ABSTRACT
Compressed CO2 has unique solvent properties that may be used advantageously in various pharmaceutical processes (1). The high volatility of CO2 can be exploited to provide rapid, energy-efficient drying of pharmaceuticals. Carbon dioxide plasticizes many biodegradable and biocompatible polymers, facilitating mass transport inside the polymers during impregnation or microencapsulation processes. It is also advantageous to use compressed CO2 because it is nontoxic and nonflammable. Since it is nonpolar and has weak van der Waals forces, both polar and nonpolar nonvolatile molecules are often insoluble. Because of this solubility limitation, only a small number of reaction, materials formation, and/or separation processes may be carried out in a homogeneous CO2-based phase. To overcome this limitation, many of the key advancements in supercritical fluid science and technology have utilized heterogeneous systems with CO2 at one or more interfaces (2, 3). The conjugate phase(s) may be either aqueous or organic solids or liquids. This
TABLE 1 Types of Interface for the Design of New Dispersions and Applications in CO2-Based Systems
Interface Process or type of dispersion Notes
Water-CO2Microemulsions 2-10nm in diameter Reactions, enzymatic catalysis, separations, materials synthesis
Water-CO2W/C and C/W macroemulsions Reactions, enzymatic catalysis, separations, materials synthesis
Organic-CO2Rapid expansion from supercritical solution (RESS)
Nucleation and growth of particles
Organic-CO2RESS with a nonsolvent (RESS-N) Microencapsulation in polymers (e.g., proteins)
Organic-CO2Precipitation with a compressed fluid Particle formation and
chapter is organized according to the type of interface, as shown in the summary in Table 1. A variety of single interfaces (e.g., water-CO2) and double interfaces (e.g., organicwater-CO2) are addressed. In some cases, the interface changes as a colloid is transferred from one phase to another: for example, a polymer latex may be transferred from CO2 to water. The phases include solid and liquid organic phases, aqueous phases, and inorganic solids and CO2 in the gas, liquid, or supercritical fluid states, including cryogenic temperatures down to the sublimation temperature of dry ice. This chapter shows how an emerging understanding of the behavior of these interfaces may serve as a framework for the discovery and development of supercritical fluid technology.
