Role of In-Situ Generated CO2 Bubbles in Heterogeneous Nucleation of Solid Solutes in the Precipitation by Pressure Reduction of Gas-Expanded Liquid (PPRGEL) Process

Abstract

Precipitation by pressure reduction of a gas-expanded liquid (PPRGEL) entails dissolution of subcritical CO2 in the solution of an organic solvent and a solid solute resulting in a gas-expanded liquid (GEL). Depressurization of a GEL leads to evolution of CO2 bubbles along with a large decrease in temperature facilitating solid precipitation. An illustrative system comprising CO2 (1), acetone (2), and cholesterol (3) is considered. It is proposed that CO2 bubbles generated from the depressurized GEL provide a substrate for heterogeneous nucleation of cholesterol. The size of nucleated CO2 bubbles and their rate of generation with time are predicted. The effective size of the CO2 bubbles ranges over 200–500 μm and their generation rate is ∼106–107 #/s. Comparing these with the corresponding solute nuclei size and nucleation rates, and considering the GEL–solid interfacial energetics, we conclude that the CO2 bubbles provide the required surface area for solid nucleation.