Compressed fluid solvents

Synthesis of Structured Polymeric Materials Using Compressed Fluid Solvents... 

Supercritical carbon dioxide (scCOa) has attracted much interest recently as an alternative solvent for materials synthesis and processing (i-i). We discuss here four areas where the use of compressed fluid solvents may offer unique advantages ... 

Gas AntisolventRecrystallizations. A limitation to the RESS process can be the low solubihty in the supercritical fluid. This is especially evident in polymer—supercritical fluid systems. In a novel process, sometimes termed gas antisolvent (GAS), a compressed fluid such as CO2 can be rapidly added to a solution of a crystalline soHd dissolved in an organic solvent (114). Carbon dioxide and most organic solvents exhibit full miscibility, whereas in this case the soHd solutes had limited solubihty in CO2. Thus, CO2 acts as an antisolvent to precipitate soHd crystals. Using C02 s adjustable solvent strength, the particle size and size distribution of final crystals may be finely controlled. Examples of GAS studies include the formation of monodisperse particles (<1 fiva) of a difficult-to-comminute explosive (114) recrystallization of -carotene and acetaminophen (86) salt nucleation and growth in supercritical water (115) and a study of the molecular thermodynamics of the GAS crystallization process (21). 

The use of SCFs as solvents influences the reacting system because it is possible to dramatically change the density of the fluid with small perturbations of temperature and pressure and, in such a way, greatly affect the density-dependent bulk properties such as the dielectric constant, solubility and diffu-sibility of these compressible fluids. 

PCA [Precipitation with a compressed anti-solvent] A process for making a solid with unusual morphology by spraying a solution of it into a supercritical fluid. The process resembles spray drying into a supercritical fluid. Used for making microspheres, microporous fibers, and hollow microporous fibers. 

Marshall s extensive review (16) concentrates mainly on conductance and solubility studies of simple (non-transition metal) electrolytes and the application of extended Debye-Huckel equations in describing the ionic strength dependence of equilibrium constants. The conductance studies covered conditions to 4 kbar and 800 C while the solubility studies were mostly at SVP up to 350 C. In the latter studies above 300°C deviations from Debye-Huckel behaviour were found. This is not surprising since the Debye-Huckel theory treats the solvent as incompressible and, as seen in Fig. 3, water rapidly becomes more compressible above 300 C. Until a theory which accounts for electrostriction in a compressible fluid becomes available, extrapolation to infinite dilution at temperatures much above 300 C must be considered untrustworthy. Since water becomes infinitely compressible at the critical point, the standard entropy of an ion becomes infinitely negative, so that the concept of a standard ionic free energy becomes meaningless. 

Micronization with supercritical fluids - Crystallization - Rapid expansion - Gas anti-solvent Recrystallization - Precipitation with compressed anti-solvent - Solution-enhanced dispersion - Particles from gas-saturated solutions 80 - 300 fine particles and powders from various products and of designed properties... 

The interest in mass transfer in high-pressure systems is related to the extraction of a valuable solute with a compressed gas. This is either a volatile liquid or solid deposited within a porous matrix. The compressed fluid is usually a high-pressure gas, often a supercritical fluid, that is, a gas above its critical state. In this condition the gas density approaches a liquid—like value, so the solubility of the solute in the fluid can be substantially enhanced over its value at low pressure. The retention mechanism of the solute in the solid matrix is only physical (that is, unbound, as with the free moisture), or strongly bound to the solid by some kind of link (as with the so-called bound moisture). Crushed vegetable seeds, for example, have a fraction of free, unbound oil that is readily extracted by the gas, while the rest of the oil is strongly bound to cell walls and structures. This bound solute requires a larger effort to be transferred to the solvent phase. 

Gas anti-solvent processes (GASR, gas anti-solvent recrystallization GASP, gas antisolvent precipitation SAS, supercritical anti-solvent fractionation PCA, precipitation with a compressed fluid anti-solvent SEDS, solution-enhanced dispersion of solids) differ in the way the contact between solution and anti-solvent is achieved. This may be by spraying the solution in a supercritical gas, spraying the gas into the liquid solution. 

Supercritical fluid solvents can act in a variety of ways to affect reaction rates. Since the reaction rate is the product of the rate constant and the concentrations of the reactants, one must consider the solvent effect on the rate constant itself (discussed below), as well as changes in concentrations. It is this second possibility that has not been addressed until this study i.e., the possible influence of changes in the local concentrations of the reactants in the compressible region near the critical point... 

Local solvent compression. The next application of the solvato-chromic data will be to determine the magnitude of the local compression of a supercritical fluid solvent in the immediate environment of the solute. The of a dye such as phenol blue can be predicted in liquids where no specific interactions are present by treating the solvent as a homogeneous polarizable dielectric (22,29). The intrinsic "solvent strength", E, °, describes dispersion, Induction, and dipole-dipole forces and is given by (22). 

Because the thermal diffusivity of SC water is comparable to that of many high quality insulation materials, gross radial temperature gradients can easily exist in a flow reactor. As shown in Figure 2, radial temperature gradients within the annular flow reactor are negligible. A computer program, which accurately accounts for the effects of the various fluid (solvent, solvent and solute, air) compressibilities on flow measurements, calculates mass and elemental balances for each experiment. A typical experiment evidences mass and elemental balances of 1.00+0.05. 

Gas antisolvent processes can be performed in a semicontinuous mode. In this case the solution and the antisolvent are continuously introduced in the system until the desired amount of the product is formed. The introduction of the solution is then stopped and the DG flux extracts the residual solvent from the system. The system is then depressurized to enable collection of the product. The solution is generally introduced through an atomization nozzle that favors the prompt expansion of the solution and the formation of small particles. Different process configurations have been utilized, i.e., co- and countercurrent introduction of the solution and antisolvent fluxes and various nozzles have been designed. The process is referred to by different acronyms such as ASES (aerosol solvent extraction system), SAS (supercritical antisolvent), SEDS (solution enhanced dispersion by supercritical fluids), PCA (precipitation with a compressed fluid antisolvent), GASR (gas antisolvent recrystallization), GASP (gas antisolvent precipitation). 

PRECIPITATION WITH A COMPRESSED FLUID ANTISOLVENT (PCA), A SUPERCRITICAL ANTISOLVENT (SAS), AND THE AEROSOL SOLVENT EXTRACTION SYSTEM (ASES) PROCESSES... 

Dozens of drugs have been recrystallized by means of the precipitation with a compressed fluid antisolvent (PCA) and the supercritical antisolvent (SAS) and aerosol solvent extraction system (ASES) processes. 

Nielsen KA. Methods and apparatus for spraying solvent-borne compositions with reduced solvent emission using compressed fluids and separating solvent. US patent, 5 290 604, 1994. 

Interestingly, the activity of C. thermocellum in SCF ethane, SCF CO2, liquid propane, and gaseous propane does not follow the log P trends seen with liquid solvents. Thus, log P may be of limited value when applied to the correlation of metabolic activity in compressed solvents. The traditional definition of log P (25 °C and 0.1 MPa) was extended to our incubation conditions (60 °C and 7 MPa) using the group contribution associating equation of state (GCA-EOS) (48,49) to calculate the mole fraction of the dissolved compressed fluids in octanol and water. Log P logio(x,- /x,- )] correlated well with the total metabolic activity (ratio of total products formed in the treatment to the total products formed in the control) in liquid hydrocarbon solvents (Fig. 2) (37). 

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