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Supercritical CO2-Based Production of Fullerene Nanoparticles Pratibhash Chattopadhyay and Ram B. Gupta* Department of Chemical Engineering, Auburn University, Auburn, Alabama 36839-5127
Fullerene nanoparticles have potential uses in a variety of applications including pharmaceuticals, lubricants, composite materials, specialized coatings, and interfacing membrane surfaces. In this study, the supercritical antisolvent process is used to reduce fullerene particle size from 40 µm to as low as 29 nm. C60 dissolved in toluene is injected into supercritical CO2, causing precipitation of C60 as fine particles. Because of the high diffusivity of CO2 in toluene, a rapid supersaturation is achieved, which results in the formation of C60 nanoparticles with a narrow size distribution. The effect of pressure, temperature, and jet velocity on particle size and morphology is studied. The particle size increases linearly with the density of supercritical CO2. A high jet velocity yields spherical particles whereas a lower jet velocity yields both spherical and rodlike particles. In most cases, a uniform thin film of the particles is obtained on the collection plate. Introduction The discovery of a new stable form of carbon called fullerene1 generated a great deal of enthusiasm in the scientific community and sparked off research efforts in its separation, isolation, and characterization. Studies were also conducted to determine the different physical and chemical properties of fullerene. A breakthrough came in 1990 when Kratschmer et al.2 discovered the electric arc method for the manufacture of large amounts of fullerene. These research efforts resulted in the opening up of avenues for potential application of fullerene in different areas. Fullerene and its derivatives can be employed in the field of superconductivity,3 optical devices,4 batteries,5 catalysts,5 and solid lubricants6 and in organic and pharmaceutical industries. Other emerging applications include the use of fullerene for nucleation of diamond films on surfaces,7 as optical limiters,8 for incorporation into photoconducting polymers,4 and also the use of its derivatives as drugs to combat AIDS.9 For many of these applications, we need processing routes that convert purified fullerene into nanoparticle form. Nanoparticles are of considerable importance in modern science and engineering. Materials having structures with grain size in the nanometer range exhibit properties that are quite different from those of the same material with larger grain sizes.10 Given this marked difference in physical, chemical, and material properties, new areas of application in various fields of science and engineering can emerge after their synthesis. Several techniques have been developed in the past for the production of fullerene nanoparticles. Gurav et al.11 proposed the spray-drying technique using fullerene/ toluene solution in nitrogen at a processing temperature of 200 °C. The fullerene particles obtained by this process were polycrystalline with a small crystallite size of 10 nm. Gurav et al.12 also introduced the generation * To whom correspondence should be addressed. Phone: (334) 844-2013. Fax: (334) 844-2063. E-mail: gupta@ auburn.edu.
of nanometer-size fullerene particles by the vapor condensation method in which fullerene was vaporized in a continuous flow reactor at temperatures of 400500 °C using nitrogen as the carrier gas. The particles obtained were spherical in shape with an average size of 40 nm and had a narrow size distribution. Joutsensaari et al.5 further investigated the production of nanophase fullerene particles via spray-drying/pyrolysis and vapor condensation routes, using gas-to-particle and droplet-to-particle conversions, respectively. Chowdhury et al.13 produced 2-30 nm fullerenic nanostructures in the flames of benzene, acetylene, or ethylene premixed with oxygen and an inert diluent gas. Lozovik and Popov14 studied the mechanism for the formation of nanoparticles of fullerene in arc discharges. Unfortunately, all these techniques involve the use of high temperatures, which make them unsuitable for the formation of composites of thermally sensitive compounds (e.g., drug molecules) with fullerene. Particle Technology Based on Supercritical Fluid Antisolvent. The use of supercritical fluids for particle formation was first proposed by Krukonis15 in 1984 for processing a wide variety of difficult-to-handle solids. Since then, several experimental studies have been conducted to develop methods of particle formation using this technology.33 One such method is by using supercritical CO2 as the gas antisolvent. In this technique the solid of interest is first dissolved in a suitable solvent and then this solution is rapidly introduced into supercritical CO2 through a narrow capillary tube. Supercritical CO2 completely extracts the solvent, causing CO2-insoluble solid to precipitate as fine particles. For many years now, this technique has been successfully used to produce microparticles of various compounds including difficult-to-handle explosives,16 lysozyme,17 trypsin,17 insulin,18 polystyrene,19 HYAFF-11 polymers,20 and numerous other organic substances.21 Supercritical fluids have higher diffusivities than liquids, higher densities than gases, and their solvent power can be controlled by adjusting the density with a small change in pressure or temperature. For example, Dixon et al.21 used the supercritical CO2 antisolvent process to make polystyrene particles ranging
10.1021/ie990729k CCC: $19.00 © 2000 American Chemical Society Published on Web 05/05/2000
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Figure 1. Schematic diagram of the capillary spray apparatus for the formation of fullerene particles using supercritical carbon dioxide as the antisolvent.
from 0.1 to 20 mm by spraying polymer/toluene solutions into CO2 of varying densities. A major advantage of the supercritical fluid precipitation process is that they can generate particles having a narrow size distribution, unlike other conventional processes that provide a wide size distribution.22 Also, the particles formed by the supercritical fluid precipitation process are free of organic solvents20 and the formation of powdered blends, thin films, and microencapsulation of materials is straightforward. The aim of the present work is to produce fullerene nanoparticles using the supercritical fluid antisolvent process. We will examine the effect of pressure, temperature, and jet velocity on the morphology and the size of fullerene particles. Because this process is carried out at near ambient temperatures, it is suitable for thermally sensitive materials such as pharmaceuticals. We will also examine the suitability of this technique in forming thin films on surfaces. Experimental Section Materials. All the materials, CO2 and N2 (99.5% pure, Airco), Buckminsters-fullerene (C60, 99.5% pure, Southern Chemical Group, Tucker, GA), toluene (99% pure, Fischer Scientific), and sodium dodecylbenzene sulfonate (Aldrich Chemical Co.) were used as received. Apparatus and Procedure. A schematic representation of the apparatus is shown in Figure 1. It consists of a cylindrical precipitation chamber (R) with an internal volume of 40 cm3. A collection plate is placed inside the precipitation chamber for collection of the particles. High pressure inside the chamber is generated using a HIP hand pump (C). Valves V1 and V2 are used to fill up the HIP hand pump with fresh CO2. The fullerene solution is injected inside the precipitation chamber using a “solution injection device” (S) which is connected to the vessel by means of a 75-µm i.d. fused quartz capillary tube. This device consists of a cylindrical reservoir separated into two chambers by a piston. Fullerene solution is filled inside one of these chambers and is pressurized by high-pressure nitrogen in the other chamber, to force the solution through the nozzle.
Supercritical CO2 is fed inside the precipitation chamber through the inlet port located at the bottom of the vessel. Valve V3 is used to control the flow of supercritical CO2 into the high-pressure chamber. A circulating water bath is used to maintain the temperature inside the precipitation chamber. Before CO2 enters the precipitation chamber, it is preheated by a 2-ft long stainless steel tube in the water bath. Pressure inside the chamber is measured using a pressure gauge, P1. The outlet port is located on top of the precipitation chamber and valve V4 is used to control the depressurization process. The pressure difference across the capillary is measured using the pressure gauge P2 and P1. All the precipitation experiments were carried out in batch mode. The first step was filling up the precipitation chamber with carbon dioxide to obtain the desired pressure. The temperature in the chamber was maintained constant by placing it in a water bath. Approximately 1.5 g of fullerene/toluene solution (C60 concentration, 0.6 mg/mL) was loaded into the “solution injection device” and then this solution was injected into the precipitation chamber through the 75-µm capillary tube. The flow rate of the solution is a function of the pressure drop (∆P) across the capillary. It was assumed that the flow characteristics across the capillary remained the same for a particular ∆P. Immediately following the injection, fullerene particles were formed as a result of the rapid removal of toluene by supercritical CO2. These particles were then allowed to settle down on the collection plate at the bottom of the chamber for about an hour. Next was the cleaning step in which the toluene that was left dissolved in supercritical CO2 was removed by continuous purging of the precipitation chamber with fresh CO2. Extra care was taken to ensure the complete removal of toluene from the chamber. If some toluene had remained inside the chamber, then it would have condensed during the depressurization process, causing particle dissolution and change in morphology. The complete cleaning process required ≈4-5 times the vessel volume of fresh CO2.
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Figure 2. Volumetric expansion of toluene in CO2 versus pressure at 35, 43, and 50 °C, from PREOS using a kij of 0.09.
Figure 3. SEM micrograph of untreated fullerene particles as obtained from the manufacturer. Average particle size is 10-40 µm.
The precipitation chamber was then allowed to slowly depressurize until it reached ambient pressure. The chamber was then opened and the collection plate was removed and taken for particle analysis. Analysis. The collection plate having the particles in the form of a thin powder film was immersed into a vial containing 10 mL of aqueous solution of 5 wt % sodium dodecylbenzene sulfonate surfactant. The vial was then placed in a sonication bath for 20 min to separate the particles that are agglomerated together. The surfactant solution appeared brownish-yellow in color because of the suspended fullerene particles. A small drop of this suspension was then placed onto the TEM grid of size 100 mesh and was examined under a TEM (Zeiss EM 10CR). Several micrographs of the sample were obtained and the particle sizes were measured. Most of the particles obtained were spherical in shape. To obtain the size distribution, diameters of about 100 randomly selected particles were measured for each experiment. To further study the particle morphology, the particles were also analyzed using SEM (Zeiss, model DSM940). For analysis the SEM samples were coated with gold/palladium using a sputter coater (Pelco, model Sc-7). Results and Discussion For any supercritical precipitation experiment to work successfully, the solubilities of the solvent and the solute in the supercritical fluid play important roles. According
Figure 4. (a, b) SEM micrograph of agglomerated fullerene particles. The particles seem loosely attached together by weak attraction forces.
to Saim et al.,23 fullerene (C60) is completely insoluble in supercritical CO2 at a wide range of conditions. This is due to the low degree of CO2 interaction with the π electrons of the 5- and 6-carbon ring units of the large fullerene molecules and also due to extremely small vapor pressures of C60 at our low experimental temperatures. The volumetric expansion of the solvent by the supercritical antisolvent is important for solute precipitation to occur. Ng and Robinson24 have reported the data for the phase behavior of CO2-toluene systems at 38.1, 79.4, 120.7, and 203.9 °C. Later, Dixon et al.25 provided additional data at 25 °C. Because the data at the temperatures of our interest were not available, we decided to use the Peng-Robinson equation of state26 (PREOS) to predict the volumetric expansions at our operating temperatures, using a binary interaction parameter, kij, of 0.09 as suggested by Ng and Robinson.24 According to PREOS, temperature (T), specific volume (ν), and pressure (P) are related as
P)
RT a + (ν - b) ν(ν + b) + b(ν - b)
(1)
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Figure 5. Fullerene particles obtained from experiments conducted at 35 °C and (a) 96.5 bar, (b) 82.7 bar, and (c) 75.8 bar.
where a and b are the characteristic parameters of the mixture given by the following mixing rules:
∑ ∑ yiyjaij b ) ∑ yibi
a)
(2) (3)
aij ) (1 - kij)xaiaj 2
aii )
(4)
2
0.45724R Tci × Pci
[
1 + (0.3764 + 1.54226ωi + 0.26992ωi2) ×
( x )] 1-
bii )
0.07780RTci Pci
T Tci
2
(5) (6)
where ωi, Tci, and Pci are the accentric factor, critical temperature, and critical pressure of the component i, respectively.
The relative volumetric expansion, ∆V%, is defined as
∆V% )
V(P,T) - V0 × 100 V0
(7)
where V is the volume of toluene after contacting CO2 and V0 is the volume of toluene alone. Using eqs 1-7, volumetric expansions were calculated at different equilibrium pressures for 35, 43, and 50 °C isotherms, as shown in Figure 2. For all three isotherms, volumetric expansion above the CO2 critical pressure is extremely high because of the high solubility of the toluene in CO2. A high volumetric expansion is desired for the proposed process, as it gives rise to rapid supersaturation. When a fullerene/toluene solution is sprayed into supercritical CO2 using the capillary tube, small droplets are formed. The Weber number (NWe), which is the ratio of inertial forces to the tension forces, can be used to describe the size of droplets formed in the spray process,27
Ind. Eng. Chem. Res., Vol. 39, No. 7, 2000 2285 Table 1. Particle Morphology Produced by Spraying Fullerene/Toluene Solution into CO2 CO2 pressure inside vessel (bar)
temp (°C)
CO2 density31 (g/mL)
pressure drop across capillary (bar)
96.5 82.7 75.8 96.5 96.5 96.5
35 35 35 43 50 35
0.70 0.56 0.29 0.35 0.50 0.70
41.4 41.4 41.4 41.4 41.4 27.6
spherical, thin film spherical, thin film spherical, agglomerated spherical, thin film spherical, thin film spherical and ellipsoidal, broken film
96.5
35
0.70
13.8
spherical and ellipsoidal, broken film
NWe )
FAu2D σ
morphology of particles, and microstructure
number-average mean particle diameter (nm)
standard deviation (nm)
83 63 29 52 51 63 maj. axis ) 364 min. axis ) 56 63 maj. axis ) 659 min. axis ) 154
25 21 7 12 18 13 17
(8)
where FA is the antisolvent density, u is the relative velocity, σ is the surface tension, and D is the jet diameter. In the precipitation process, a small interfacial tension, high density, and high jet velocity leads to an extremely large NWe. A large Weber number indicates that deforming forces are dominant compared to reforming forces, thus leading to breakup of the spray into tiny droplets. Rapid transfer of CO2 into these droplets and toluene out of these droplets causes them to expand rapidly, leading to a drop in the concentration of toluene. This in turn causes a decrease in the ability of the droplet to keep fullerene dissolved, causing fullerene to precipitate out as fine particles. Precipitation starts at the supercritical fluid-liquid interface and then propagates inside the liquid, attracting the solute toward the interface.28,29 This causes the formation of the porous spherical balloon-like structures made up of tiny fullerene particles coalesced together. Details of these structures, obtained by precipitation at 96.5 bar and 35 °C, are shown in Figure 4a,b. Here, one can see the circular rings of fullerene particles caused by the breaking of the hollow spherical balloon-like structures. Similar structures were also obtained by Revechon et al.29 in the case of AcY particles and Dixon et al.21 in the case of polystyrene particles. Figure 3 shows a SEM image of untreated fullerene particles as obtained from the manufacturer where the particle size ranges from 10 to 40 µm. A comparison of Figures 3 and 4 elucidates the change in morphology and size of fullerene particles because of the precipitation process. Prior to the treatment, fullerene particles appeared to be large, black in color, and irregular in shape with a wide size distribution. After the processing of the fullerene particles by a supercritical fluid antisolvent technique, very tiny spherical particles of yellowish-brown color with a narrow size distribution were obtained. Figure 4a,b shows SEM micrographs of the collection plate having a thin film of fullerene particles obtained by precipitation at 96.5 bar and 35 °C. The particles are agglomerated and coalesced together in the form of an open airy network. Upon visual inspection the film seems discontinuous and subject to collapse and damage when mechanically crushed. The particles are coalesced together by weak physical forces and can be separated apart quite easily. The agglomerated particles are immersed in a suitable surfactant solution and placed in a sonication bath to separate them. No change in the morphology of the individual particles was observed as a result of the mild sonication in our case.
Figure 6. Number of particles versus mean particle size at a constant temperature of 35 °C and at pressures of (a) 96.5 bar, (b) 82.7 bar, and (c) 75.8 bar.
Effects of Different Process Parameters on Particle Morphology and Size. A considerable amount of research has been done in the past to determine the effect of different process parameters on the antisolvent precipitation process, but still there is little knowledge on the quantitative aspects of the process. The size of the particles obtained in the precipitation process depends on several factors, including pressure, density of CO2, nucleation kinetics, initial droplet size, masstransfer rates, and particle growth rate. Contradictory results have been obtained by different authors in the past about the influence of pressure and temperature on the particle size during the precipitation process.18,21,30 Some researchers observed a reduction in particle size with reducing pressures; others found the process to be independent of pressure and yet others obtained a particle size increase with pressure. Similar differences in the results were also observed with temperature and flow rate. Keeping this in mind, we decided to study the effect of change in pressure, temperature, and flow rate in our process, to understand the main factors controlling fullerene particle size and morphology.
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Figure 7. TEM micrographs of fullerene particles obtained from experiments conducted at 96.5 bar and (a) 35 °C, (b) 43 °C, and (c) 50 °C.
Effect of CO2 Pressure and Temperature. The effect of pressure was investigated by conducting several experiments at different pressures at 35 °C. At a pressure of 75.8 bar, toluene dissolved immediately, forming particles of an average size of 29 nm with a standard deviation of 7 nm. As the pressure was increased, the average size of the particles increased to 63 nm for 82.7 bar and 83 nm for 96.5 bar with standard deviations of 21 and 25 nm, respectively. Hence, there is an increase in the size of fullerene particles with an increase in the pressure. Figure 5a-c shows transmission electron micrographs (TEM) of particles obtained in the experiments conducted at these three pressures. Most of the particles are spherical in shape and have a very narrow size distribution as listed in Table 1. Upon visual inspection, in all the cases thin films of fullerene particles were obtained except at 75.8 bar where the film seemed broken and discontinuous because of agglomeration. Figure 6a-c show a comparison between the size distribution of particles obtained in the experiments. To study the influence of temperature, the pressure was kept constant at 96.5 bar while the temperature was varied from 35 to 50 °C. At 35 °C, the particles
formed had an average size of 83 nm and a standard deviation of 25 nm. The size of the particles changed, with an increase in temperature, to 52 nm at 43 °C and to 51 nm at 50 °C with standard deviations of 12 and 18 nm, respectively. Figure 7a-c shows TEMs of the experiments conducted at the three temperatures. Also, Figure 8a-c shows a comparison between the size distributions of particles obtained in these experiments. There is a drop in particle size with an increase in temperature. Because this drop is almost negligible at higher temperatures, no definite trend can be identified in this range. In all the cases spherical particles having a narrow size distribution are obtained. Thin films of fullerene particles were obtained in all the cases (Table 1). When the particle size is plotted versus density31 (Figure 9), most of the points follow a straight line. The particle size is related to the density by the following equation:
S ) 110.58F + 2.68
(9)
where S is the mean size of the fullerene particle and F is the density of supercritical CO2 in g/mL.
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as discrete units and precipitate out as nanoparticles. At higher temperatures and a constant pressure, even though there was a substantial decrease in density, the particle size does not change much because of the temperature being moved further away from the critical point. The trends observed here are similar to those obtained by Randolph et al.,30 that the particle nucleation growth rather than initial droplet size25 was the major determining factor of particle size. Particle sizes increase with increases in density, even though the Weber number of the droplets formed inside the vessel is higher. It is interesting to note that the particles obtained here are in the nanometer range, as opposed to the micrometer range particles obtained in most of the supercritical CO2-based studies. Perhaps, the main reason behind this is the nature of fullerenes as compared to that of other organic/polymeric/drug molecules studied earlier. At our experimental temperature, fullerene is far removed from its melting point. Effect of the Injection Rate of the Fullerene Solution. Another relevant parameter in controlling particle size and morphology is the velocity of solution through the capillary. According to Grant and Middleman,32 the jet breakup length for Newtonian fluids is related to the jet velocity through NWe and the Reynolds number (NRe),
L/d ) CxNWe(1 + 3Z)
Figure 8. Number of particles versus mean particle size at a constant pressure of 96.5 bar and varying temperatures of (a) 35 °C, (b) 43 °C, and (c) 50 °C.
Figure 9. Mean (number-average) particle size versus CO2 density at different conditions described in Table 1.
This shows that the density is the key parameter in controlling the particle size. Density influences the mass-transfer characteristics between supercritical CO2 and toluene, which is the main driving force in precipitation. At the lower densities toward the critical point, there is faster diffusion of CO2 into and toluene out of the droplet, causing rapid precipitation, leading to the formation of nucleation sites. Because the fullerene concentration is very low and the rate of precipitation rapid, chances of growth for these nuclei by diffusion of fullerene into or by interaction with another such nuclei is fairly minimal. These individual nuclei thus behave
(10)
where L/d is the dimensionless jet breakup length, Z (Ohnesorge number) is equal to xNWe/NRe, and C is a constant. On one hand, for high values of Z and NWe, the jet breaks into droplets much smaller in size than the jet diameter. On the other hand, the low values of Z and NRe result in droplet diameters of the order of the jet diameter. Mass transfer between the droplet and the supercritical antisolvent is an important factor affecting particle size and morphology. Because jet velocity controls the droplet diameter, it is therefore an important tuning parameter for size and morphology control. Randolph et al.30 observed an increase in particle size with increasing flow rates. To observe the effect of flow rate in our system, we studied the morphology and the size of particles obtained at different pressure drops (∆P) across the capillary at constant pressure and temperature conditions of 96.5 bar and 35 °C. The mean sizes of the particles for ∆P of 41.4, 27.6, and 13.8 bar were 83, 63, and 64 nm, respectively. No particular trend was deduced as to the effect of particle size with flow rate because the particles maintain fairly close to the mean size with small deviations. However, there was an appreciable change in the morphology of the particles at lower flow rates as shown in Figures 10ac. A mixture of rodlike and spherical particles was obtained when ∆P was 27.6 or 13.8 bar. The mean sizes of the rodlike particles were as follows: major axis, 659 nm, and minor axis, 171 nm, for a ∆P of 27.6 bar, and major axis, 364 nm, and minor axis, 56 nm, for ∆P of 13.8 bar. Upon visual inspection, one noticed much agglomeration of particles at lower ∆P and the film of particles seemed nonuniform and broken. Conclusions Fullerene nanoparticles can be successfully produced by a supercritical CO2 antisolvent precipitation process.
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Figure 10. TEM micrographs of fullerene particles obtained from experiments conducted at 96.5 bar and 35 °C and at different flow rates: ∆P ) (a) 41.4 bar, (b) 27.6 bar, and (c) 13.8 bar.
Supercritical CO2 density can be used to control the size of the particles. An increase in particle size was obtained with an increase in CO2 density away from the critical point. In all cases a very narrow size distribution of particles was obtained. The flow rate of the solution through the capillary can be used to control the morphology of the particles. Both spherical and rodlike particles were obtained at lower flow rates. The process was also found suitable for forming a thin, uniform film of nanometer-size fullerene particles on surfaces. Acknowledgment We thank Dr. Michael E. Miller of Auburn University for help with the SEM and TEM micrographs. Financial support from NSF (CST-9801067), NIH (James A. Shannon Director’s award to R.B.G.), the U.S. Civilian Research and Development Foundation (RCI-170), and NSF-EPSCOR (Young Faculty Career Enhancement Award to R.B.G.) are deeply appreciated. Literature Cited (1) Kroto, H. W.; Heath, J. R.; O’Brian, S. C.; Curl, R. F.; Smalley, R. E. C60: Buckminsterfullerene. Nature 1985, 318, 162.
(2) Kratschmer, W.; Lamb, L. D.; Fostiropoulos, K.; Huffman D. R. Solid C60: A New form of Carbon. Nature 1990, 347, 354. (3) Yildirim, T.; Barbedette, L.; Kniaz, K., Fischer, J. E.; Lin, C. L.; Bykovetz, N.; Stephens, P. W.; Sulewski, P. E.; Erwing, S. C. Fullerene Superconductors: Effects of Molecular Orientation and Valence. Mater. Res. Symp. Proc. 1995, 359. (4) Wang, Y. Photoconductivity of Fullerene Doped Polymers. Nature 1992, 356, 585. (5) Joutsensaari, J.; Ahonen, P.; Tapper, U.; Kauppinen, E. I.; Laurila, J.; Kuokkala, V. T.; Generation of Nanophase Fullerene Particles via Aerosol routes. Synth. Met. 1996, 77, 85. (6) Wei, Z.; Jinke, T.; Puri, A.; Seany R. L.; Yuxin, L.; Liquan, C. Tribiological Properties of Fullerene C60 and C70 Microparticles. J. Mater. Res. 1996, 11, 2749. (7) Meilunas, R. J.; Chang, R. P. H. Nucleation of Diamond Films on Surfaces Using Carbon Clusters. Appl. Phys. Lett. 1991, 59, 3461. (8) Tutt, L. W.; Kost, A. Optical Limiting Performance of C60 and C70 Solutions. Nature 1992, 356, 225. (9) Friedman, S. H.; DeCamp, D. L.; Sijbesma, R. P.; Srdanov, G.; Wudl, F.; Kenyon. G. L. Inhibiton of the HIV-1 Protease by Fullerene Derivatives: Model Building Studies and Experimental Verification. J. Am. Chem. Soc. 1993, 115, 6506. (10) Seigel, R. W. Nanostructured Materials. Mater. Sci. Eng. 1993, A158, 189. (11) Gurav, A. S.; Duana, Z.; Wang, L. Synthesis of FullereneRhodium Nanocomposites via Aerosol Decomposition. Chem. Mater. 1993, 5, 214.
Ind. Eng. Chem. Res., Vol. 39, No. 7, 2000 2289 (12) Gurav, A. S.; Kodas, T, T.; Lu-Ming; W, Kauppinen; E. I., Joutsensaari; J. Generation of Nanometer-Size Fullerene Particles via Vapor Condensation. Chem. Phys. Lett. 1994, 218, 304. (13) Chowdhury, K. D.; Howard, J. B.; VanderSande, J. B. Fullerenic Nanostructures in Flames. J. Mater. Res. 1996, 11, 341. (14) Lozovik, Yu. E.; Popov, A. M. Formation Mechanism of Nanoparticles in Arc Discharge. Mol. Mater. 1996, 7, 89. (15) Krukonis, V. J. Supercritical Fluid Processing: Current Research and Operations. In Proceedings of the International Symposium on Supercritical Fluids; Perrut, M. Ed.; INPL: Nice, France, 1998; p 541. (16) Gallagher, P. M.; Coffey, M. P.; Krukonis, V. J.; Klasutis, N. Gas Antisolvent Re-crystallization: New Process To Recrystallize Compounds Insoluble in Supercritical Fluids. Am. Chem. Soc. Symp. Ser. 1989, 405, 334. (17) Winters, M. A.; Barbara, L. K.; Debenedetti, P. G.; Sparks, H. G.; Przybycien, T. M.; Stevenson C. L.; Prestrelski, S. J. Precipitation of Proteins in Supercritical Carbon Dioxide. J. Pharm. Sci. 1993, 85, 586. (18) Yeo, S.-F.; Lim, G.-B.; Debenedetti, P. G.; Bernstein, H. Formation of Microparticle Protein Powders Using Supercritical Fluid Antisolvent. Biotechnol. Bioeng. 1993, 41, 341. (19) Chang C. J.; Randolph A. D. Precipitation of Microsize Organic Particles from Supercritical Fluids. AIChE J. 1989, 35, 1876. (20) Benedetti, L.; Bertucco, A.; Pallado, P. Production of Micronic Particles of Biochemical Polymer Using Supercritical Carbon Dioxide. Biotechnol. Bioeng. 1997, 53, 232. (21) Dixon, D. J.; Johnston K. P.; Bodmeier, R. A. Polymeric Materials Formed by Precipitation with a Compressed Fluid Antisolvent. AIChE J. 1993, 39, 127. (22) Debenedetti, P. G. Homogeneous Nucleation in Supercritical Fluids. AIChE J. 1990, 35, 1289. (23) Saim, S.; Kuo, K. C.; Stalling, D. L. Supercritical Fluid Extraction of Fullerenes C60 and C70 from Carbon Soot. Sep. Sci. Technol. 1993, 28, 1509.
(24) Ng, H. J.; Robinson D. B. Equilibrium Phase Properties of the Toluene-Carbon Dioxide System. J. Chem. Eng. Data 1978, 24, 325. (25) Dixon, D. J.; Johnston K. P. Molecular Thermodynamics of Solubilities in Gas Antisolvent Crystallization. AIChE J. 1991, 37, 1441. (26) Peng, D.-Y., Robinson, D. B. A Two Constant Equation of State. Ind. Eng. Chem. Fundam. 1976, 15, 59. (27) Lefebvre, A. H. Atomization and Sprays; Hemisphere Publishing: NewYork, 1989. (28) Bertucco, A. Precipitation and Crystallization Techniques. In Chemical Synthesis Using Supercritical Fluids; Jessop, P. G., Leitner, W., Eds.; Wiley-VCH Verlag GmbH: Weinheim, Germany, 1999; pp 108-126. (29) Reverchon, E.; Della Porta G.; Di Trolio, A.; Pace, S. Supercritical Antisolvent Precipitation of Nanparticles of Superconductor Precursors Ind. Eng. Chem. Res. 1998, 37, 952. (30) Randolph, T. W.; Randolph, A. D.; Mebes, M.; Yeung, S. Sub-Micron-Sized Biodegradable Particles of Poly(l-Lactic Acid) via the Gas Antisolvent Spray Precipitation Process. Biotechnol Prog. 1993, 9, 429. (31) Ely, J. F.; Magee, J. W.; Haynes, W. M. Thermophysical Properties for Special High CO2 Content Mixtures; Research Report RR-110, Gas Processors; Association: Tulsa, OK, 1987 [all data]. (32) Grant, R. P.; Middleman, S. Newtonian Jet Stability. AIChE J. 1955, 12, 559. (33) Eckert, C. A.; Knutson, B. L.; Debenedetti, P. G. Supercritical Fluids as Solvents for Chemical and Material Processing. Nature 1996, 383, 313.
Received for review October 5, 1999 Revised manuscript received February 29, 2000 Accepted March 8, 2000 IE990729K
