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Supercritical fluid cleaning - McHardy J.

McHardy J., Sawan P.S. Supercritical fluid cleaning - Noyes publications, 1998. - 304 p.
Download (direct link): spercrificalfluidcleaning1998.pdf
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A universal property of surfactant solutions is the existence of a critical micelle concentration (CMC) representing the minimum amount of surfactant required to form aggregates. The CMC also represents the solubility of the surfactant unimer in the oil or continuous phase solvent. At surfactant concentrations above the CMC,
surfactant unimers exist in equilibrium with the surfactant making up the micellar aggregates. The CMC in reverse micelle systems formed with ionic surfactants is typically 10-4 to 10"3 molar. For a reverse micelle system in a supercritical fluid, the CMC is dependent upon the type or density of the fluid, the amount of water, and the temperature and pressure of the system. In order to obtain enhanced solvent powers of a microemulsion for a cleaning operation, one must operate well above the CMC of the particular surfactant. Also the capacity of the microemulsion to solvate polar or ionic species is naturally dependent upon the concentration of reverse micelles in the solution.
Finally, in the discussion of reverse microemulsion systems, mention should be made of one of the most widely studied systems. The surfactant, sodium bis(2-ethylhexyl) sulfosuccinate or Aerosol-OT (AOT), is one of the most thoroughly studied reverse micelle-forming surfactants since it readily forms reverse micelle and microemulsion phases in a multitude of different solvents without the addition of cosurfactants or other solvent modifiers. The phase behavior of AOT in liquid alkane/water systems is already well documented. Indeed, the first report of the existence of the formation of microemulsions in a supercritical fluid involved an AOT/alkane/ water system.^ The spherical structure of an AOT/nonpolar-fluid/ water microemulsion droplet is shown in Fig. 1. In the now well-known structure, it can be seen that the two hydrocarbon tails of each AOT molecule point outward into the nonpolar phase (e.g., supercritical fluid). These tails are lipophilic and are solvated by the nonpolar continuous phase solvent whereas the hydrophilic head groups are always positioned in the aqueous core.
3.0 MICROEMULSIONS IN SUPERCRITICAL FLUIDS
Most of the early work involving microemulsions in supercritical fluids utilized the supercritical alkanes, ethane and propane, with the surfactant AOT. Table 1 gives a summary of the surfactant systems that have been studied in supercritical hydrocarbon solvents. More recently, there has been some success with the formation of
microemulsions in supercritical C02, and these systems are summarized in Table 2.
Table 1. Supercritical Fluid Surfactants and Solution Conditions
Gas Surfactant Temperature Pressure Reference
(C) (bar)
Methane AOT 25 1650 (27)
(Tc=-82C) (Pc=46 bar)
Ethane AOT 37 250-850 (23)
(Tc=32C) (Pc=49 bar)
u 250 (1)
220-350 (35)
250-350 (2)
Up to 1500 (31)
37-100 (57)
37 380-500 (58)
37-50 100-300 (34)
C12EO, 40 Up to 400 (59)
Ci2E02 (59)
Dodecanol u (59)
c12eo0 90-450 (28)
C12E03 u (28)
C]2E05 (28)
C12EOg ct a (28)
Propane AOT 103 100-300 (23)
(Tc=97C) (Pc=43 bar)
u 103-110 100-350 (1)
tt 110 200 (60)
up to 150 10-200 (22)
103 Up to 350 (2)
a 100 70-600 (61)
DDAB it 120-500 (29)
44 350 (25)
Propylene Lecithin 35 200-500 (62)
(Tc=92C) (Pc=46 bar)
Table 2. Supercritical Carbon Dioxide Surfactants and Solution Conditions
Surfactant Temperature Pressure W Reference
(C) (bar) [HjOl/Isurf.]
Perfluorooctanoic acid 50 97 (47)
Polyoxyethylene 4-lauiyl ether 388 --- (47)
Polyoxyethylene 3-
(Cn.]4-iso-C13 rich) alkyl ether 257 --- (47)
Polyoxyethylene 2-cetyl ether 304-432 --- (47)
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