Parametric analysis of homogeneous and heterogeneous nucleation in subcritical CO2-mediated antisolvent crystallization

• Subcritical CO2-antisolvent crystallization of cholesterol from acetone is modeled.
• Homogeneous and heterogeneous nucleation is considered.
• Process conditions are selected for micro-particles and their sizes computed.
• Parametric effects predict decrease in particle size with antisolvent dissolution rate.
• Decrease in temperature and substrate-nuclei contact angle decreases particle size.

The present work investigates the tunability of antisolvency effect of subcritical carbon dioxide (CO2), in a solution of cholesterol in acetone, which facilitates high and rapid supersaturation needed for producing cholesterol micro-particles. A thermodynamic analysis is proposed for selection of operating conditions that result in high solid solute supersaturation. This is further coupled to a computational analysis of homogeneous and heterogeneous nucleation, and crystal growth kinetics. A numerical strategy has been evolved and for verifying its consistency the predicted particle size has been compared with that obtained experimentally. In addition, the effects of pressure (60–70 bar), temperature (291–303 K), initial solute concentration (90–100%), specific dissolution rate of CO2 (0.095–6.0 min−1), and nuclei-substrate contact angle (30–50°) on average particle size have been ascertained. In the case of homogeneous nucleation, the particle size increases with temperature, while pressure has a negligible effect. Further, the particle size decreases when the antisolvent dissolution rate and initial solute loading in solvent are increased. For heterogeneous nucleation, an enhancement in contact angle increases the particle size. These trends are in agreement with the experimental observations reported in the literature. The computational method thus elucidates a generalized approach for engineering desired particle size in subcritical CO2-mediated antisolvent crystallization.

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