Supercritical Fluid Processing of Nanoscale Particles from

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Supercritical Fluid Processing of Nanoscale Particles from Biodegradable and Biocompatible Polymers Mohammed J. Meziani, Pankaj Pathak, Tarang Desai, and Ya-Ping Sun* Department of Chemistry and Laboratory for Emerging Materials and Technology, Clemson UniVersity, Clemson, South Carolina 29634-0973

The supercritical solution rapid expansion technique, rapid expansion of a supercritical solution into a liquid solvent (RESOLV), was applied to the processing of poly(L-lactic acid) and poly(methyl methacrylate) into nanoscale particles (100 nm or less). Neat supercritical carbon dioxide or with ethanol as a cosolvent was used as solvent, and an aqueous medium was used at the receiving end of the rapid expansion. The asproduced polymeric nanoparticles were suspended, and various strategies for the stabilization of the suspension to protect the nanoparticles from agglomeration were evaluated. It was found that, in the presence of a watersoluble polymer or surfactant, aqueous suspensions of the well-dispersed polymeric nanoparticles could remain stable for an extended period of time to allow potentially further processing into the desired products. Introduction Polymeric nanoparticles (100 nm or less), especially those that are biocompatible and/or biodegradable, have attracted much attention for their various applications.1,2 In the field of drug delivery, as an example, these nanoparticles may be used to carry a wide range of drugs, proteins, vaccines, or other biological species and for the purpose of controlled release of drugs.1,2 Among commonly used methods for preparing polymeric microspheres and nanoparticles are those based on the in situ particle formation in emulsion or other polymerization of appropriate macromonomers.3-5 Polymeric nanoparticles have also been produced via solvent-in-emulsion evaporation, phase separation, and spray drying.6 However, there is still the need for developing other more effective and versatile techniques for the processing of polymeric nanoparticles, in particular, those with advantages such as low solvent and surfactant uses, minimum unwanted residues, colloidal stability, etc.6 Several supercritical fluid processing techniques have been applied to the production of polymeric particles, yielding promising results.7-19 Particularly popular has been the rapid expansion of a supercritical solution (RESS) for the processing of particles from a variety of polymeric materials.7-17 In RESS, the solute is dissolved in a supercritical fluid to form a solution, followed by the rapid expansion of the solution into air. Because of the rapid pressure reduction in the expansion, significant supersaturation is reached in a short period of time. The resulting rapid decrease in solubility accompanied with homogeneous nucleation of the solute leads to well-dispersed particles. However, the RESS process generally produces micrometersized (occasionally submicrometer-sized) particles as primary products, despite the fact that theoretical modeling and calculations have been predicting for the formation of nanoscale particles.7-17,20,21 A widely acknowledged explanation for the observation of larger particles in traditional RESS is the presence of substantial condensation and coagulation of initially formed smaller particles in the expansion jet.17,22 Thus, our laboratory has recently developed a simple but critical modification to the traditional RESS by expanding the supercritical solution into a liquid instead of air, or the rapid expansion of a supercritical * To whom correspondence should be addressed. Tel.: (864) 6565026. Fax: (864) 656-5007. E-mail: [email protected].

solution into a liquid solvent (RESOLV).18,19,23 The liquid at the receiving end of the rapid expansion suppresses the particle growth, making it possible to obtain exclusively nanoscale particles.18,19,23 The successful use of RESOLV for the processing of nanoparticles from a CO2-soluble fluoropolymer has been demonstrated.19a Here, we report applications of RESOLV to the production of exclusively nanoscale particles from biodegradabe and biocompatible polymers, including poly(L-lactic acid) (PLA) and poly(methyl methacrylate) (PMMA), for their stable aqueous suspensions. Results from the characterization of the polymeric nanoparticles and the protection of the suspensions are presented and discussed. Experimental Section Materials. PLA (MW ≈ 2000), PMMA (MW ≈ 35 000), poly(vinyl alcohol) (PVA, MW ≈ 100 000), sodium dodecyl sulfate (SDS), and sodium chloride were purchased from Aldrich. Pluronic L-31 was kindly provided by BASF. Ethanol was obtained from Fisher Scientific and was distilled over molecular sieves and then filtered before use. Carbon dioxide (high-purity SFC grade) was supplied by Air Products. Water was deionized and purified by being passed through a Labconco WaterPros water purification system. Measurement. Solubilities of PLA and PMMA in carbon dioxide without and with ethanol as cosolvent were evaluated by visual inspection of the cloud point in a high-pressure optical cell.24 A miniature magnetic stirring bar was placed in the cell chamber for effective mixing. The system pressure was generated by a syringe pump and monitored by a precision pressure gauge (Heise 901A). The system temperature was controlled and monitored by an RTD temperature controller (Omega 4200A) coupled with a pair of cartridge heaters (Gaumer 150 W) inserted into the optical cell body. The experimental setup for RESOLV is illustrated in Figure 1. A syringe pump was used to generate and maintain the system pressure, which was monitored by a precision pressure gauge. The heating unit consisted of a cylindrical solid copper block of high heat capacity in a tube furnace, and the copper block was tightly wrapped with the stainless steel tubing coil to ensure efficient heat transfer. The nozzle was a laser-drilled orifice (50µm in inner diameter and the aspect ratio of 5), one end of

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Figure 1. The RESOLV setup for the processing of polymeric nanoparticles.

which was attached to the pre-expansion section of the tubing and the other end was inserted into the collection chamber containing the ambient receiving solution. Scanning electron microscopy (SEM) images were obtained on Hitachi S4700 field-emission SEM and Hitachi HD-2000 STEM (in the SEM mode) instruments. The energy-dispersive X-ray (EDX) analysis was performed in situ on the same SEM system. The specimen for SEM analyses was prepared by depositing a few drops of a dilute suspension of nanoparticles onto a carbon tape, followed by evaporating the solvent under ambient condition and then coating the sample with platinum to minimize charging effects. For the suspension containing NaCl and/or surfactant, it was first filtered through polycarbonate “Track Etch” membrane filter (pore size 30 nm), followed by washing the filter with water and then attaching it to a carbon tape. After a complete removal of residual water at ambient temperature, the specimen was coated with platinum. Results and Discussion A general requirement for RESOLV processing is solubility of the polymer under consideration in the selected supercritical fluid. PLA is soluble in ethanol (solubility ≈ 17 wt %), which makes ethanol a suitable cosolvent in supercritical CO2.16 In CO2 with 10 vol % ethanol, the solubility of PLA is higher than 4 mg/mL at 80 °C and 350 bar, sufficient for RESOLV processing. In a typical RESOLV experiment, PLA was dissolved in CO2 with 10 vol % ethanol as cosolvent in the syringe pump. The solution (concentration 0.1 mg/mL) was equilibrated at 80 °C, and then rapidly expanded at a preexpansion pressure of 200 bar through a 50-µm orifice into an ambient aqueous medium. Because the PLA polymer has no meaningful solubility in ambient water, it precipitated into particles. The as-formed particles were all nanoscale, although their harvesting as nanoparticles in the final products was dependent on the nature of the receiving aqueous medium. In the case of only water at the receiving end, the formation of aggregates became visible as the rapid expansion progressed (within minutes). An aliquot of the suspension was used to prepare a specimen for SEM analysis. The results suggest that the sample contained predominantly aggregated PLA nanoparticles. The agglomeration of the initially formed PLA particles was similar in rate and appearance to that of the fluoropolymer particles reported earlier,19a which is probably understandable because the postexpansion agglomeration is likely dictated by diffusional processes in the same aqueous receiving medium. In an effort to mitigate the aggregation, an aqueous NaCl solution (0.5 M) instead of neat water was used at the receiving end of the rapid expansion. The increased ionic strength in the suspension of the initially formed nanoparticles had some stabilization effects.

Figure 2. SEM images of the PLA nanoparticles obtained with the rapid expansion of the solution of PLA in neat CO2 (0.1 mg/mL, 80 °C, 550 bar) into neat water (top) and an aqueous NaCl solution (bottom).

According to the SEM imaging of the specimen from the suspension, the PLA nanoparticles can be identified, but still in aggregates. The use of relatively low PLA concentration was to avoid the formation of any nanofiber products, as reported previously.19c In fact, the low PLA concentration could also be achieved in supercritical CO2 at 80 °C and a higher pressure of 550 bar without the use of cosolvent. In RESOLV with the solution of PLA in neat CO2 (0.1 mg/mL), the rapid expansion was at a pre-expansion pressure of 550 bar through a 50-µm orifice into an ambient aqueous medium. Again, with only water at the receiving end, the initially formed PLA nanoparticles aggregated significantly (Figure 2). An increase of ionic strength in the suspension with the addition of NaCl (0.5 M) had some stabilization effects, but did not stop the particle agglomeration. Nevertheless, it is clear from the SEM images that the products contained primarily aggregates of PLA nanoparticles with sizes in the range of 30-200 nm (Figure 2). These results are similar to those reported previously on the RESOLV processing of the CO2-soluble fluoropolymer.19a It was rationalized that in RESOLV the initially formed polymer particles are all nanoscale, but these suspended nanoscale particles quickly agglomerate in the absence of any protection from a stabilization agent. Indeed, the harvesting of exclusively nanoscale polymer particles could be achieved by including a commonly used stabilization agent in the liquid receiving solution in RESOLV. The surfactant SDS was found to be an effective stabilization agent for the protection of the initially formed PLA nanoparticles in RESOLV. For example, when an aqueous solution of SDS (9 mM) was used at the receiving end of the rapid expansion (PLA concentration 0.1 mg/mL in neat CO2, 80 °C, 550 bar), the resulting suspension of PLA nanoparticles remained stable without precipitation (at least several days). The specimen from the stable suspension was analyzed by SEM, and the results suggested no significant presence of aggregates. As shown in Figure 3, the PLA nanoparticles are relatively well-dispersed.

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Figure 5. SEM images of the PMMA nanoparticles obtained with the rapid expansion of the PMMA in CO2-ethanol solution (0.1 mg/mL, 80 °C, 350 bar) into aqueous NaCl (top) and SDS (bottom) solutions. Figure 3. Top: an SEM image of the PLA nanoparticles obtained with SDS as stabilization agent (9 mM) in RESOLV (PLA concentration 0.1 mg/mL in neat CO2, 80 °C, 550 bar). Bottom: a collection of various regions in SEM images of the same specimen.

Figure 4. An SEM image of the PLA nanoparticles obtained with PVA as stabilization agent (3.5 mg/mL) in RESOLV (PLA concentration 0.1 mg/ mL in neat CO2, 80 °C, 550 bar).

According to a statistical analysis of multiple SEM images, these PLA nanoparticles have an average size of 92 nm in diameter, with a size distribution standard deviation of 35 nm. Many other stabilization agents could be used to protect the PLA nanoparticles from aggregation in the RESOLV process, including PVA polymer and pluronic L-31 (PLA concentration 0.1 mg/mL, 80 °C, 550 bar). Shown in Figure 4 are PLA nanoparticles obtained from RESOLV with PVA polymer (3.5 mg/mL) in the aqueous receiving solution. The average particle size and size distribution standard deviation of these PVA polymer-protected PLA nanoparticles are similar to those obtained with SDS as the stabilization agent. PMMA is an important commercial polymer, also known to be biocompatible. However, the solubility of PMMA in supercritical CO2 is low even under higher pressure conditions, so that the use of cosolvent is necessary. In CO2 with 10 vol %

ethanol at 80 °C and 350 bar, the solubility of PMMA was found to be at least 1 mg/mL, again sufficient for RESOLV. The rapid expansion of the PMMA solution (concentration 0.1 mg/mL) in supercritical CO2-10 vol % ethanol was at a pre-expansion temperature and pressure of 80 °C and 350 bar, respectively, through the same 50-µm orifice into an ambient aqueous NaCl solution (0.5 M). The observed formation of PMMA nanoparticles and subsequent agglomeration (Figure 5) were similar to those found in the preparation of PLA nanoparticles. The PMMA nanoparticles could also be protected by SDS, added in the aqueous receiving medium (9 mM) in RESOLV. The suspension of SDS-protected PMMA nanoparticles remained stable (at least several days) and appeared homogeneous. SEM images of the specimen prepared from the stable suspension show well-dispersed PMMA nanoparticles (Figure 5), from which an average size of 112 nm and a size distribution standard deviation of 40 nm can be obtained from the statistical analysis. According to the results presented above, the RESOLV process is obviously capable of producing exclusively nanoscale particles from polymers of different structures and properties. This distinguishes RESOLV from the traditional RESS, in which larger (mostly micrometer-sized) polymeric particles are generally produced. For example, the use of RESS with supercritical CO2 for the processing of PLA and PMMA polymers resulted in primarily micrometer-sized particles, with particle diameters ranging from 1 to 25 µm.13c,16 Because the two processes are identical down to the tip of the expansion nozzle, it seems likely that both RESOLV and traditional RESS produce nanoscale particles as “transient” products (upstream of the Mach disk in the supersonic free jet region).25,26 It is the dynamic and complex process following the initial particle formation that determines whether nano- or microparticles are harvested. The use of liquid at the receiving end of the rapid expansion in RESOLV effectively quenches the predicted particle growth in the expansion jet, allowing the “capturing” of the initially formed

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nanoscale particles. Because of the use of liquid, however, the nanoparticles from RESOLV are suspended (instead of being in the solid state for particles from RESS). Suspended nanoscale particles, with their large surface areas, typically undergo agglomeration in the absence of any stabilization agent, as observed here with the initially formed PLA and PMMA nanoparticles in aqueous suspension. Conceptually, the stabilization of the nanoparticles should probably be considered as a separate issue, which is common with any colloidal systems. The use of increased ionic strength by adding the inorganic salt sodium chloride to the suspension for stabilization is obviously preferred for not introducing organic “foreign” substance into the polymeric system, but the strategy appears not so effective for the polymeric nanoparticles in this work. On the other hand, the PLA and PMMA nanoparticles from RESOLV can be protected from agglomeration by many commonly employed polymeric and surfactant stabilization agents, offering various options for selection in terms of requirements in specific applications. The production of polymeric nanoparticles via RESOLV is versatile and broadly applicable. A distinctive feature of the technique is that no nano-templating agents (such as surfactants used in most other methods)3-5 are required for the nanoparticle formation. Instead, the supersaturation over a very short period of time and homogeneous nucleation, characteristic of supercritical solution during the rapid expansion, are responsible for the nanosizing of the polymeric particles. In this work, polymeric and surfactant stabilization agents were used to protect the initially formed nanoparticles from agglomeration, but only in relatively small quantities. In principle, even this level of surfactant use might be avoided with potentially other schemes for the harvesting of the nanoscale polymeric particles, such as rapid dilution, solvent evaporation, or solidification that can effectively compete with the particle agglomeration process. The nanoparticles of biodegradable and biocompatible polymers largely free from unwanted “impurities” are valuable to many applications, especially in the formulation and delivery of drugs and other therapeutic agents or biological species. Acknowledgment Financial support from the NSF and the Center for Advanced Engineering Fibers and Films (NSF-ERC at Clemson University) is gratefully acknowledged. Literature Cited (1) (a) Soppimath, K. S.; Aminabhavi, T. M.; Kulkarni, A. R.; Rudzinski, W. E. Biodegradable Polymeric Nanoparticles as Drug Delivery Devices. J. Controlled Release 2001, 70, 1. (b) Panyam J.; Labhasetwar, V. Biodegradable Nanoparticles for Drug and Gene Delivery to Cells and Tissue. AdV. Drug DeliVery ReV. 2003, 55, 329. (2) (a) Mainardes, R. M.; Silva, L. P. Drug Delivery Systems: Past, Present, and Future. Curr. Drug Targets 2004, 5, 449. (b) Bramwell, V. W.; Perrie, Y. Particulate Delivery Systems for Vaccines. Crit. ReV. Ther. Drug Carrier Syst. 2005, 22, 151. (c) Del Valle, E. M. M.; Galan, M. A. Supercritical Fluid Technique for Particle Engineering: Drug Delivery Applications. ReV. Chem. Eng. 2005, 21, 33. (3) (a) Ugelstad, J.; Mork, P. C.; Kaggerud, K. H.; Ellingsen, T.; Berge, A. Swelling of Oligomer-Polymer Particles. New Methods of Preparation of Emulsions and Polymer Dispersions. AdV. Colloid Interface Sci. 1980, 13, 101. (b) Vanderhoff, J. W.; El-Aasser, M. S.; Tseng, C. M. Preparation of Large-Particle-Size Monodisperse Latexes in Space: Polymerization Kinetics and Process Development. J. Dispersion Sci. Technol. 1984, 5, 231. (4) Landfester, K. Miniemulsions for Nanoparticle Synthesis. Top. Curr. Chem. 2003, 227, 75.

(5) (a) Miller, C. M.; El-Aasser, M. S. Recent AdVances in Polymeric Dispersions; NATO ASI Series E: Applied Science, 335; Kluwer Academic Publisher: Dordrecht, 1997. (b) Candau, F. Polymerization in Organized Media; Gordon & Breach: Philadelphia, 1992; Chapter 4. (6) (a) Pichot, C. Recent Developments in the Functionalization of LatexParticles. Makromol. Chem., Macromol. Symp. 1990, 35, 327. (b) Kawaguchi, H. Functional Polymer Microspheres. Prog. Polym. Sci. 2000, 25, 1171. (c) Zhang, G. Z.; Niu, A. Z.; Peng, S. F.; Jiang, M.; Tu, Y. F.; Li, M.; Wu, C. Formation of Novel Polymeric Nanoparticles. Acc. Chem. Res. 2001, 34, 249. (7) Jung, J.; Perrut, M. Particle Design using Supercritical Fluids: Literature and Patent Survey. J. Supercrit. Fluids 2001, 20, 179. (8) Kompella, U. B.; Koushik, K. Preparation of Drug Delivery Systems using Supercritical Fluid Technology. Crit. ReV. Ther. Drug Carrier Syst. 2001, 18, 173. (9) Yeo, S. D.; Kiran, E. Formation of Polymer Particles with Supercritical Fluids: A Review. J. Supercrit. Fluids 2005, 34, 287. (10) Krukonis, V. J. Presented at the AIChE Annual Meeting, 1984. (11) (a) Matson, D. W.; Petersen, R. C.; Smith, R. D. The Preparation of Polycarbosilane Powders and Fibers during Rapid Expansion of Supercritical Fluid Solutions. Mater. Lett. 1986, 4, 429. (b) Petersen, R. C.; Matson, D. W.; Smith, R. D. Rapid Precipitation of Low Vapor-Pressure Solids from Supercritical Fluid Solutions - The Formation of Thin-Films and Powders. J. Am. Chem. Soc. 1986, 108, 2100. (c) Matson, D. W.; Fulton, J. L.; Petersen, R. C.; Smith, R. D. Rapid Expansion of Supercritical Fluid Solutions-Solute Formation of Powders, Thin Films, and Fibers. Ind. Eng. Chem. Res. 1987, 26, 2298. (12) Mohamed, R. S.; Halverson, D. S.; Debenedetti, P. G.; Prud’homme, R. K. Supercritical Fluid Science and Technology; ACS Symposium Series 406; American Chemical Society: Washington, DC, 1989; p 355. (13) (a) Lele, A. K.; Shine, A. D. Morphology of Polymers Precipitated from a Supercritical Solvent. AIChE J. 1992, 38, 742. (b) Lele, A. K.; Shine, A. D. Effect of RESS Dynamics on Polymer Morphology. Ind. Eng. Chem. Res. 1994, 33, 1476. (c) Tom, J. W.; Debenedetti, P. G. Formation of Bioerodible Polymeric Microspheres and Microparticles by Rapid Expansion of Supercritical Solutions. Biotechnol. Prog. 1991, 7, 403. (14) (a) Mawson, S.; Johnston, K. P.; Combes, J. R.; DeSimone, J. M. Formation of Poly(1,1,2,2-tetrahydroperfluorodecyl acrylate) Submicron Fibers and Particles from Supercritical Carbon-Dioxide Solutions. Macromolecules 1995, 28, 3182. (b) Luna-Barcenas, G.; Mawson, S.; Takishima, S.; DeSimone, J. M.; Sanchez, I. C.; Johnston, K. P. Phase Behavior of Poly(1,1-dihydroperfluorooctylacrylate) in Supercritical Carbon Dioxide. Fluid Phase Equilib. 1998, 146, 325. (15) Blasig, A.; Shi, C. M.; Enick, R. M.; Thies, M. C. Effect of Concentration and Degree of Saturation on RESS of a CO2-Soluble Fluoropolymer. Ind. Eng. Chem. Res. 2002, 41, 4976. (16) (a) Mishima, K.; Matsuyama, K.; Yamaguchi, S.; Tanabe, D.; Young, T. J.; Johnston, K. P. Microencapsulation of Proteins by Rapid Expansion of Supercritical Solution with a Nonsolvent. AIChE J. 2000, 46, 857. (b) Matsuyama, K.; Mishima, K.; Umemoto, H.; Yamaguchi, S. Environmentally Benign Formation of Polymeric Microspheres by Rapid Expansion of Supercritical Carbon Dioxide Solution with a Nonsolvent. EnViron. Sci. Technol. 2001, 35, 4149. (c) Matsuyama, K.; Donghui, Z.; Urabe, T.; Mishima, K. Formation of L-poly(lactic acid) Microspheres by Rapid Expansion of CO2 Saturated Polymer Suspensions. J. Supercrit. Fluids 2005, 33, 277. (17) Weber, M.; Thies, M. C. In Supercritical Fluid Technology in Materials Science and Engineering: Synthesis, Properties, and Applications; Sun, Y.-P., Ed.; Marcel Dekker: New York, 2002; p 387. (18) Sun, Y.-P.; Rollins, H. W.; Bandara, J.; Meziani, M. J.; Bunker, C. E. In Supercritical Fluid Technology in Materials Science and Engineering: Synthesis, Properties, and Applications; Sun, Y.-P., Ed.; Marcel Dekker: New York, 2002; p 491. (19) (a) Meziani, M. J.; Pathak, P.; Hurezeanu, R.; Thies, M. C.; Enick, R. M.; Sun, Y.-P. Supercritical-Fluid Processing Technique for Nanoscale Polymer Particles. Angew. Chem., Int. Ed. 2004, 43, 704. (b) Sun, Y.-P.; Meziani, M. J.; Pathak, P.; Qu, L. W. Polymeric Nanoparticles from Rapid Expansion of Supercritical Fluid Solution. Chem.-Eur. J. 2004, 11, 1366. (c) Meziani, M. J.; Pathak, P.; Wang, W.; Desai, T.; Patil, A.; Sun, Y.-P. Polymeric Nanofibers from Rapid Expansion of Supercritical Solution. Ind. Eng. Chem. Res. 2005, 44, 4594. (20) Domingo, C.; Berends, E.; Van Rosmalen, G. M. Precipitation of Ultrafine Organic Crystals from the Rapid Expansion of Supercritical Solutions over a Capillary and a Frit Nozzle. J. Supercrit. Fluids 1997, 10, 39. (21) Kro¨ber, H.; Teipel, U.; Krause, H. Manufacture of Submicron Particles via Expansion of Supercritical Fluids. Chem. Eng. Technol. 2000, 23, 763.

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ReceiVed for reView June 14, 2005 ReVised manuscript receiVed July 19, 2005 Accepted July 21, 2005 IE050704N