This Is AuburnElectronic Theses and Dissertations

Specific Interactions of Carbon Dioxide in Supercritical Fluid Processes and Nanoparticle Processing




Bell, Philip

Type of Degree



Chemical Engineering


Specific interactions of molecules with supercritical carbon dioxide (CO2) are of fundamental importance to the understanding of current processes and to the development of new processes. While relatively weak interactions are important for solubility and dissolution concerns, strong interactions play a major role in reaction chemistry. Chapter 1 provides background information on the solvent characteristics of CO2, FTIR and NMR spectroscopy methods to examine high-pressure solutions and specific solvent-solute interactions. It also gives motivation to further examine solute-solvent interactions in CO2 and to apply knowledge of these interactions to various processes. Chapter 2 discusses the utilization of Fourier transform infrared (FTIR) spectroscopy to study the effect of CO2 density on the dimerization of various carboxylic acids. The interaction of CO2 with the carboxylic acid functional group is shown to be relatively strong. These strong interactions can be used to manipulate reactions involving carboxylic acids. Chapter 3 discusses the use of high pressure nuclear magnetic resonance (NMR) spectroscopy to probe the nature of the interactions between CO2 and carboxylic acids. The change of the chemical shift of the acidic proton of a carboxylic acid with CO2 density is shown to be nonlinear, suggesting a specific interaction with CO2. Unfortunately, the acid peak is the composite of the monomer and the dimer peaks, so the changing equilibrium constant with CO2 density also results in a nonlinear change with CO2 density. These effects on the peak shift are therefore indistinguishable. Chapter 4 discusses the visualization of the supercritical fluid antisolvent (SAS) precipitation process. This process takes advantage of the interactions of CO2 with an injected solvent to cause mutual diffusion leading to the precipitation of an insoluble solute. This process can show drastically different behavior by simple adjustments of the processing parameters. Chapter 5 discusses new methods of polymerization in a reactive SAS process. The interaction of CO2 with amine groups is used to form nylon 6 during the spray process. The mass transport in this process is also developed. In a different process, the interaction of CO2 with carboxylic acids is used to dissolve methacrylic acid so that sonication induced polymerization can occur during the SAS process. The polymethacrylic acid formed in this process is precipitated because of its insolubility in CO2. Each of these polymerizations resulted in low yields. More experiments need to be performed to advance the development of these processes. Chapter 6 discusses the stable dispersion of silver nanoparticles in CO2 using a non-fluorinated ligand, isostearic acid. The ability of CO2 to solvate the tails of the short, stubby, methylated ligand allows for steric stabilization of the nanoparticles in CO2. Silver nanoparticles are stably dispersed in both a CO2/hexane mixture and in pure CO2. Particles are also synthesized and dispersed in CO2 in a single step without the use of any fluorinated compounds. This process again takes advantage of the ability of CO2 to solvate branched, methylated tails by using a branched, methylated silver precursor. These results are the first reports of stable steric dispersions of nanoparticles in CO2 without the use of any fluorinated compounds. Chapter 7 provides conclusions. The IR studies showed a relatively strong interaction between CO2 and carboxylic acids. High pressure NMR studies were used to probe the nature of this interaction. High resolution imaging of the SAS process showed that the spray characteristics could be significantly changed by adjusting the processing parameters. Two new polymerization techniques were explored in a reactive SAS process that takes advantage of specific interactions with CO2. Finally, the ability of CO2 to effectively solvate the ta