning solvent density and ligand type allowed the nanocrystal growth mechanism to be controlled from a mix of coagulation and condensation at conditions of strong steric stabilization, leading to small monodisperse particles, to coagulation at poor stabilization conditions, leading to large polydisperse particles. Superlattice formation was examined by assembling gold nanocrystals from liquid carbon dioxide. The resulting structures varied from disorganized liquids at fast evaporation rates to hexatic states with highly ordered regions at slower evaporation rates. Comparison with a computer simulated reference state showed that the crystallization kinetics were slower than diffusion limited, likely due to ensemble rearrangement during the late stages of assembly. Finally, gold and indium manganese arsenide nanocrystal dispersions were drop-cast from volatile solvents under humid conditions to form macroporous nanocrystal thin films. The porous structures were templated by condensed water droplets as a result of solvent evaporation. Prevention of droplet coalescence by interfacially active nanocrystals, which adsorbed onto the surface of the droplets, led to the formation of highly ordered pore structures.
Relationship between interfacial properties and formation of microemulsions and emulsions of water and supercritical carbon dioxide
(2001-08) Psathas, Petros; Johnson, Keith P.
The utilization of supercritical (SC) CO2 as an alternative green solvent has attracted significant research devotion in the last decade. Its uniqueness lies on the fact that CO2 is a non-FDA regulated solvent mainly generated as the sideproduct of industrial process, is easily recyclable, readily available, non flammable and essentially non toxic. Dense CO2 is non-polar (unlike water), has weak van der Waals forces (unlike oils) and as such may be considered a third type of fluid phase in nature, somewhat similar to fluorocarbons. The use of SC CO2 has expanded into broad technological areas one of which is the stabilization of water-in-CO2 dispersions that offer new possibilities for separations on the basis of polarity, and as media for reactions between polar and nonpolar molecules. The formation of stable emulsions of water-in-CO2 (W/C) so far has been hampered by the lack of suitable surfactants. The synthesis of various molecularly engineered surfactants is demonstrated in this study, among which are polydimethylsiloxane (PDMS)-based block copolymer ionomers, ionic and nonionic perfluoropolyether (PFPE) and nonionic perfluorooctylmethacrylate (PFOMA)-based ones. The concentrated W/C emulsions are characterized with electrical conductivity, optical microscopy and multiwavelength turbidity technique. The emulsion stability is assessed as a function of formulation variables that influence the surfactant monolayer curvature, such as temperature, pH, salinity and pressure. The response of the interfacial activity of the surfactant to changes in the variables above is monitored with interfacial tension (γ) measurements and is correlated to emulsion stability. Moreover, salinity is used to tune the surfactant aggregation characteristics, resulting in spontaneous microemulsion formation upon crossing the critical microemulsion concentration (cµc). Based on guidelines provided by γ versus temperature, stable concentrated (50:50 by mass) C/W miniemulsions consisting of 200 nm droplets are formed with the phase inversion temperature (PIT) method. Finally, the formation of unflocculated and stable dilute W/C emulsions is studied with a homologous series of PFOMA-based nonionic surfactants, and mapping of γ with surfactant hydrophilicity provide useful pathways for the synthesis of the optimum structure.
Supercritical fluid spray processes for microencapsulation and formation of submicron aqueous dispersions of pharmaceutical compounds
(2003-12) Young, Timothy John; Johnston, Keith P., 1955-
Precipitation with a Compressed Fluid Antisolvent (PCA) and Rapid Expansion from Supercritical Solution (RESS) are two processes based on supercritical fluids that are capable of producing submicron particles. Novel variations of these basic processes have been examined to produce stable particles of various pharmaceutical compounds. PCA is an antisolvent precipitation technique where an organic solution of drug + polymer in solvent is atomized (sprayed) into supercritical (SC) CO₂. Upon liquid mixing, the solute materials precipitate to form microparticles. A Vapor-over-Liquid technique has been used to produce larger, uniform particle sizes of biodegradable polymers. By suspending a protein in the solvent phase, the protein can be encapsulated/coated by the precipitating polymer. RESS is a process by which a homogeneous solution at supercritical conditions is sprayed through an expansion nozzle to atmospheric conditions. The resultant change in phase leads to the precipitation of the solute materials. The production of extremely small particles (<50 nm) have been predicted but rarely demonstrated. Typically, particle growth occurs to form larger (~1 μm) particles. A novel adaptation was developed, dubbed RESAS (Rapid Expansion from Supercritical to Aqueous Solution), wherein the expansion is conducted within an aqueous environment. The aqueous phase can contain surfactant or lipid stabilizers to capture and preserve submicron particles of water-insoluble drug actives in the form of a suspension.