Basic Aspects of Supercritical Fluids

As shown in the schematic projection of the phase diagram of a pure substance (Fig. 1), the phase transition from liquid to gas ends at the critical point. The term supercritical fluid strictly characterizes a substance that is above both its critical temperature and its critical pressure, but is often used improperly to describe a fluid in the relative vicinity of its critical point, mainly a fluid at a pressure over its critical pressure. In the SC state, the substance is neither a gas nor a liquid but possesses properties of both, whereby the density is liquidlike, the viscosity is gas-like, and the diffusion coefficient lies between that of a gas and a liquid [8]. One distinguishing feature of SCFs is that, though almost liquidlike in density, they possess a very high compressibility. The density behavior of carbon dioxide is shown in Fig. 2. At the critical point, the fluid compressibility diverges, as seen from the vertical slope of the critical isotherm at the critical point, resulting in an infinite rate of change in density with respect to the isothermal pressure change. The solvent power of SCF is thus highly dependent on the variations of temperature or pressure. This behavior allows the use of pressure or temperature as very sensitive parameters for tuning the properties of the fluid, and in particular its solvent power. The ratio of the experimental solubility in an SCF to the solubility predicted by assuming ideal

FIG. 2 Isothermal density behavior of CO2 as a function of pressure.

gas behavior at the same pressure and temperature can be as high as 106 for nonvolatile solutes [15]. For polymer processing, this feature is exploited for the fractionation of polymers by programming the fluid density [16]. The properties of SCF, including the solubility and phase equilibria calculation methods, have been extensively reviewed [8,17,18].

Carbon dioxide (CO2) is the most widely used SC solvent because it possesses easily accessible critical parameters (Table 1) and is inexpensive, inert toward most compounds, and nontoxic. Its low critical temperature allows the processing of thermolabile compounds, such as pharmaceuticals. A limitation of CO2 in comparison with other SC solvents is its poor solvent power for highly polar, ionic, or high molecular weight compounds. Under readily achievable conditions, most of the high molecular weight polymers are not soluble in SC CO2. Only some fluoropolymers or silicones [8] show a good solubility in pure CO2. The addition of small amounts of a volatile cosolvent, such as acetone or ethanol, can dramatically increase the solvent power of an SCF [19]. Many therapeutic compounds or carriers are polar compounds and/or have high molecular weights, neither of these properties being conducive to high solubility in SC CO2.

One major advantage of using SCFs is that they can be efficiently separated, by decompression, from both organic solvents or solid products, facilitating sin-

TABLE 1 Critical Temperature (Tc) and Critical Pressure (Pc) of Commonly Used Supercritical Solvents

Fluid

Tc (°C)

Pc (bar)

Ethylene

9.3

50.4

Carbon dioxide

31.1

73.8

Nitrous oxide

31.3

73.8

Ethane

32.3

48.8

Chlorodifluoromethane

96.0

51.9

Propylene

91.8

46.2

Propane

96.7

42.5

Ethanol

240.8

61.4

Water

374.1

217.7

gle-step, clean, and recycling process engineering. In the case of manufacturing techniques using pure SC solvents, a solvent-free product is recovered. This unique feature is a key point for the processing of pharmaceuticals. When SCFs are used as antisolvent, the process conditions are set up so that the SCF extracts most of the organic solvent during the particle formation. Taking advantage of their high diffusivity and solvent power, SCFs are then used for further extraction of residual solvents from the final product. The efficiency of this solvent extraction, currently used in the manufacturing of particles by SCF antisolvent techniques, is illustrated in a recent coating technology [20]. After coating tablets with an organic solution of coating agent, an SCF is contacted with the coated tablets so as to strip the solvent.

Due to their unique features and to their ability to allow the reduction of organic solvents, SCFs have been intensively used for the development of polymer particle engineering over the past 20 years.

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