Supercritical antisolvent methods for particle formation: Mass transfer theory and protein precipitation experiments

This dissertation describes theoretical and experimental investigations of the fundamental phenomena governing precipitation by a compressed antisolvent. The supercritical antisolvent (SAS) and gas antisolvent (GAS) methods are novel alternatives to traditional precipitation techniques. However, their applicability is limited by an incomplete understanding of the precipitation process and the resulting effect on particle characteristics.; The main focus of this work is a numerical study of the thermodynamic and mass transfer aspects of SAS. Models developed to predict the isothermal mass transfer between a toluene (solvent) droplet and its compressed carbon dioxide (antisolvent) environment showed that droplet swelling occurs when the solvent is the denser component, while shrinking occurs at very high pressures for which the antisolvent is denser. Furthermore, mass transfer is faster and less sensitive to temperature and pressure when the solvent and antisolvent are miscible (supercritical conditions), as compared to conditions for which two phases exist (subcritical conditions). The results suggest that the solvent-antisolvent critical locus and equal-density locus can be useful guides for selecting SAS operating conditions.; A numerical study of mixing effects indicated that the common assumption of SAS as an isothermal process is often inaccurate at the droplet scale. Calculations for carbon dioxide and toluene, ethanol, or hexane showed that mixing ranges from very exothermic near the critical point of carbon dioxide to very endothermic near the solvent's critical point. Isothermal mass transfer models will be most accurate at conditions away from pure-component and mixture criticality, thereby reinforcing that supercritical operation is desirable for well-controlled precipitation.; The second part of this dissertation describes an experimental investigation of the relationship between a protein's structure and its ability to recover biological activity after GAS precipitation. Proteins with similar secondary structure content showed similar degrees of distortion after GAS, as measured by circular dichroism spectroscopy. However, the extent of distortion was not always correlated with activity loss, indicating that knowledge of secondary structure is insufficient for predicting activity. No one parameter was uniquely correlated with activity recovery; instead, the protein's molecular weight, secondary structure, disulfide bonds, prosthetic groups, and mechanism of biological activity may all influence recovery after GAS.