The purpose of this study is to better understand the use of carbon dioxide (CO₂) supercritical fluid based separation or fabrication techniques to produce high-quality, high-value added materials, particularly from a solubility perspective. Supercritical fluid is environmentally friendly as compared with organic solvents and possesses properties such as liquid-like densities and high diffusivities that are favorable to various processing operations. In such an operation, the raw materials are processed to yield special properties and improved product attributes to benefit the eventual application. However, due to the different roles that a supercritical fluid takes in different processes, a thorough understanding of the physicochemical phenomena to utilize it properly in each process i...[ Read more ]

The purpose of this study is to better understand the use of carbon dioxide (CO₂) supercritical fluid based separation or fabrication techniques to produce high-quality, high-value added materials, particularly from a solubility perspective. Supercritical fluid is environmentally friendly as compared with organic solvents and possesses properties such as liquid-like densities and high diffusivities that are favorable to various processing operations. In such an operation, the raw materials are processed to yield special properties and improved product attributes to benefit the eventual application. However, due to the different roles that a supercritical fluid takes in different processes, a thorough understanding of the physicochemical phenomena to utilize it properly in each process is essential to obtain a successful product. Indeed, the successful development and design of each technique requires detailed phase behavioral information of the components involved and understanding of the process mechanisms. In this research, we highlight the potentials of supercritical fluid technologies for the production of high-value added materials by developing methods involving chromatography, crystallization, extraction and reaction. Four different processes were investigated to address the issues involved in developing the separation and fabrication processes that can lead to the output of targeted quality products.

The first project demonstrates the course of developing a separation process by supercritical fluid chromatography (SFC) that can simultaneously separate high purity single enantiomers of (-)Tröger’s Base 99.19% and (+)Tröger’s Base 98.82% from a racemic mixture. Supercritical CO₂ is used as the mobile phase and slight changes in the fluid dynamics would alter separation quality. Solubility parameter calculations and the ternary phase diagram of the system, assist in developing a base case scenario that can be used as a starting point for the separation. An iterative parameter optimization approach then follows to guide decision making to achieve separation performance target. This procedure relies on two causal tables, a constructed compilation of own experiments and those obtained from the literature, that summarizes the qualitative relationship between chromatographic characteristics and separation performance and those between chromatographic characteristics and operating parameters.

The second process focuses on obtaining nanoparticles with narrow particle size distributions for the development of a more effective pharmaceutical formulation. The particle formation process by precipitation through compressed anti-solvent (PCA) technique was carried out on salicylic acid as the model drug, ethanol and acetone as the cosolvent, and supercritical CO₂ as the anti-solvent. Solubility determination by experiments, and visualization of phase diagrams facilitated the identification of the regions for operation and to effectively precipitate small and uniform particles. It was discovered that particles in the nano-range could be produced in the higher supersaturation region without changing the nozzle size of the PCA system. Uniformed particles as small as 63.35 nm were obtained and the dissolution results revealed a substantially better performance of the PCA processed nanoparticles compared to that of the unprocessed.

The third is a polymer encapsulation process by supercritical fluid extraction of emulsion (SFEE) for the development of a drug polymer composite particle. The practicality of this process was illustrated with ibuprofen as the drug to be encapsulated by biodegradable polymer, polylactic-glycolic acid while supercritical CO₂ acted as the solvent to remove the oil phase from the emulsion. A standard emulsion was formulated and used throughout the study to investigate how the conditions for the supercritical CO₂ extraction affected the final encapsulated product. The operating conditions of the second step in the SFEE process were shown to have direct relation with the drug release characteristics, namely drug content, encapsulation efficiency and drug release profile. It was found that the drug content of the composite particle was affected by the solubility of the drug compound in CO₂. The rate of release was affected by the surface area and the T_g of that composite polymer particle. Round particles of 100 – 300 nm were generally obtained for this study.

In the fourth project, we designed and built an apparatus for the synthesis of metallic nanostructures by the templating method using supercritical CO₂ as the reaction medium in chemical fluid deposition (CFD) process. Ordered platinum nanowire bundles were molded out from mesoporous silica template, SBA-15, with a pore channel diameter of 7 nm. Solubility study of the platinum precursor, platinum (II) dimethylcyclooctadiene, in supercritical CO₂ was carried out to obtain a phase diagram in order to optimize the operating parameters required for the metal deposition process. High purity and uniform nanowires with high aspect ratios (1:200) were obtained at moderate temperature and pressure conditions.