Biomedical Engineering Reference
In-Depth Information
Fig. 1
Schematic phase diagram of CO
2
indicating the compound sublimation point (
N
), triple
point (
) and supercritical point (
•
), respectively, using experimental data from NIST Chemistry
Webbook
Tabl e 1
Typical ranges for some physical properties of gases, liquids and supercritical fluids [
49
]
Property
Gas
SCF
Liquid
Density (g cm
3
)
6
10
4
-2
10
3
0.2-0.9
0.6-1.6
Diffusion coefficient
(cm
2
/s)
0.1-0.4
2
10
4
-7
10
4
2
10
6
-2
10
5
Viscosity (cP)
0.01-0.03
0.01-0.09
0.2-3.0
Another important advantage of SCFs over conventional solvents includes their
density-dependent solvent power which controls its ability to dissolve solutes [
50
].
Moreover, as the density is related to pressure and temperature and as a SCF is
highly sensitive to changes in pressure, it is possible to control the solute-solvent
interactions in SCFs by varying the process conditions. This high sensitivity induces
an ability to tune the density and solvent strength of a SCF and consequently its
selectivity to dissolve various compounds.
Any substance can be used as a supercritical fluid however, particular attention
has been given to supercritical carbon dioxide (scCO
2
) since it is an environmentally
clean solvent with mild critical point value (Tc
D
31.1
ı
CandPc
D
73.8 bar) making
its operation safe and cheap. Besides its intensive use as an alternative solvent in
extraction of nutraceuticals from vegetal matrices [
51
], scCO
2
is being extensively
used in the field of medical polymers and bioblends, which are very sensitive to
residual solvents, benefiting from the use of a non-toxic, inert, and environmentally-
friendly processing media [
46
]. The advantageous mass transfer properties of
dense CO
2
allows it to impregnate amorphous polymers with various CO
2
-soluble
compounds, including monomers, catalysts, initiators, and various bio-actives that
