Handbook of Solvents - George Wypych - ChemTech - Ventech!

1454 Aydin K. Sunol and Sermin G. Sunol
Figure 21.1.18. Homogeneous supercritical solution
path to materials synthesis.
Figure 21.1.19. Anti-solvent path to materials synthesis.
entrainers or not, the particle size, shape, and particle size distribution changes. The primary
particle creation mechanism is nucleation and growth. Usually the amount solubilized in the
SCF is not very high for particle synthesis. For many applications such as pharmaceutical
and energetic materials, the particle base material is not soluble in dense gasses. In those instances,
these substances are solubilized in a liquid solvent and in a subsequent stage the
supercritical anti-solvent is used for synthesis of particles.
21.1.4.7 Encapsulation - generic application
Micro-encapsulation of drug-polymer systems using the RESS (Rapid Expansion of Supercritical
Fluid Solutions) techniques have been initiated with limited success due to poor understanding
of the complex phenomena involved in co-nucleation of components. Not only
do the particles have to be nucleated with the desired particle size and shape but also encapsulate
the material simultaneously in an uniform fashion.
An alternative approach 99 depicted in Figure 21.1.17 involves a sequential method
where synthesized particles are coated with polymeric thin films by simultaneous nucleation
of polymeric material out of a supercritical fluid, encapsulating the particles fluidized
in the supercritical fluid, followed by further polymerization and binding of the encapsulating
material on the particle surface. The method involves a recirculatory system that includes
dissolution of the polymer in the supercritical solvent and coating the particles
through a temperature swing operation in the fluidized bed that contains the particles. The
particulate material coated with the tailor polymeric material possesses unique timed-release
characteristics, improved stability, and often-enhanced behavior.
21.1.4.8 Spraying and coating - generic application
The RESS and SAS approaches can be applied to coating and spraying applications if the
inhomogenization step is exercised on a surface (a base material).
21.1.4.9 Extrusion - generic application
The addition of a supercritical fluid to a Newtonian or non-Newtonian fluid reduces the viscosity
of the fluid improving its processibility and end materials morphology. Supercritical
fluids can aid in food and polymer extrusion.
21.1.4.10 Perfusion (impregnation) - generic application
Excellent transport characteristics, solubilization power, and sensitivity to process variables
all contribute to the success of the methodology. The generic scheme as shown in Fig-